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Title: Water Reptiles of the Past and Present
Author: Williston, Samuel Wendell
Language: English
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THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS
_Agents_
THE CAMBRIDGE UNIVERSITY PRESS
LONDON AND EDINBURGH
THE MARUZEN-KABUSHIKI-KAISHA
TOKYO, OSAKA, KYOTO
KARL W. HIERSEMANN
LEIPZIG
THE BAKER & TAYLOR COMPANY
NEW YORK
WATER REPTILES OF THE PAST AND PRESENT
BY SAMUEL WENDELL WILLISTON
_Professor of Paleontology in the
University of Chicago_
[Illustration]
THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS
COPYRIGHT 1914 BY
THE UNIVERSITY OF CHICAGO
All Rights Reserved
Published October 1914
Composed and Printed By
The University of Chicago Press
Chicago, Illinois, U.S.A.
PREFACE
It was just forty years ago that the writer of these lines, then an
assistant of his beloved teacher, the late Professor B. F. Mudge,
dug from the chalk rocks of the Great Plains his first specimens of
water reptiles, mosasaurs and plesiosaurs. To the youthful collector,
whose first glimpse of ancient vertebrate life had been the result of
accident, these specimens opened up a new world and diverted the course
of his life. They were rudely collected, after the way of those times,
for modern methods were impracticable with the rifle in one hand and
the pick in the other. Nor was much known in those days of these or
other ancient creatures, for the science of vertebrate paleontology was
yet very young. There were few students of fossil vertebrates—Leidy,
Cope, and Marsh were the only ones in the United States—and but few
collectors, of whom the writer alone survives.
Those broken and incomplete specimens, now preserved in the museum of
Yale University, will best explain why this little book was written.
The author offers it, so far as lies within him, as an authoritative
and accurate account of some of the creatures of earlier ages which
sought new opportunities by going down from the land into the water.
So far as possible he has endeavored to make the text understandable,
and, he hopes, of interest also, to the non-scientific reader. He will
not apologize for such scientific terms as remain, since only by their
use can precision be attained: there are no common English equivalents
for them. The reader will find their explanations in the chapter on the
skeleton of reptiles, and especially in the illustrations.
The author has had the opportunity during recent years of critically
studying nearly all the reptiles described in the following pages,
but, if that were the only source of his information, the accounts of
many would have been meager. He has endeavored, briefly at least, to
mention the names of all those to whom we are chiefly indebted for our
knowledge, but in such a work as this it is manifestly impracticable to
give due credit to every one.
To the friends who have been of assistance in various ways he tenders
his sincere thanks: to Professor E. Fraas for photographs and the kind
permission to reproduce some of his excellent illustrations; to Dr.
Dreverman, of the Senckenberg Museum, for several excellent photographs
for reproduction or restoration; to Dr. Hauff, of Holzmaden, for an
excellent photograph of an ichthyosaur; to Dr. H. F. Osborn, of the
American Museum, for permission to reproduce the spirited restoration
of ichthyosaurs drawn by Mr. Knight; to Professors Schuchert and Lull,
and Dr. Wieland, of Yale University; to Dr. Hay and Mr. Gilmore, of the
National Museum, to Mr. Barnum Brown and Dr. McGregor, of the American
Museum, and to Professor Merriam, of the University of California, for
photographs and other favors.
SAMUEL W. WILLISTON
UNIVERSITY OF CHICAGO
July, 1914
CONTENTS
CHAPTER PAGE
I. INTRODUCTION 1
II. CLASSIFICATION OF REPTILES 13
III. THE SKELETON OF REPTILES 19
IV. THE AGE OF REPTILES 44
V. ADAPTATION OF LAND REPTILES TO LIFE IN THE WATER 59
VI. ORDER SAUROPTERYGIA 73
Plesiosauria.
Nothosauria.
VII. ORDER ANOMODONTIA 102
Lystrosaurus.
VIII. ORDER ICHTHYOSAURIA
107
IX. ORDER PROGANOSAURIA 126
Mesosaurus.
X. ORDER PROTOROSAURIA 132
Protorosaurus.
Pleurosaurus.
XI. ORDER SQUAMATA 138
Lizards.
Mosasaurs.
Snakes.
XII. ORDER THALATTOSAURIA
171
XIII. ORDER RHYNCHOCEPHALIA 176
Choristodera.
XIV. ORDER PARASUCHIA 184
Phytosauria.
XV. ORDER CROCODILIA 194
Eusuchia.
Mesosuchia.
Thalattosuchia.
XVI. ORDER CHELONIA 216
Side-necked Turtles.
Snapping Turtles.
Fresh-water or Marsh Tortoises.
Land Tortoises.
Sea-Turtles.
Ancient Sea-Turtles.
Leather-back Marine Turtles.
River Turtles.
CHAPTER I
INTRODUCTION
In most persons the word reptile incites only feelings of disgust and
abhorrence; to many it means a serpent, a cold, gliding, treacherous,
and venomous creature shunning sunlight and always ready to poison. Our
repugnance to serpents is so much a part of our instincts, or at least
of our early education, that we are prone to impute to all crawling
creatures those evil propensities which in reality only a very few
possess. Were there no venomous serpents—and there are but two other
venomous reptiles known—we should doubtless see much to admire in those
animals now so commonly despised; because a few dozen kinds, like the
rattlesnakes, copperheads, and cobras, protect themselves in ways not
unlike those used by man to protect himself, we unjustly abhor the
thousands of other kinds, most of which are not only innocent of all
offense toward man, but are often useful to him.
There are now living upon the earth more than four thousand kinds or
species of cold-blooded animals which we call reptiles, all of which
are easily distinguishable into four principal groups: the serpents
and lizards, the crocodiles, the turtles, and the tuatera. Their
habits and forms are very diverse, but they all possess in common
certain structural characters which sharply distinguish them from
all other living creatures. A reptile may be tersely defined as a
cold-blooded, backboned animal which breathes air throughout life.
And yet, it is not quite certain that this definition is strictly
correct when applied to all the reptiles of the past, since it has
been believed that certain extinct ones may have been warm-blooded.
By this definition, short as it is, we at once exclude a large number
of cold-blooded, air-breathing, backboned animals which were formerly
included by scientific men among the true reptiles, and even yet are
popularly often so included—the amphibians or batrachians. These
animals, now almost wholly represented by the despised toads, frogs,
and salamanders, were, very long ago, among the rulers of the land,
of great size and extraordinary forms. But they have dwindled away,
both in size and in numbers, till only a comparatively few of their
descendants are left, none of them more than two or three feet in
length, and all of them sluggish in disposition and of inoffensive
habits. While we may speak of the amphibians as air-breathing, they
are, with few exceptions, water-breathers during the earlier part of
their existence. Some may pass their whole lives as water-breathers,
while a few begin to breathe air as soon as hatched from the egg; but
these are the marked exceptions.
In many respects the internal structure of the amphibians of the
present time is widely different from that of reptiles, though there
can be no doubt that the early amphibian ancestors of the modern
toads, frogs, and salamanders were also the ancestors of all living
and extinct reptiles, and it is a fact that the living amphibians
differ more from some of the ancient ones than those early amphibians
did from their contemporary reptiles. Discoveries in recent years have
bridged over nearly all the essential differences between the two
classes so completely that many forms cannot be classified unless one
has their nearly complete skeletons. We know that some of the oldest
amphibians, belonging to the great division called Stegocephalia,
were really water-breathers during a part of their lives, because
distinct impressions of their branchiae, or water-breathing organs,
have been discovered in the rocks with their skeletal remains, but we
are not at all sure that some of the more highly developed kinds were
not air-breathers from the time they left the egg; indeed, we rather
suspect that such was the case.
We are also now quite certain that, from some of the early extinct
reptiles—the immediate forbears probably of the great dinosaurs—the
class of birds arose, since the structural relationships between
birds and reptiles are almost as close as those between reptiles and
amphibians.
Huxley believed that the great class of mammals arose directly from
the amphibians, and there are some zoölogists even yet who think that
he was right. But paleontologists are now quite sure that they were
evolved from a group of primitive reptiles, known chiefly from Africa,
called the Theriodontia; quite sure because nearly all the connecting
links between the two classes have already been discovered—to such an
extent, indeed, that really nothing distinctive of either class is left
save the presence or absence of the peculiar bone called the quadrate,
the bone with which the lower jaw articulates in birds and reptiles;
and certain elemental parts of the lower jaw itself. And even these
bones, in certain mammal-like reptiles, had become mere vestiges.
Even the double condyle of the mammal skull, with which the vertebrae
articulate, so like those of the amphibian skull that Huxley based his
belief of the amphibian origin of the mammals chiefly upon it, has now
been found in certain reptiles. Warm-bloodedness, one of the diagnostic
characters of birds and mammals, is not really very important, since
it must have arisen in these two classes independently, and we may
easily conceive that the earliest mammals were cold-blooded or that the
immediate ancestors of the mammals were warm-blooded.
It is an interesting fact in the history of the vertebrates, as of
all other groups of animals and plants, that the chief divisions
arose early in geological history. Every known order of amphibians
and reptiles, unless it be that including the blind-worms, was
differentiated by the close of the Triassic period. The frogs are now
known from the Jurassic. The mammals and birds also quite surely date
their birth from the Triassic. And this early differentiation of the
chief groups is doubtless due to the fact that the potentialities of
diverse evolution are limited by specialization. It is apparently a law
that evolution is irreversible, that it never goes from the special to
the general, that an organism or an organ once extinct or functionally
lost never reappears. And it is also a law in evolution that the parts
in an organism tend toward reduction in number, with the fewer parts
greatly specialized in function, just as the most perfect human machine
is that which has the fewest parts, and each part most highly adapted
to the special function it has to subserve. And these laws explain why
it is that no highly specialized organism can be ancestral to others
differing widely from it. The more radically distinct an organism
is from its allies, the earlier it must have branched off from the
genealogical tree.
The many new discoveries of extinct forms so often intermediate, not
only between the larger groups, but between many of the lesser ones as
well, are making the classification of the vertebrates increasingly
difficult. At one time it was sufficient to define a reptile as a
cold-blooded animal with a single occipital condyle, that is, with
a single articular surface between the skull and the first vertebra
of the neck; a mammal as a warm-blooded animal with two articular
surfaces; but these definitions are no longer strictly correct.
Connecting links do not break down classification, as one might think,
but they do often spoil our fine systems and compel our classifiers to
take a wider view of nature than their own narrow province affords.
We can never hope that most, or even the greater part, of all the
animals which have lived in the past will ever become known to us,
even imperfectly. Doubtless the species of the past geological ages
outnumbered many times, perhaps hundreds of times, all those now
living, since many of these latter are merely the remnants of far
more varied and extensive faunas. At times the conditions for the
preservation of the remains of animal life have been more favorable
than at others, and, under such favorable conditions, a fairly good
glimpse is sometimes given us of the fauna of some isolated epoch
and locality in the earth’s history. Those animals which lived in
and about the water have been preserved in greater numbers and more
perfectly than the strictly land animals, since fossils are due to
the preserving action of water, with few exceptions. Of those animals
which lived upon the land or in the air only the rarest of accidents
carried the skeletons into the lakes, seas, and oceans. And, even when
they had been covered by sediments at the bottoms of lakes and seas
and hidden away from adverse agencies, it has often happened that the
great erosions of later ages have carried away and destroyed the rocks
in which they were inclosed. The records of long intervals of time
have thus been lost in all parts of the world. That we are able to
obtain even an imperfectly continuous history is due to the fact that
the intervals thus lost are not everywhere contemporaneous, that the
missing records of one place may be filled out in part elsewhere. But
this substitution of records from a distance can never make the history
complete. If, in human history, we had only the records for one century
in China, for another in England, and for yet another in South America,
how imperfect indeed would be our knowledge of human progress. Animals
and plants are never quite alike in remote regions, and they never have
been. The living reptiles of North and South America are today almost
entirely different, and, were their fossil remains to be discovered a
million years hence, it would be very difficult to decide that they had
once lived contemporaneously; difficult, though perhaps not impossible,
since some are so nearly alike that their relationships or possible
identity would probably be established after long search. This will
serve to make clear how very difficult it is, for the most part, to
correlate exactly the geological formations in remote regions of the
earth, or even sometimes in adjacent regions where the fossils are
scanty, or the conditions under which the animals had lived were very
different.
There are long periods of time, millions of years at a stretch perhaps,
throughout which our knowledge amounts to little or nothing concerning
many land reptiles which we are sure must have existed abundantly. No
better example of our oftentimes scanty knowledge can be cited than
the following. Until within the past fifteen years it was thought that
true land lizards, of which there are about eighteen hundred species
now living, dated back in their history no farther than about the
close of the great Secondary Period, or the Age of Reptiles. But a
single skull of a true land lizard has been discovered in the Triassic
deposits of South Africa, a skull of a form so nearly like that of the
modern iguana of America that its discoverer, Dr. Broom, has called
it _Paliguana_. The lizards must have been in existence, probably
many thousand species of them, during all the great interval of time
between the Middle Triassic and the close of the Cretaceous, since it
is a law which can have no exception, that a type of life once extinct
never reappears. The “ancient iguanas” of the Trias must have been the
forbears of many, if not all, of the lizards of later times, though
nothing is known of their descendants through a period of time which
can be measured only by millions of years.
However, notwithstanding these imperfections of our geological records,
we know very much more about extinct reptiles than we do about living
ones, so far at least as those parts capable of preservation in the
rocks are concerned. Were our knowledge of reptiles confined to the
forms now living upon the earth it would be relatively very incomplete
since, aside from the lizards and snakes, they are merely the remnants
of what was once a mighty class of vertebrates.
Not only do we learn from the remains preserved in the rocks the
precise shape and structure of the bones of the skeleton and their
precise articulations, but we are often able to determine not a little
regarding the forms which the living animals had by the impressions
made by the dead bodies in the soft sediment which inclosed them
before decomposition of the softer parts had ensued, sediments which
afterward solidified into hard rock. But these impressions are, with
rare exceptions, only those of profiles or of flattened membranes.
The rounded bodies of life do not retain their shape long enough for
the sediment to harden; in most cases the flesh has decomposed before
being entirely covered by sediment. Sometimes the integument and
scales in a carbonized condition are actually preserved, retaining
some of the actual structure of the organized material. The carbon
pigment of the skin has sometimes been preserved in patterns indicating
the color-markings in some of these ancient reptiles; and even the
microscopic structure has been detected in carbonized remains of
organs. Fossil stomach contents, the bony remains of unhatched young,
as well as the delicate impressions of skin and membrane, all add to
our knowledge of the structure and habits of the animals which lived so
long ago. Many other things also may be learned, or at least inferred,
concerning the living animals and their habits from the positions in
which the skeletons are found, from the nature of the rocks which
inclose them, or from the character and abundance of other fossils
found with them. The frequent discovery of bones which had been injured
and mended during life, or the living amputation of members, often
tell of the characteristics of the creatures. So, too, the climatic
conditions under which the animals lived may often be inferred with
tolerable certainty; the presence of “stomach-stones” reveals something
of the food habits, and even of the structure of the alimentary canal,
etc.
All this information is gained slowly, often very slowly, and with
much labor and pains. Rarely or never is it the case that all the
information obtainable concerning any one kind of an extinct animal
is furnished by a single specimen. Skeletons are very seldom, perhaps
never, found quite complete, with all their parts in their natural
positions; and the nature of the matrix inclosing them usually prevents
a study of all parts of any specimen. If a newly discovered fossil
is widely different from the corresponding parts of any creature
previously known, whether living or extinct, we cannot infer very much
from a few bones as to what the remainder of the skeleton is like.
Such inferences or guesses in the past have often resulted in grievous
error, and self-respecting paleontologists are now very reluctant to
speculate much concerning extinct animals from fragments of a skeleton,
no matter what those fragments or bones may be; future discoveries are
sure to reveal errors. It is, therefore, only by the accumulation of
much material, and by the careful study and comparison of all known
related animals, that reliable conclusions can be reached. Often it
requires scores of specimens to determine the exact structure of a
single kind of animal, and, as the collection and preparation of
fossil skeletons are tedious and expensive, our knowledge sometimes
increases very slowly. In recent years, however, there have been many
more students of extinct backboned animals than formerly, and there are
now many museums and universities which spend annually large sums of
money in the collection and preparation of such fossils. This greater
activity of the last twenty years is bringing to light many new and
strange forms, as well as completing our knowledge of those previously
imperfectly known.
It is commonly, but erroneously, believed that the bones of extinct
animals are usually found in excavations made for the purpose. It
is true that not a few specimens of fossils have been discovered
in excavations made for other purposes, such as railway cuttings,
quarries, wells, etc., but if no others were found our knowledge of the
animals of the past would be very meager indeed. Fossils are, for the
most part, found by deliberate search over the denuded rocks in which
they occur. Methods of search and collection will best be understood by
the following description of the noted fossil-bearing rocks of western
Kansas.
[Illustration: FIG. 1.—A characteristic chalk exposure in western
Kansas, a hundred acres or more in extent.]
About the middle of Cretaceous times, there extended from the Gulf of
Mexico on the south to or nearly to the Arctic Ocean on the north a
narrow inland ocean or sea, a few hundred miles in width, covering what
is now the western part of Kansas and the eastern part of Colorado,
and separating the North American continent into two distinct bodies
of land. This ocean, because of its location, bordered on both sides
by low-lying lands—the Rocky Mountains had not then been pushed
up—doubtless was comparatively calm and placid, free from violent
storms and high tides. That the climate, in the region of Kansas at
least, was warm or even subtropical is fairly certain, since plants
allied to those now living in warm, temperate, or subtropical regions
were then living much farther to the north; and since the animals
which then lived in this sea were only such as would be expected
in waters of warm temperature. Its tributary rivers could have been
neither large nor swift-flowing, since the sediment at its bottom was
free, or nearly free, from in-brought material. This was at least the
case not very far from its shores. Its slowly falling sediment was
composed, almost exclusively, of microscopic shells of animals and
plants, foraminifera and coccoliths. The deposits thus made are almost
identical with those now forming in various parts of the world in clear
but not deep waters, away from the immediate coasts of the continents,
almost pure chalk. Animals dying in this inland sea fell slowly to
the bottom during or after decomposition of their softer parts, and
the slowly increasing sediments covered up and buried the preservable
parts. The many predaceous fishes and other scavengers with which the
waters abounded often tore the decomposing bodies apart, separating
and displacing the bones of the skeleton; and the currents of the
shallow waters washed others apart. Often the teeth of fishes and other
carnivorous animals are found imbedded in the bones, and many are the
scars and toothmarks observed in the fossil bones.
After the ocean had dried up and the bottom had been raised far above
the present level of the oceans, other deposits made in lakes and by
the winds covered deeply the consolidating sediments, burying them for
millions of years with all that they contained. Long-continued erosion
by winds and rains has again laid bare many parts of the old ocean
bottom, and has washed them out into ravines and gullies. Many hundreds
of square miles of this chalk are now laid bare in western Kansas, upon
which the growth of vegetation has been prevented by the arid climate.
Here and there may now be discovered protruding from the sloping or
precipitous surfaces of this exposed chalk bones or parts of bones of
the old animals buried so long ago in the soft sediment of the ancient
ocean bottom.
The sharp-eyed searcher after fossils detects these protruding, often
broken and weather-worn, petrified bones, which themselves betray the
presence often of other parts of their skeletons still concealed in the
chalky hillside. Fortunate is he if he has discovered a specimen soon
after it appeared at the surface, before the rains have washed away
and destroyed most of the remains that had been there preserved. Still
more fortunate is he if all or nearly all of the original skeleton has
been preserved together in its natural relations. After days, perhaps
weeks, of labor, the specimen is secured and shipped to the laboratory.
Those parts which have been washed out of the chalky rock before the
discovery of the specimen are always more or less injured and for the
most part lost, their fragments strewn down the hillside, for erosion
is always slow and many years may have elapsed since first the specimen
had appeared at the surface. More frequently, perhaps, a few strokes
of the pick and shovel disclose but one, two, or three bones remaining
in the rocks. The specimen, if large, or composed of many bones, is
carefully uncovered sufficiently to show its extent, and then, so far
as possible, removed in large blocks of the rock. The bones themselves,
notwithstanding their petrifaction, are usually soft and easily broken,
and their separate removal from the matrix may require weeks or even
months of labor, work which cannot be done prudently in the field.
Of many specimens the rock matrix is so hard that the task of removing
it from the bones is slow and difficult, indeed well-nigh impossible,
for the bones are usually softer than their surrounding matrix. On the
other hand, the matrix may be so soft and friable that it cannot be
quarried out in blocks. In such cases the separate divisions, as large
as they can be excavated and safely handled, are carefully covered with
thick bandages of burlap and plaster-of-paris, often strengthened with
rods of iron or boards. The skeleton of a single animal treated in this
way may require weeks and even months to collect, prepare, and mount in
the museum.
From what has been said the reader will understand how it is possible
to make an approximately accurate picture of extinct animals as they
appeared in life—approximately accurate, never absolutely so. The
flesh and other soft parts of an animal are never petrified, though
it is a common belief that they may be. Petrified men and women are
still occasionally shown in cheap museums, but they are always frauds.
Many times has the writer been called upon to express an opinion as
to the nature of some concretion which the discoverer was sure was a
petrified snake, turtle, or even some part of the human body, because
of fancied resemblances in shape and size. Not too emphatically can it
be said that anything dug from the earth having the shape of a living
animal and alleged to be petrified is either an accidental resemblance
or a deliberate humbug—if we except such extraordinary casts as those
of Pompeii. The Cardiff Giant and the Muldoon are still fresh in the
memory of some of us. There have been a few instances where flesh has
been preserved in the North, frozen for thousands of years, but frozen
fossils are very different from petrified fossils. Flesh decays before
it possibly can be petrified, and only rarely is the residue of flesh,
tendons, and skin, that is, the carbon and mineral matters, preserved.
[Illustration: FIG. 2.—Removing a specimen of fish in a block from the
chalk of western Kansas.]
One may sometimes restore extinct animals as in life, knowing fully
the shape and structure of the skeleton, and still be far from the
real truth. All elephants of the present time have a bare or nearly
bare skin. If all that we knew of the extinct mammoth were derived
from the skeleton we should never have suspected that the creature
was clothed during life with long and abundant hair, such as has been
found with the frozen bodies in Siberia. Nor should we suspect that
the dromedary and Bactrian camels of today have large masses of fat
on their backs, if we knew only their skeletons. It must therefore be
remembered that all restorations of extinct animals, representing them
as in life, are merely the sum of our knowledge concerning them, as
close approximations to the real truth as it is possible to make. Or,
rather, they should be such approximations; unfortunately many such
restorations have been made by artists wholly unacquainted with the
anatomy of the creatures they attempt to represent, often adorned with
appendages drawn from a too vivid imagination.
CHAPTER II
CLASSIFICATION OF REPTILES
There is very much doubt, very much uncertainty, among paleontologists
about the classification of reptiles. No two writers agree on the
number of orders, or the rank of many forms. Some recognize twenty or
more orders, others but eight or nine. And this doubt and uncertainty
are due chiefly to the many discoveries of early forms that have been
made during the past twenty years. The many strange and unclassifiable
types which have come to light in North America, South Africa, and
Europe have thrown doubt on all previous classificatory schemes,
have weakened our faith in all attempts to trace out the genealogies
of the reptilian orders; and classification is merely genealogy.
It is only the paleontologist who is competent to express opinions
concerning the larger principles of classification of organisms,
and especially of the classification of reptiles. The neozoölogist,
ignorant of extinct forms, can only hazard guesses and conjectures
as to the relationships of the larger groups, for he has only the
specialized or decadent remnants of past faunas upon which to base
his opinions. About some things we can be quite confident; about some
groups opinions have crystallized, and we all agree, except perhaps on
trifles. The dinosaurs, the pterodactyls, the crocodiles, for instance,
offer only minor problems to perplex the systematist, but the origin
and the relations, not only of these, but also of nearly all the
others, are still involved in obscurity. The question, whence came the
ichthyosaurs, the plesiosaurs, the turtles, etc., seems almost as far
from solution as it did fifty years ago. With every problem solved a
dozen more intrude themselves upon us. Hence, classification simply
represents the present condition of our knowledge, our present opinions
as to genealogies. It was the fashion a dozen years ago to draw all
sorts of genealogical trees on the slightest pretext, to trace in
beautifully clear lines the precise descent of all kinds of animals;
and very few have been worth the paper on which they were printed.
When facts are numerous enough, conclusions are patent even to the
novice; when facts are few and obscure, one can guess about as well as
another. In general, it may be said that the older a group of animals
is the more abstruse are the problems presented; first, because of the
lack of abundant material; second, because the forms speak to us in an
unfamiliar language that we cannot easily interpret. The classification
of the mammals approaches more nearly the ultimate truth than does that
of any other group of organisms, because we know more about the extinct
forms than we do of any other class, and also because we know more
about the living forms than we do about any other living animals.
Species of reptiles are, for the most part, vague quantities in
paleontology; they can be determined with assurance only by the
comparison of abundant material. Adult characters in mammals are
apparent in the ossification of the skeleton, and size can be used
within moderate limits in the determination of species; but size in
reptiles means but little; no one could possibly say that the skeleton
of an alligator six feet in length is not that of an adult animal if
he knew nothing else about the Crocodilia. So also the compression and
malformations of bones from the processes of fossilization obliterate
specific characters in great part. Nor are specific characters easily
distinguishable in the skeletons of living reptiles. The genus,
therefore, among fossil reptiles is practically the unit, and we may
be sure that for every well-defined genus we discover there existed
numerous minor variations, which, had we the living animals to study,
we should call species. We classify the living Crocodilia into two
families, about four well-defined genera—perhaps even five or six—and
about twenty-five species. Of the living lizards there are about
eighteen hundred species, twenty families, and four larger groups
or suborders. In all probability the lizards have never been more
abundant and more varied than they are at the present time. Possibly
these proportions of species, genera, families, and suborders may
represent approximately the proportions that have existed at some time
or other in most of the other groups which we call orders—approximately
only, for we can never be quite sure that we evaluate the structural
characters of different groups of organisms quite equally. The absence
of a molar tooth in a mammal would ordinarily indicate a genus, the
absence of a tooth in a reptile might not indicate even a variety or
a race. Whence it follows that classification of organisms is not
and never will be an exact science. The value of characters used in
classification is very unequal, as we have seen. No two persons see
these characters from the same viewpoints, and in consequence no
two persons whose opinions are worth while ever wholly agree as to
classification.
The following scheme differs only in minor details from the more
conservative of the generally accepted views, and those differences
are, for the most part, the writer’s own opinions, to be taken for
what they are worth. It may be said decisively that no classification
of the reptiles into major groups, into super-families or subclasses
that has so far been proposed is worthy of acceptance; there is no
such subclass as the Diapsida or Synapsida, for instance. And we have
very much more to learn about the early reptiles before any general
classification of the reptiles can be securely founded. It is very
probable that the primary radiation of the reptiles into the various
lines of descent, into its main branches, occurred much earlier than
we have been disposed to believe; that before the close of Paleozoic
time, perhaps before the close of the Carboniferous, all the great
groups of reptiles had gone off from the main stem, and that since then
only smaller and smaller branches have appeared. There have been no new
orders of reptiles in all probability since Triassic times, and perhaps
none since Permian.
Taxonomists are often disposed to cut the Gordian knots of
relationships by raising the ranks of the animals they study to
independent positions. More than thirty independent orders of reptiles
have been proposed by different students, and quite as many of mammals
and of birds; possibly after more forms have been discovered there will
be as many proposed for the amphibians. Sometimes, indeed, it is better
to make such independent groups than to unite lesser ones on doubtful
evidence. But the writer, for one, believes that it is more worthy of
the thoughtful scientific student to seek for relationships than for
differences. It is far easier to destroy than to construct, to make
new genera, families, and orders than to unite those already proposed.
To raise every proposed suborder of reptiles to an order, as has been
proposed by various writers, and the orders to subclasses, only leaves
classification where it was; nothing has been added to taxonomy save a
lot of new names to perplex and annoy the student.
In the following scheme of classification three groups provisionally
called orders are prefixed by an asterisk.
CLASS REPTILIA
Order =COTYLOSAURIA=
Primitive reptiles with notochordal vertebrae, imperforate
temporal region, persistent intercentra; two coracoids;
plate-like pelvis, with all or most of the amphibian skull
elements; short legs and short neck; phalangeal formula
primarily 2, 3, 4, 5, 3(4).
Suborder =Diadectosauria= Permocarboniferous, North America.
=Pantylosauria= Permocarboniferous, North America.
=Labidosauria= Lower Permian, North America.
=Pareiasauria= Upper Permian, Europe, Africa.
=Procolophonia= Triassic, Europe, Africa.
Order =CHELONIA=
Temporal region imperforate. Head and limbs more or less
retractile within a box formed chiefly by the exoskeleton.
Suborder =Pleurodira= Triassic to recent.
=Cryptodira= Jurassic to recent.
=Trionychoidea= Cretaceous to recent.
Order =THEROMORPHA=
Primitive reptiles with notochordal vertebrae, perforate
temporal region, persistent intercentra; two coracoids;
plate-like pelvis with median vacuity; no free
dermosupraoccipitals in skull; longer legs and neck;
phalangeal formula 2, 3, 4, 5, 3(4).
Suborder =Pelycosauria= (_sens. lat._) Permocarboniferous,
North America, Europe.
=Dromasauria= Upper Permian, Africa.
=Dinocephalia= Middle and Upper
Permian, Africa.
Order =THERAPSIDA=
Reptiles with a single temporal perforation on each side;
vertebrae not notochordal; intercentra not persistent;
pelvis with vacuity; skull bones reduced; teeth heterodont;
phalangeal formula, 2, 3, 3, 3, 3.
Suborder =Anomodontia= Permo-Trias, Africa, North America.
=Therocephalia= Upper Permian, Africa.
=Theriodontia= Trias, Africa.
Order =SAUROPTERYGIA=
Aquatic reptiles with a single temporal vacuity; no
supratemporal bone, or quadratojugal; ribs single-headed,
diapophysial; coracoids large, meeting in middle line,
single; neck long, tail short.
Suborder =Nothosauria= Triassic, Europe.
=Plesiosauria= Triassic to close of Cretaceous,
cosmopolitan.
⃰Order =PROGANOSAURIA=
Primitive aquatic reptiles; single (? upper) temporal
perforation; neck elongate; nares posterior; vertebrae
notochordal; intercentra persistent; pelvis plate-like;
phalangeal formula 2, 3, 4, 5, 4(6). Permocarboniferous,
Africa, South America.
Order =ICHTHYOSAURIA=
Reptiles with all aquatic adaptations; a single, upper
temporal perforation; both supratemporal and squamosal
present; a single coracoid. Middle Triassic to Benton
Cretaceous, cosmopolitan.
⃰Order =PROTOROSAURIA=
A single, upper temporal vacuity, quadrate fixed (neck
vertebrae elongate); bones hollow; cervical ribs
single-headed, articulating with centrum; pelvis plate-like.
Permian, North America, Europe.
Order =SQUAMATA=
A single, upper temporal vacuity, or, secondarily none;
quadrate loosely articulated with cranium; teeth on palate;
intercentra more or less persistent; a single coracoid; ribs
single-headed, central.
Suborder =Lacertilia= Trias to recent.
=Mosasauria= Upper Cretaceous, cosmopolitan.
=Ophidia= Upper Cretaceous to recent.
⃰Order =THALATTOSAURIA=
Aquatic reptiles; two (?) temporal vacuities; ribs
single-headed, attached to centrum; single coracoid; no
intercentra. Trias, California.
Order =RHYNCHOCEPHALIA=
Two temporal vacuities on each side; palate with teeth;
intercentra persistent; a single coracoid; teeth acrodont;
ribs articulating with centrum and arch.
Suborder =Rhynchosauria= Triassic, Europe.
=Sphenodontia= Triassic to recent.
=Choristodera= Uppermost Cretaceous, lowermost
Eocene, North America, Europe.
Order =PARASUCHIA=
Subaquatic reptiles, with two temporal vacuities; an
antorbital vacuity; no false palate; pubis entering
acetabulum; ribs double-headed, diapophysial.
Suborder =Phytosauria= Upper Trias, cosmopolitan.
=Pelycosimia= Trias, Africa.
=Pseudosuchia= Trias, Europe, North America.
Order =CROCODILIA=
Two temporal vacuities; teeth thecodont; a false palate;
pubis excluded from acetabulum; single coracoid; ribs
double-headed, diapophysial; subaquatic or aquatic.
Suborder =Eusuchia= Jurassic to recent.
=Thalattosuchia= Upper Jurassic, Europe.
Order =DINOSAURIA=
Ambulatory reptiles, with two temporal vacuities; no false
palate; pubis entering acetabulum; ribs double-headed,
diapophysial.
Suborder =Theropoda= Upper Trias to close of Cretaceous,
cosmopolitan.
=Orthopoda= Close of Trias to close of Cretaceous,
cosmopolitan.
=Sauropoda= Upper Jurassic, Lower Cretaceous,
cosmopolitan.
Order =PTEROSAURIA=
Volant reptiles; fourth finger greatly elongated to support
patagium; neck vertebrae elongated; bones hollow; ribs
double-headed, diapophysial; a single coracoid; no clavicles
or interclavicle; two temporal vacuities.
Suborder =Pterodermata= Jurassic, Europe.
=Pterodactyloidea= Upper Jurassic to Upper Cretaceous,
Europe, North America.
CHAPTER III
THE SKELETON OF REPTILES
The bony framework, or skeleton, that which gives form and stature
to the body, and which serves for the support of the soft parts and
the attachment of muscles, is, with rare exceptions, all that is ever
preserved of fossil animals. Because, therefore, students of extinct
animals must rely so much, if not exclusively, upon the skeleton
much attention has been given to the study of comparative osteology,
the science of bones. Not only are most of the bones of the skeleton
characteristic of the genus to which they belong, but the more
general plan of the skeleton, or parts of the skeleton, is likewise
characteristic of the larger groups. The paleontologist may become so
expert in deciphering the characters of single bones, or even parts
of bones—often all that are known of animals new to science—that he
is able to hazard guesses as to the general structure of the skeleton
to which they belong. But such guesses usually will approximate the
real truth only in the degree that the bones upon which they are based
approximate like bones of other animals that are better known. Not all
parts of the skeleton are equally characteristic of the type of animal
which possessed them. A tooth of a mammal may positively determine the
species to which it belongs, while the toe bone of the same animal
might not enable one to guess at its family, even. As a rule one can
seldom be quite sure of the species of a reptile unless the larger part
of the skeleton, or at least the skull, is available, although almost
any bone of the skeleton, if one is expert, will permit a decision as
to the family, if not the genus.
One must often depend upon the positions and relations of the bones,
as found in the rocky matrix, for the final determination of many
characters. One can, for instance, never be sure of the number of
bones in the neck, trunk, tail, or feet of a reptile, until specimens
have been found with all such bones in position. It is for this reason
that much care is exercised in the collection of specimens of fossil
animals, and especially of fossil reptiles, to preserve all parts of
the skeleton, so far as possible, in the relations they occupied in the
rocks until they can be studied in the laboratory. Many grievous errors
have been made in the past by hasty inferences from fragmentary and
poorly collected specimens.
[Illustration: FIG. 3.—_Limnoscelis_, a subaquatic cotylosaur, from the
Permocarboniferous of New Mexico.]
Because of the reliance which must be placed upon the skeleton it
will be necessary to speak somewhat in detail of its structure in
the reptiles, and to use not a few terms in its description that are
unfamiliar to the general reader. So far as possible technical terms
will be avoided, though some must be used, as there are no equivalents
in the English language for them. The reader may use this chapter as a
sort of explanatory index or glossary for the better elucidation of the
necessary details of the following chapters.
It is needless to say that the skeleton of a reptile is arranged on
essentially the same plan as that of our own; the bones have the
same names that they have in our own skeleton, but there are more of
them, and the individual bones, as a general rule, are less highly
specialized, that is, are not so well adapted for special functions.
In a word, the skeleton of a reptile for the most part is generalized,
though particular parts may be highly specialized for particular uses.
As a rule, if not as a law, the course of evolution has been to reduce
the number of parts and to adapt those which remain more closely to
their special uses, either by increase in size, or by modifications of
their shape and structure.
SKULL AND TEETH
The skull of reptiles is much more primitive or generalized in
structure than is that of mammals, to such an extent, indeed, that
there is yet much doubt as to the precise homologies of some of the
bones composing it; and, inasmuch as the names were originally given,
for the most part, to the bones of the human skull, there is still
some confusion among students as to the proper names in all cases, a
confusion that doubtless will not be wholly dissipated until we know
much more about the early or more primitive reptiles than we do at
present.
[Illustration: Fig. 4 Fig. 5]
[Illustration: FIG. 4.—_Seymouria_, a primitive cotylosaurian.
Skull, from above: _pm_, premaxilla; _n_, nasal; _l_, lacrimal; _p_,
prefrontal; _f_, frontal; _pf_, postfrontal; _it_, intertemporal;
_st_, supratemporal; _sq_, squamosal; _ds_, dermosupraoccipital; _t_,
tabulare; _j_, jugal; _po_, postorbital; _m_, maxilla; _s_, surangular;
_ang_, angular; _pa_, parietal.
FIG. 5.—_Seymouria_, skull from the side. Explanations as in fig. 4.]
As in other parts of the skeleton, there has been a reduction in the
number of parts of the reptile skull from that of the more primitive
forms, and a better adaptation of those which remain for the special
uses they subserve. This reduction in number has been caused in part
by the actual loss of bones, in part by the fusion of contiguous ones.
The most primitive reptiles had no less than seventy-two separate bones
in the skull;[1] the human skull has but twenty-eight inclusive of the
ear bones. There is but little variation, either in the number or in
the relations of bones, in the mammalian skull. If one knows the human
skull thoroughly he can easily understand the structure of the skull
of any mammal. The same cannot be said of the skulls of reptiles; one
would be greatly puzzled in the comparison of the skulls of turtles
and crocodiles, if he knew nothing about other forms. And it is safe
to formulate another general law in evolution here: Characters which
have been longest inherited are least liable to change. The earliest
reptiles had at least four pairs of bones which have disappeared
in all later reptiles; and they had some bones in pairs which have
fused in later reptiles, either with their mates or with contiguous
bones. The crocodile has at least two pairs of bones which have
disappeared in turtles. On the other hand, the turtle has at least one
pair of free bones which have been fused with adjacent bones in the
crocodiles, and one pair that is fused which is free in the latter.
The lizard has one pair of bones that has been wholly wanting in other
reptiles for millions of years, while on the other hand it has lost
some bones that are present in all other modern reptiles. The four
parts of the occipital bone of mammals, basioccipital, exoccipitals,
and supraoccipital, are almost invariably free and there is a single
occipital condyle, except in the Theriodontia.
[1] Paired maxillae, premaxillae, nasals, prefrontals, lacrimals,
frontals, parietals, dermosupraoccipitals, tabularia, supratemporals,
intertemporals, squamosals, jugals, quadratojugals, postorbitals,
postfrontals, quadrates, exoccipitals, paroccipitals, vomers,
palatines, pterygoids, sphenomaxillae, stapes, transverse, alisphenoids
or orbitosphenoids, epipterygoids, articulars, prearticulars, angulars,
surangulars, coronoids, splenials, dentaries, one supraoccipital, one
basioccipital, one basisphenoid, one ethmoid.
[Illustration: FIG. 6.—_Labidosaurus_, a cotylosaur. Skull from above:
_pm_, premaxilla; _n_, nasal; _m_, maxilla; _l_, lacrimal; _p_,
prefrontal; _fr_, frontal; _pf_, postfrontal; _po_, postorbital; _j_,
jugal; _pa_, parietal; _sq_, squamosal; _ds_, dermosupraoccipital;
_pf_, parietal foramen.]
In this reduction or fusion of parts, or in addition thereto, there has
been a general lightening-up of the whole skull-structure in reptiles
from the rather massive and protected form of the older to the lighter,
less protected, and more fragile type of the later ones, since speed,
greater agility, better sense organs, and doubtless greater brain power
have rendered unnecessary or useless the older kinds, just as modern
methods and modern arms have rendered useless the coat of mail of the
Middle Ages.
[Illustration: FIG. 7.—_Edaphosaurus_, a theromorph reptile from the
Permian of Texas. Skull with single temporal vacuity.]
The old reptiles had a continuous covering or roof for the skull,
pierced only by the openings for the nostrils in front—the nares—the
orbits for the eyes near the middle, and a smaller median opening
back of them for the so-called “pineal eye.” The temporal region,
that is, the region back of the orbits on each side, was completely
roofed over by bone for the support and protection of the jaw muscles.
In later reptiles this region has been lightened, either by holes
that pierce it or by the emargination of its free borders, as in the
turtles. The openings have occurred in different ways, and with the
loss of different bones in various lines of descent. In one large
group of reptiles, comprising the pterodactyls, dinosaurs, phytosaurs,
crocodiles, and rhynchocephalians, there are two openings on each side,
called the supratemporal and lateral temporal vacuities. In another
still larger group there is a single vacuity on each side, all members
of which it has been thought were markedly related to each other.
Some of these, the lizards, snakes, and mosasaurs, the ichthyosaurs,
and probably the proganosaurs, have the single opening high up on
the side, corresponding apparently to the supra temporal vacuity of
the double-arched forms, as those with two openings are called. Many
others, however, like the whole order Therapsida and the Theromorpha,
have the single opening lower down and bounded differently; their
relationships are doubtful, since it is very much of a question how the
single opening has arisen. There have been many theories to account for
the origin of the temporal vacuities, but all are yet speculations.
Notwithstanding these doubts, which more recent discoveries have
intensified, there can be none that the structure of this region of the
skull offers important and reliable characters for the classification
of the reptiles into the larger groups, but, unfortunately, we are
very uncertain yet as to what this classification should be. We are
confident that all those reptiles having two temporal vacuities on each
side are related to each other; we are yet very much in doubt as to the
classification of all other reptiles, or at least all others having
only a single temporal vacuity on each side.
[Illustration: FIG. 8.—_Sphenodon_ (tuatera). Skull from side and
above: _pm_, premaxilla; _n_, nasal; _prf_, prefrontal; _f_, frontal;
_pf_, postfrontal; _p_, parietal; _po_, postorbital; _sq_, squamosal;
_m_, maxilla; _j_, jugal; _qj_, quadratojugal; _q_, quadrate; _c_,
coronoid; _sa_, surangular; _art_, articular; _pa_, prearticular; _d_,
dentary; _an_, angular.]
Better evidences of relationships, or the absence of relationships, are
offered by the presence of certain bones in the skulls in some orders
that are lost in others, since it may be accepted as an axiom that new
bones have not appeared in the skulls of reptiles, birds, or mammals;
and that no bone which has once disappeared has ever been functionally
regained by the descendants of those that lost it. The presence,
then, of an extra bone in the temporal region of the lizards or the
ichthyosaurs is proof that they have had a long and independent descent
from reptiles which possessed it.
The mandible of the earliest reptiles was composed of not less than
seven separate and distinct bones, as shown in the accompanying
figures. The mandible of no modern reptile has more than six, and some
have fewer. The mandible of mammals is composed of a single bone,
the dentary; those reptiles, the Theriodontia, which doubtless were
ancestral to the mammals in Triassic times, have all the bones, except
the dentary, much reduced, or even vestigial. The prearticular bone, as
shown, so far as known, has been absent in all reptiles since Triassic
times, except the ichthyosaurs, plesiosaurs, _Sphenodon_, and turtles,
all reptiles of ancient origin. The coronoid bone primitively extended
the whole length of the teeth on the inner side; in all reptiles,
except the plesiosaurs, since Triassic times it is either reduced to
a small bone back of the teeth or is absent. So also the splenial has
been greatly reduced in size in all later reptiles and may be wanting
as in _Sphenodon_ and modern turtles. The articular of reptiles, it is
now generally believed, is represented in mammals by one of the ear
bones, the quadrate by another.
[Illustration: FIG. 9.—Mandible of _Trimerorhachis_, a stegocephalian
amphibian, ancestrally related to the reptiles: _A_ from within;
_B_ from without. The coronoid is composed of three bones, the true
coronoid (_cor_), the intercoronoid (_icor_), and the precoronoid
(_pc_). The splenial is composed of two, the true splenial (_sp_) and
the postsplenial (_psp_). The prearticular (_pa_) is broad, the dentary
(_d_) is small; and the angular (_an_) is only slightly visible on the
inner side.]
[Illustration: FIG. 10.—Mandible of _Labidosaurus_, a cotylosaur
reptile: _A_ from within; _B_ from without. The coronoid (_cor_) is a
single bone, but extends far forward. The splenial (_sp_) is also a
single bone, replacing the two of the amphibians. The _prearticular_
(_pa_) is narrower, and the angular (_ang_) appears broadly on the
inner side. The dentary (_d_) is much larger and the surangular (_sa_)
is distinct. The articular (_art_) is small.]
[Illustration: FIG. 11.—Mandible of _Alligator_, a modern, highly
specialized reptile, from within. The coronoid (_cor_) is small and
is situated far back; the splenial (_sp_) does not extend to the
symphysis; the prearticular (_pa_) has disappeared, or has fused with
the angular (_an_) or articular (_art_). The dentary (_d_) has become
the chief bone of the mandible.]
The teeth of reptiles are of much less importance, as a rule, in the
determination of relationships than are the teeth of mammals. Rarely
are their shapes of specific, and often not of generic, importance,
though their number and relative sizes may be. The teeth of mammals,
as a rule, are forty-four or less in number, and they are always
inserted in distinct sockets in the jaw bones. Among reptiles they
are indefinite in number, and may be attached to any of the bones
of the palate and sometimes also to the coronoid of the mandibles.
Furthermore, except in those reptiles related to the immediate
ancestors of the mammals, they are alike or nearly alike in the
jaws, that is, homodont, not distinguishable into incisors, canines,
and molars. They may be inserted in separate sockets (thecodont),
in grooves, or simply be co-ossified to the surface of the bone
(acrodont). And they are usually reproduced indefinitely by new teeth
growing at the side of the base or below them. More usually they are
pointed and curved; sometimes they are flattened, with sharp cutting
edges in front and behind in the more strictly carnivorous reptiles;
in those of herbivorous habits they are more dilated and roughened on
the crown, not pointed; in not a few they are low, broad, and flat
and are used only for crushing the hard shells of invertebrates. With
the very few exceptions among certain dinosaurs, they never have more
than one root for attachment. The evolutional tendency for reptiles,
as for the mammals, is to loose teeth, especially those of the palate.
Among living reptiles it is only the most primitive types, such as
the lizards, snakes, and the tuatera, which have teeth on the palatal
bones, and in none are there teeth on the vomers, as was the rule in
the ancient reptiles. The lizards may have them on pterygoids and
palatines, and the tuatera has them on the palatines only. There may be
as many as eighty on each jaw, above and below, and hundreds of smaller
ones on the palate, or they may be reduced in number to five or six,
or even to a single one; some reptiles, like the turtles and later
pterodactyls, have none. The teeth of reptiles are composed of the same
kinds of tissues as are the teeth of mammals, that is, of dentine and
enamel, but the enamel is always thin, perhaps because the teeth are
so easily replaced that a thicker protective covering is not needed.
The arrangement of the dentine in primitive reptiles is complicated,
that is, plicated or folded in labyrinthine figures, like that of
many stegocephalian amphibians, the Labyrinthodontia, especially.
This labyrinthine structure of the dentine persisted longest in the
ichthyosaurs.
VERTEBRAE AND RIBS
The spinal column or backbone of reptiles, as in all air-breathing
vertebrates, is made up of a variable number of separate segments
called vertebrae, permitting flexibility. Each vertebra is composed of
a body, or centrum, and an arch on the dorsal side for the protection
of the spinal cord. Various projections from the vertebra, called
processes, serve for the attachment of ligaments or muscles, for
articular union with adjacent vertebrae, or for the support of ribs,
and these processes have characteristic differences in different
reptiles. The pair in front and behind, for articulation with the
adjoining vertebrae, may become obsolete or even lost in swimming
reptiles, as we shall see; they are called zygapophyses. In not a few
reptiles there is an additional pair for zygapophysial articulation
in front and behind, called zygosphene and zygantrum, for the greater
strengthening of the column; they are especially characteristic of
snakes and certain lizards. In certain other reptiles, especially the
long-necked dinosaurs, there is an additional pair arranged differently
from the zygophene, that have received the names hyposphene and
hypantrum.
[Illustration: FIG. 12.—Procoelous vertebra of snake: _za_, zygantrum;
_zs_, zygosphene; _ps_, posterior zygapophysis.]
On the top of the arch is the spine or spinous process, which may vary
enormously in size and length; sometimes it is flattened or dilated
above for the support of an exoskeleton, or it may be heavy and massive
for the attachment of strong muscles and ligaments. In the modern
basilisk lizards and in the ancient _Dimetrodon_ and _Edaphosaurus_
from the Permian rocks of Texas these spines are of enormous length,
some of them nearly four feet long in reptiles not twice that length.
Slender crawling reptiles usually have no spines, or only vestigial
ones. On the sides of the arch there may be a distinct transverse
process for the articulation of the rib.
In all early reptiles the ends of the body or centrum are concave,
as they are in nearly all fishes. Such a conformation, called
amphicoelous, gives great flexibility to the spinal column, but only
moderate strength, since the intervening spaces are filled with
cartilage in life. In all living reptiles, with few exceptions, the
body is concave, like a saucer, in front and correspondingly convex
behind, and the intervening cartilage has largely disappeared. Such a
mode of union, called procoelous, adds greatly to the strength of the
backbone, enabling it to receive greater shocks or greater pressure
without dislocation; or to sustain the greater strain of muscles used
in running swiftly or in climbing. Among living reptiles, only the
gecko lizards and the tuatera have biconcave vertebrae. Some extinct
reptiles, such as some of the dinosaurs, animals that walked erect upon
their legs, had their vertebrae convex in front and concave behind
(opisthocoelous). Birds, though walking erect, have a very different
and more complicated articulation of the cervical vertebrae, and
certain reptiles, like the turtles, have very complicated cervical
vertebrae.
In the embryos of all vertebrate animals there appears first an
elongated fibrous rod, called the notochord, in the place of the future
spinal column. This rod may persist through life, never ossifying, as
was the case with all the earliest fishes, and is the condition in some
living ones. As the embryo grows, however, the separate segments, or
vertebrae, ossify about this rod in all reptiles, forming bony rings,
perforate at first in the middle for the more or less constricted
notochord. This stage was the permanent condition in all the earliest
reptiles and in some later ones. Such animals are said to have
notochordal vertebrae, the notochord more or less continuous, like a
string of beads, the beads representing the enlargements between the
contiguous vertebrae.
[Illustration: FIG. 13.—Notochordal cervical vertebrae, with
intercentra, of _Ophiacodon_, a primitive theromorph reptile from the
Permocarboniferous of New Mexico: _pa_, proatlas; _an_, arch of atlas;
_o_, odontoid; _ax_, axis.]
In many early amphibians, and probably in all the earliest ones, as
well as in the fishes from which they were derived, the vertebra
is more complicated in that it is composed of at least three pairs
of separate bones, two of which united with each other, the third
finally disappearing in modern animals, or at the most represented
by a mere vestige called the intercentrum. The dorsal pair of these
bones, called the neurocentra, forms the arch of the vertebra. The
ventral posterior pair, called the pleurocentra, increases in size and
unites to form the centrum or body of the vertebra; while the ventral
anterior pair, early united with each other, is called the hypocentrum
or intercentrum, persistent in all early reptiles as a vestige between
the centra on the ventral side. This divided condition of the vertebra
is persistent in the first vertebra, the atlas of all higher animals,
in which the so-called body is the hypocentrum or intercentrum, the
arch is the neurocentrum, while the pleurocentra have fused more or
less with the anterior part of the next vertebra, the axis, to form the
so-called odontoid. That this is the real explanation of the structure
of the atlas is proved by the various stages of its evolution in the
reptiles, from the earliest (Fig. 15) in which it scarcely differs
from rhachitomous—as this structure is called—vertebrae of an early
amphibian, to the modern in which the structure is nearly like that of
mammals.
[Illustration: FIG. 14.—Rhachitomous dorsal vertebra of _Eryops_:
_n_, neurocentrum or arch; _pl_, pleurocentrum; _i_, intercentrum or
hypocentrum; _az_, anterior zygapophysis; _pz_, posterior zygapophysis;
_d_, diapophysis, for tubercle of rib; _p_, parapophysis, for head of
rib.]
In front of the atlas, that is, between it and the skull, there was, in
all early reptiles, as well as in some later ones, like the crocodiles
and tuatera, the remnant of what is believed to have been another
vertebra, of which only the arch remains, and which is called the
proatlas. In its earliest condition it articulated with the skull in
front and the arch of the atlas behind.
As in mammals, the vertebrae of the different regions have received
distinctive names, cervical, dorsal, lumbar, sacral, and caudal. The
numbers of each region are far more variable than they are among
mammals, the total number of vertebrae in the column varying from
about thirty to more than five hundred, in certain snakes. Nor are the
different regions always easily distinguishable, especially those in
front of the sacrum. In the earliest reptiles there was practically
no neck, and only two vertebrae, the atlas and axis, that properly
can be called cervical. Very soon, however, the reptiles developed a
longer neck with seven vertebrae, a number that has remained singularly
constant in higher animals, especially in the mammals. In most modern
reptiles there are from seven to nine; in a few lizards, five. But the
number was much more inconstant among the older reptiles; some of the
plesiosaurs had as many as seventy-six cervical vertebrae; some of the
older lizards even had as many as eighteen.
Ordinarily the cervical vertebrae differ from those behind them only
in the small size or fusion of their ribs; sometimes, however, as in
the Protorosauria and Pterosauria, the vertebrae may be much elongated.
The dorsal vertebrae of reptiles vary in number from ten in turtles
and some dinosaurs to forty-three in _Pleurosaurus_; and under the
name dorsal we include the so-called lumbar, as there is seldom any
real distinction between the two series, save the smaller size or the
co-ossification of the ribs of the latter.
[Illustration: FIG. 15.—_Ophiacodon_, a primitive theromorph reptile:
proatlas, atlas, and axis, with ribs.]
[Illustration: FIG. 16.—Sacrum of _Chelone_.]
The sacrum in reptiles primitively consisted of a single vertebra,
which bore a large rib on each side for the support of the pelvis. Very
early, however, a second or even a third vertebra was added to it from
behind. The number two is the rule among reptiles, both ancient and
modern; among crawling reptiles the number never exceeds three, but
among ambulatory and flying reptiles the number may be as great as in
any mammal.
The number of caudal vertebrae in reptiles is exceedingly variable,
from a dozen or fifteen up to a hundred and fifty or more. In snakes
but two regions are distinguishable, the caudal and precaudal, and the
number altogether may reach nearly five hundred. With the exception
of the first few basal caudal vertebrae (pygals) and the minute ones
at the extreme tip, all caudal vertebrae of reptiles bear a slender,
usually =Y=-shaped bone below in the interval between the centra,
for the protection of the vessels and nerves. Because of their shape
they have been called chevrons, and are really outgrowths from the
intercentra.
The ribs of reptiles are of more importance in classification than
one would suppose. The primitive rib was a slender, curved bone,
with the vertebral end dilated to articulate continuously with the
intercentral space—that between the centra and the anterior part of
the arch. And this is the condition still remaining in the tuatera.
Very soon, however, the lower end of the articular surface (capitulum)
became separated from the upper (tubercle) by a notch, and the ribs
became distinctly double-headed. And this mode of articulation is
the rule among mammals. Among later reptiles, however, there were
many modifications. In nearly all the head migrated a little backward
on the centrum. By the loss of the tubercle in lizards, the head
became truly single-headed, and attached solely to the body; and this
condition is characteristic of the order Squamata. In another large
group the head of the rib gradually migrated up on the arch and on
the transverse process (diapophysis), so that both head and tubercle
are attached to the diapophysis; and this condition is equally
characteristic of the orders of reptiles known collectively as the
Archosauria—the crocodiles, pterodactyls, dinosaurs, and phytosaurs.
In the Sauropterygia, the ribs are single-headed and attached to the
end of the diapophysis. Finally in most ichthyosaurs the capitulum and
tubercle both articulate with the body of the vertebra.
[Illustration: FIG. 17.—_Ostodolepis_, a primitive theromorph reptile.
Vertebrae from in front and side, with primitive double-headed rib and
intercentrum.]
Ribs primitively were probably attached to all the vertebrae to the end
of the tail. In the earliest reptiles that we know they are present on
all vertebrae as far back as the tenth or twelfth caudal only, those of
the caudal for the most part co-ossified with the centra. The ribs of
the neck vertebrae more quickly disappeared, or became fused with the
vertebrae, and only in the crocodiles among living reptiles are there
ribs on the atlas. The sacral ribs, on the other hand, became much
larger and stouter and developed an articulation at their outer ends
for the support of the ilium (Fig. 16).
The so-called ventral ribs are slender ossifications in the connective
tissue under the skin, on the under side of the body, and are
characteristic of most reptiles. The anterior ones doubtless fused
together more or less to form the sternum or breast bone, which was
otherwise absent in the early reptiles.
PECTORAL OR SHOULDER GIRDLE
Those bones which form the framework for the support of the anterior
extremity in vertebrate animals are known collectively as the pectoral
girdle. In our own skeleton there are but two on each side, or four in
all, the scapula or shoulder-blade, and the clavicle or collar-bone. A
third bone, however, is represented in all mammals by a mere vestige
which early unites with the scapula and is called the coracoid
process. In the lowest forms of mammals, the Monotremata, of which the
_Ornithorhynchus_ and _Echidna_ are the only examples, not only is
this coracoid bone largely developed, articulating with the sternum
or breast bone, but there is an additional coracoid bone in front of
this; and there is also an interclavicle. Indeed, the pectoral girdle
in these mammals is more primitive or generalized in structure than it
is in any living reptiles, composed of scapula, coracoid, metacoracoid,
and clavicle on each side and an interclavicle in the middle. No living
reptiles have the metacoracoid, and, as is the case with many mammals,
some reptiles have no clavicles.
Primitively, that is, in all the old reptiles, the girdle is composed
of scapula, coracoid, metacoracoid, clavicles, and interclavicle,
while in some of the very oldest there is yet another bone, more or
less of a vestige, derived from the ancestral amphibians and called
the cleithrum or supraclavicle. The scapula is more or less elongated
in crawling and climbing reptiles; more slender and bird-like in those
which walked erect after the manner of birds and mammals; shorter and
more fan-shaped in the swimming reptiles, as we shall see. In some
pterodactyls, unlike all other known animals, the scapula articulated
at its upper end with the backbone, giving a much firmer support
for the anterior extremities. Only in those reptiles allied to the
ancestors of the mammals has the scapula ever had a spine or projection
on its dorsal side.
[Illustration: FIG. 18.—_Cacops_, a Permian stegocephalian, ancestrally
allied to the primitive reptiles, with rhachitomous vertebrae and large
cleithrum above the scapula.]
Of the two coracoid bones in the original pectoral girdle the posterior
one began to disappear early and is entirely lost in all reptiles that
lived later than Triassic times, though it still persists in the lowest
mammals, as we have seen. In most later reptiles the remaining coracoid
has become less firmly attached to the scapula than it was in the older
reptiles. It usually has a small foramen piercing it near the middle of
the upper border or end, the supracoracoid foramen. The clavicle, while
more constant among reptiles than among mammals, has been lost in some,
the Crocodilia, for instance, as also the dinosaurs and pterodactyls.
The interclavicle is more constant in reptiles, a more or less
=T=-shaped bone underlying the coracoids where they join, or the breast
bone; but there were some reptiles that lost it, the dinosaurs and
pterodactyls, for instance. In the turtles both the clavicles and the
interclavicle form a part of the under shell or plastron.
[Illustration: FIG. 19.—Scapula (_sc_), coracoid (_cor_), and
metacoracoid (_mcor_) of _Dimetrodon_]
The cleithrum is known in only a few of the old reptiles; it is a
more or less slender bone which lies along the upper front margin of
the scapula, articulating at its lower end with the upper end of the
clavicle on each side.
The breast bone or sternum, while not properly a part of the pectoral
girdle, may be mentioned here. In reptiles it is rarely well developed
or even ossified, the flying reptiles known as the pterodactyls being
the most notable exceptions. It was a comparatively late development in
this class, the earliest ones not possessing it even in a cartilaginous
condition. It was doubtless evolved from the more or less numerous and
slender ossifications on the under side of the body called ventral
or abdominal ribs, after the coracoids had become reduced and more
slender. Whenever it is present the coracoid articulates with it
on each side in front. In most lizards it remains as a cartilage
throughout life.
[Illustration: FIG. 20.—Clavicles and interclavicle of _Ophiacodon_, a
theromorph reptile from the Permocarboniferous of New Mexico.]
ANTERIOR EXTREMITY
The upper arm bone, or humerus, like most other bones of the
extremities, has been greatly modified by the habits of the different
reptiles. In running and climbing reptiles it is always slender, while
in burrowing reptiles it is short and stout and much expanded at the
extremities, like the humerus of the mole among mammals. And we shall
also see how greatly modified it was among the swimming reptiles.
The humerus of flying reptiles has an enormous process on the side,
corresponding to the attachment of the deltoid muscle. The head of the
humerus, for articulation with the glenoid cavity of the scapula, is
rounded in all reptiles, except the pterodactyls, and the articulation
is always at the extremity. At the lower extremity the protuberance
at the outer or radial side is called the ectocondyle; that on the
inner or ulnar side, the entocondyle. Between the two at the end are
the articular surfaces for the radius and ulna, the capitellum and
trochlea. A little above each of these condyles there is usually, on
one side or the other or on both, a foramen or hole for the passage of
arteries or nerves. That on the inner side, which is characteristic of
all early reptiles and of many mammals, is called the entepicondylar
foramen; that on the outer side, the ectepicondylar foramen; the latter
is present in the lizards, and both are found in the tuatera and some
of the early reptiles.
[Illustration: FIG. 21.—Anterior extremity of _Ophiacodon_.]
The radius and ulna are always distinct bones in reptiles, and always
freely movable on each other; they are usually shorter than the
humerus, but in some springing and climbing reptiles they are quite as
long.
The carpus or wrist of reptiles consists primitively of eleven
distinct, irregularly shaped bones, which articulate more or less
closely with each other in three rows. Those of the first row, all true
carpals, are known usually as the radiale, intermedium, ulnare, and
pisiform, corresponding quite with the bones of the human wrist known
as the scaphoid, lunar, cuneiform, and pisiform. The second row has
but two bones, on the radial side, known as the centralia; while the
third row has a bone to correspond to each of the metacarpals, five
in number, and collectively known as the carpalia. Some or indeed all
of these bones may be either absent or unossified, that is, remaining
through life as nodules of cartilage. Seldom, however, are there less
than nine bones in the carpus of reptiles.
The metacarpals, like the digits, primitively were five in number, and
seldom are there less, though the fifth is sometimes lost, and rarely
also the first. They are more or less elongate bones, increasing in
length from the first to the fourth, with the fifth usually shorter.
The first and the fifth are usually more freely movable on the wrist
than are the other three.
The number of joints or phalanges in the fingers of all primitive
reptiles is that of the modern lizards and the tuatera, that is, two
on the first finger or thumb, three on the second, four on the third,
five on the fourth, and three on the fifth. The crocodiles have one
less phalange on the fourth digit; the turtles have usually two less
on the fourth and one less on the third, that is, with precisely the
same arrangement that is found in our own fingers and that of mammals
in general, two on the thumb and three on each of the other fingers.
As exceptions the river turtles have four bones in the fourth digit.
And this mammal-like and turtle-like arrangement of the phalanges was
that of those early reptiles, the Theriodontia, from which the mammals
arose. The last or ungual phalange of reptiles is usually claw-like,
that is, sharp, curved, and pointed, but sometimes it is more nail-or
hoof-like.
PELVIC OR HIP GIRDLE
The pelvic girdle or pelvis in reptiles and higher animals consists
of three bones on each side, often closely fused in adult reptiles
and together known as the innominate bone. The upper or dorsal one of
these three bones—that to which the sacrum is attached—is the ilium;
the one on the lower or ventral side in front is the pubis; and that on
the ventral side behind is the ischium. On the outer side, where these
three bones meet, is a cup-like depression, sometimes a hole, called
the acetabulum, for the articulation of the head of the thigh bone,
homologous with the glenoid articulation of the pectoral girdle, which,
as we have seen, was originally formed by three bones, the scapula,
coracoid, and metacoracoid, the two latter bones, like the pubis and
ischium, meeting in the middle line below. In all the primitive and
early reptiles the pubis and ischium form a continuous plate of bone
without holes in it, except a small one just below the acetabulum in
the pubis, called the obturator foramen, and corresponding to the
supracoracoid foramen of the coracoid. One may almost always recognize
these two bones by the presence of the foramen. This “plate-like”
condition of the pelvis has been lost in all late and modern reptiles
by the appearance of a larger or smaller vacuity between the pubis
and ischium, either paired, when it corresponds quite with the
so-called obturator opening of mammals, or singly in the middle. This
old-fashioned character, like the old-fashioned type of pectoral
girdle, disappeared entirely about the close of the Mesozoic period,
the Choristodera, described in the following pages, being the last of
the kind.
[Illustration]
[Illustration: FIG. 22.—Pelvis of _Ophiacodon_: _A_ from side; _B_ from
above; _pu_, pubis; _il_, ilium; _is_, ischium.]
The ilium in reptiles usually has a more or less prolonged process
or projection turned backward by the side of the anterior caudal
vertebrae, but in those animals which walked erect on the hind legs,
the dinosaurs and pterodactyls, as also some of the more erect-walking
reptiles ancestral to the mammals, this process was directed forward,
as in birds and mammals. The crocodilia, unlike all other known
reptiles, have the pubes excluded from the acetabulum, and they do not
meet in a median symphysis. This character alone will distinguish any
crocodilian from all other reptiles. But there is some doubt as to the
homology of the bones usually called pubes in the crocodiles. Some of
the bipedal dinosaurs have the pubis forked, the anterior part directed
downward and forward, and not meeting its mate in a symphysis, the
posterior process long and slender, lying below the long ischium, as
in birds. Indeed, when this peculiarity of the dinosaurian pubis was
first discovered, it was thought to be an evidence of the immediate
relationship of birds; its structure is now interpreted differently.
POSTERIOR EXTREMITY
The thigh bone or femur in reptiles, like the humerus, is variable
in size and shape. Only in those reptiles that walked erect is the
articulation of the head set off from the shaft of the bone by a
distinct neck. In others the articulation is at the extreme top of
the bone, since the thigh bones are habitually turned more or less
directly outward from the acetabulum and the long axis of the body.
The more or less pronounced rugosities at the upper end of the femur,
for the attachment of muscles, called trochanters, are not easily
distinguishable into the greater and lesser, as in mammals. Sometimes,
as in the erect-walking dinosaurs, there is a more or less pronounced
process on the shaft lower down, called the fourth trochanter, for the
attachment of caudal muscles. On the back part of the shaft there is a
ridge or line for the attachment of muscles, corresponding to the linea
aspera of the mammalian femur. The projections at the lower end on the
sides are called condyles.
The two bones of the leg, or shin, are usually shorter than the thigh
bones, though in running and leaping animals they may be quite as long
or even longer. That on the inner or big toe side is called the tibia,
and articulates with the distal end of the femur, but chiefly with its
inner condyle. It has a more or less well-developed crest in front
above for the attachment of the extensor muscles directly, since there
never is a patella in reptiles, and only rarely sesamoid bones of any
kind. The fibula, at the little-toe side of the leg, is usually more
slender than the tibia, though it may be larger in swimming reptiles
and even in some running forms. It disappeared in some of the later
pterodactyls. Its upper articulation has a more gliding and somewhat
rotary motion on the outer condyle of the femur, turning the foot
outward in extension of the leg.
[Illustration: FIG. 23.—Right hind foot of _Ophiacodon_: _a_,
astragalus; _c_, calcaneum; _c1_, _c2_, centralia; 1, 2, 3, 4, 5,
tarsalia.]
The tarsus of reptiles differs from that of mammals, in that the
chief movements of extension and flexion of the foot upon the leg
occur within the tarsus rather than between the tarsus and leg bones.
Primitively the tarsus of reptiles consisted of nine bones, two in
the first row, two in the second, and five in the third, but in all
modern reptiles the bones of the middle row and the fifth one in the
third row have disappeared; in some lizards and turtles the two of
the first row are fused. The two bones of the proximal row correspond
quite to the astragalus and calcaneum, the astragalus articulating with
both tibia and fibula proximally, the calcaneum with the latter only.
The oldest known tarsus of any vertebrated animal, one from the Coal
Measures of Ohio, has this structure, while in all the early amphibians
there were three bones, the tibiale, intermedium, and fibulare. Some
of the later swimming reptiles, like the ichthyosaurs and plesiosaurs,
have apparently this amphibian structure, with three bones that are
usually called tibiale, intermedium, and fibulare, but it is very
doubtful indeed whether they are homologically the same. In the middle
row two centralia are known in one or two very ancient reptiles, but
for the most part there is only a single centrale, and even that is
usually lost in later reptiles. The third row, like the third row of
the carpus, had a distinct bone for each digit originally, but the
fifth one was very soon lost and has never reappeared. The structure
of the digits and number of bones are quite like those of the hands,
except that the fifth toe has four bones instead of three, that is,
the phalangeal formula was 2, 3, 4, 5, 4. As a rule in terrestrial
reptiles, as in terrestrial mammals, the hind foot is more specialized
than the front ones.
Most reptiles have an external covering or exoskeleton of horny plates
or scales or bony scutes. Horny scales are of course not preservable
as petrifactions, though in many instances their actual carbonized
remains or their impressions have been detected. Such information comes
only rarely, though doubtless in the course of time we shall obtain
it for most extinct reptiles. In the mosasaurs, for instance, very
perfect impressions showing the detailed structure of the scales have
been frequently found. Similar impressions were long since observed
by Lortet in _Pleurosaurus_, and in not a few dinosaurs impressions
of most wonderful perfection have been found. It is only in the water
reptiles, probably, that all external coverings tended to disappear.
Bony dermal plates or scutes are less common among reptiles, though by
no means rare. The turtles, as is well known, are almost completely
inclosed in such an exoskeleton, bones which have coalesced more or
less to form a box or carapace within which the head and limbs may be
withdrawn for protection. In the modern crocodilians also the body is
more or less protected by small bone plates forming rows on the back
and sometimes on the under side. The ancient phytosaurs had similar
plates. Not a few of the dinosaurs were more or less covered with bony
scutes and sometimes with large bony plates or spines. Some modern
lizards have bony plates over the body instead of horny scales.
CHAPTER IV
THE AGE OF REPTILES
Geologists divide the history of the earth, since life first appeared
upon it, into four general eras, the Proterozoic, Paleozoic, Mesozoic,
and Cenozoic, that is, into eras of first life, ancient life, middle
life, and recent life. These divisions were made long ago by geologists
when it was believed that extraordinary changes, great cataclysmic
revolutions, marked their limits.
With a fuller knowledge of the life of the past we know that evolution
has been continuous and uninterrupted; possibly accelerated or retarded
at times, but without break. Were the earth’s history to be written
anew, with our present knowledge, and with an unbiased mind, it is
very doubtful whether many of the time divisions would have the same
limits that they have now—whether the Paleozoic would terminate with
the Carboniferous, or the Permian, or the Trias, or whether indeed we
should think it necessary to make any primary divisions whatsoever. In
other words, our greater knowledge of living and extinct organisms,
and of the rocks which contain fossils, has made the problems of
classification much more complex than they seemed to be formerly.
It is much easier to classify organisms or rocks, or anything else,
when we know only a few isolated kinds—much easier to draw divisional
lines. Geological history is like a volume in which pages, leaves, and
even whole chapters either are missing or are printed in languages
which we understand only imperfectly. Where the lost or unknown parts
belong, the largest divisions may be made, and possibly such may have
been epochs of unusual activity, of diastrophic changes which greatly
accelerated organic evolution. No one can say just where the dividing
line should be drawn between the rocks of Paleozoic and Mesozoic age,
or between the Mesozoic and Cenozoic, for there is none; the most
that we can hope for is to make the divisions everywhere in the world
conform to those first made for local reasons.
------------------------------------+----------------------------+----------------------------------------+-------
PALEOZOIC | MESOZOIC | CENOZOIC |
----------------------------+-------+--------+--------+----------+------+---------+-------+--------+------+-------
CARBONIFEROUS |PERMIAN|TRIASSIC|JURASSIC|CRETACEOUS|EOCENE|OLIGOCENE|MIOCENE|PLIOCENE|PSTCNE| RECENT
----------------------------+-------+--------+----+---+-----+----+------+---------+-------+--------+------+-------
PTEROSAURIA Pterodactyloidea| | |----|--████████████| | | | | |
| | | | | | | | | | | |
Pterodermata····| | |███████ ? | | | | | | |
| | | | | | | | | | | |
DINOSAURIA Theropoda·······| | ███████████████████████████ ? | | | | |
| | | | | | | | | | | |
Sauropoda·······| | | █████████ | | | | | | |
| | | | | | | | | | | |
Orthopoda·······| | |? ███████████████████ ? | | | | |
| | | | | | | | | | | | Birds
BIRDS ················| | | | ██████████████████████████████████████████████████████████████
| | | | | | | | | | | |
CROCODILIA Thalattosuchia··| | | |███| | | | | | | Crocodiles
| | | | | | | | | | | |
Eusuchia········| | | ███████████████████████████████████████████████████████████████████
| | | | | | | | | | | |
PARASUCHIA Phytosauria·····| |████████| | | | | | | | | |
| | | | | | | | | | | |
Pseudosuchia····| | ███████| | | | | | | | | |
| | | | | | | | | | | |
RHYNCHO- Rhynchosauria···| | ██████| | | | | | | | | Sphenodon
| | | | | | | | | | | |
CEPHALIA ················| | █████ █ █ █ █████::|::::|::::::|:::::::::|:::::::|::::::::|██████████████
Choristodera····| | | | | | ████ | | | | |
| | | | | | | | | | | |
THALATTOSAURIA··············| | ███| | | | | | | | | |
| | | | | | | | | | | | Snakes
Ophidia·····| | | | | | ██████████████████████████████████████████████████
| | | | | | | | | | | |
SQUAMATA Lacertilia··| | ███|::::|::████?::|::███████████████████████████████████████████████████
| | | | | | | | | | | |
Mosasauria··| | | | | |██████ | | | | |Lizards
| | | | | | | | | | | |
PROTOROSAURIA···············|████████::::::::|::::|████| | | | | | | |
| | | | | | | | | | | |
PROGANOSAURIA··············████ | | | | | | | | | | |
| | | | | | | | | | | |
ICHTHYOSAURIA···············| | ██████████████████████████| | | | | |
| | | | | | | | | | | |
THEROMORPHA Caseasauria····|███ | | | | | | | | | | |
| | | | | | | | | | | |
Pelycosauria::███████████::::███|? | | | | | | | | |
| | | | | | | | | | | |
Diadectosauria█████ | | | | | | | | | | |
| | | | | | | | | | | |
COTYLOSAURIA Pantylosauria███ | | | | | | | | | | |
Procolophonia··|█████████:::::::|::::|████|(Labidosauria) | | | | |
| | | | | | | | | | | |
Pareiasauria···|·██████| | | | | | | | | | |
Microsauria | | | | | | | | | | | |
██████████████████ | | | | | | | | | | |
AMPHIBIANS | | | | | | | | | | | Amphibians
==================================================================================================================
Pleurodira········| | ████████████████████████████████████████████████████████████████████████
| | | | | | | | | | | |Turtles
CHELONIA Cryptodira········| | | |███████████████████████████████████████████████████████████████
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Trionychoidea·····| | | | | | █████████████████████████████████████████████████
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SAUROPTERYGIA Nothosauria···| | █████ | | | | | | | | | |
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Plesiosauria··| | ██████████████████████████| | | | | |
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Dromasauria | ████ | | | | | | | | | | |
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Dinocephalia | ████ | | | | | | | | | | |
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THERAPSIDA Anomodontia | ████████████| | | | | | | | | |
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Therocephalia | ████ | | | | | | | | | | |
Theriodontia | | █████ | | | | | | | | | |
| | | | | | | | | | | |Mammals
MAMMALS·····················|· | ██|::::|:██████:::|:███████████████████████████████████████████████████
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FIG. 23_a_.—Range of the Reptilia. Heavy lines indicate occurrence in North America.
The periods of the Paleozoic era are the Cambrian, Ordovician,
Silurian, Devonian, Carboniferous, and Permian, in the order as given;
those of the Mesozoic era are the Triassic, Jurassic, and Cretaceous;
those of the Cenozoic era, the Eocene, Oligocene, Miocene, Pliocene,
Pleistocene, and Recent. As a relic of an old classification we still
often divide the Cenozoic into two quite arbitrary divisions, the
Tertiary and the Quaternary, the latter including the Pleistocene and
Recent only. The same may be said regarding the limits of each of
these periods as of the eras; the sole problem is to make each period
contemporaneous throughout the world, an exceedingly difficult problem,
because no faunas or floras have ever been the same over the whole
earth. Indeed, with the exception of some of the lowliest and most
generalized forms, or man himself, no species are the same throughout
the earth today. Inasmuch as we must depend upon the fossils in the
rocks for the determination of the ages, where none is quite the
same in strata of remote localities the identification becomes very
difficult or even impossible. Nor are the periods, as accepted, of
equal or even approximately equal duration; the Cretaceous period, for
instance, was longer than all the remainder of the Mesozoic, longer
perhaps than all the time which has elapsed since its close.
The earliest animals with a backbone, or rather the earliest that we
call vertebrates—for some vertebrates have no vertebrae—began their
existence, so far as we know, in late Ordovician times, as attested
by fish bones in Ordovician rocks of Colorado and Utah. The first
evidences of the existence of air-breathing vertebrates in geological
history are footprints preserved in the uppermost Devonian rocks of
Pennsylvania. We call them amphibian because they resemble footprints
associated with amphibian skeletons in later formations, and because
the foot itself is still the most important difference we know between
fishes and the higher animals.
[Illustration: FIG. 24.—Permocarboniferous landscape (adapted from
Neumayr) with restoration of _Eryops_, a stegocephalian amphibian
ancestrally allied to the reptiles; and _Limnoscalis_, a cotylosaur (in
water).]
In the rocks of the next great time division, the Mississippian, as we
call it in America, corresponding more or less closely with the Lower
or Subcarboniferous of other parts of the world, numerous footprints
of amphibians have been discovered, but no fossil remains except a
few from near its close in Scotland. From the Upper Carboniferous, or
Pennsylvanian, however, not only numerous footprints but the actual
skeletons, or impressions of skeletons, have long been known in Europe
and America. Until recently all these footprints and skeletons were
supposed to be exclusively amphibian. We are now almost sure that some
of them belonged to reptiles of lowly type, the earliest coming from
near the middle of the Pennsylvanian of Linton, Ohio. The amphibians of
this period were, for the most part, salamander-like creatures of from
a few inches to two or three feet in length. They all belong to the
group collectively known as the Stegocephalia, except that very near
the close of the period there appeared small, slender, small-legged
aquatic forms which seem to be the ancient representatives of the real
salamanders of modern times. Some of the Stegocephalians had become
greatly specialized as legless, snake-like, or eel-like creatures.
[Illustration: FIG. 25.—Restoration of _Seymouria_, the most primitive
of known cotylosaur reptiles. From the Permian of Texas, about two feet
long.]
By the beginning of Permian times tremendous changes had taken place
in the land life. The small amphibians of the Carboniferous types
dwindled away, soon to disappear, and their places were taken by others
of peculiar types, for the most part larger; and by many and diverse
kinds of reptiles—water reptiles, marsh reptiles, land reptiles, and
even climbing tree reptiles. From the uppermost Carboniferous and
Lower Permian rocks of the United States more than fifty genera and
twice that many species of amphibians and reptiles have been made known
in recent years, and doubtless as many more will be discovered in the
future. From other parts of the world the history of reptiles of the
Lower Permian is yet scanty, two or three forms from South America, as
many more from Africa, and a half-dozen or so from Europe are all; and
of these very few are known at all well.
[Illustration: FIG. 26.—_Captorhinus_, a cotylosaur reptile from Texas.]
We classify all the known forms of reptiles from the Lower Permian
under three or four orders, the Cotylosauria, Theromorpha or
Pelycosauria, Proganosauria, and possibly the Protorosauria, but the
classification is yet provisional, representing merely the present
stage of our knowledge. The Proganosauria and Protorosauria, including
distinctively aquatic reptiles, will be more fully described in
the following pages. To give even a brief description of the more
terrestrial reptiles of this, the earliest known reptilian fauna, would
be beyond our purpose; the accompanying life restorations by the author
of some of the more typical and better known forms, based upon nearly
perfect skeletons, will suffice.
From the reptiles and amphibians of the Lower Permian of Texas and New
Mexico to the ichthyosaurs of the Middle Triassic of California there
is a complete gap in the records of the land life of North America. We
do not know what became of all the remarkable animals of the Permian.
There are few traces of their descendants elsewhere known, unless it
be in South Africa. From the Middle and Upper Permian of South Africa
and Russia, a marvelous reptilian fauna has been made known in recent
years. More than a hundred species of six or seven groups, and at least
two orders have been described. Of these the Cotylosauria are the
continuation of the American order, but include more specialized forms,
the Pareiasauria and the Procolophonia, all of them, like the more
primitive American forms, characterized by the imperforate temporal
region. The Therapsida, likewise, seem to be the continuation of the
American Theromorpha, so closely allied to them that it is difficult
to draw a distinguishing line between them. On the other hand, these
African reptiles merge through the Theriodontia into the mammals in the
Triassic. They are all terrestrial, crawling reptiles, except a few
which are described on a later page under the Anomodontia.
[Illustration: FIG. 27.—Restoration of _Labidosaurus_, a cotylosaur
reptile from Texas, about three feet long.]
The records of the lower part of the Triassic period are scanty
everywhere in the world, save perhaps in Africa. Before the close of
the period, however, probably every important group of cold-blooded
air-breathing animals had made its appearance in geological history,
if we except the snakes; even the mammals had appeared, and possibly
the birds. The Cotylosauria, Theromorpha, and Therapsida disappeared,
the latter giving birth to the mammals; the nothosaurs and plesiosaurs,
the ichthyosaurs, dinosaurs, crocodiles, phytosaurs, rhynchocephalians,
lizards, and turtles have all left records of their existence in Upper
Triassic rocks; and the pterodactyls had also, in all probability,
begun their career, though none is surely known till the Jurassic.
During Jurassic times all these orders of reptiles waxed prosperous
and powerful, and branched out in many ways and in countless numbers;
many new kinds of each appeared—the marine crocodiles, the quadrupedal
dinosaurs, etc.—but no order or suborder, so far as we know,
disappeared before its close. And this prosperity continued on into
the Lower Cretaceous and for many even into the Upper Cretaceous. The
largest dinosaurs disappeared in the Lower Cretaceous, so far as our
knowledge goes, but the old-fashioned crocodiles continued on into
the Upper, to give place to the new-fashioned kinds. The ichthyosaurs
lingered on for a while on the western continent, but the mosasaurs
appeared, and the plesiosaurs reached their highest evolution and
continued to the end. The flying reptiles attained the zenith of
their evolution, but disappeared before the close. The marine turtles
attained the maximum of specialization and size. The upright-walking
dinosaurs continued on unabated to the close of the period; and a new
kind of dinosaurs appeared near the end.
[Illustration: FIG. 28.—Restoration of _Dimetrodon_, a pelycosaur
reptile from the Permian of Texas; about eight feet long.]
With the opening of the next great era—the Cenozoic or Tertiary—the
reptiles dwindled away to their present insignificant position, while
the birds and mammals appeared in great numbers and varied forms. The
Age of Reptiles was closed and the Age of Mammals had begun.
The history of the reptiles during the Cenozoic is an uneventful one;
they ceased their dominion upon land, in the water, and in the air.
Their remains are scanty, for the most part, in the rocks of the
Tertiary, and such as are known differ only in details from those now
living. The land tortoises only, like the mammals of Oligocene and
Miocene times, seized the opportunities of open prairies and prospered.
A few of the late Mesozoic forms continued a short while into the
Eocene. No new groups, perhaps few new families, came into existence
during the greater part of this time; it was the age only of land
tortoises and the poisonous snakes among reptiles.
EXTINCT REPTILES OF NORTH AMERICA
The oldest known fossil reptile of North America, or indeed of the
world, is represented by a single specimen, lacking the skull, from
black shales of Middle Pennsylvanian age overlying a coal seam at
Linton, Ohio. The specimen was originally described as an amphibian,
but was later recognized by Professor Cope as a true reptile. It was
more fully described by the writer under the name _Eosauravus Copei_,
who agreed with Cope as to its reptilian nature. Until the skull is
discovered, however, the precise relationships of the animal must
remain doubtful.
The next later rocks that have yielded reptilian remains are those
of Illinois and Texas formerly supposed to be of Permian age. Later
evidence, furnished by invertebrates, however, seems to prove that
the lowermost of the strata are of uppermost Carboniferous age. The
Illinois deposits, so far as known, are of very limited extent,
consisting practically of a single bone-bed in black shale in the
immediate valley of the Kaskaskia River near Danville. The known
fossils from this bone-bed—all isolated bones—are preserved in the
museum of the University of Chicago, and include the types of several
genera later recognized in the Texas deposits.
The deposits of Texas, extending northward through Oklahoma to the
south line of Kansas, are of considerable extent, for the most part
lying along the Wichita River and its tributaries, north of Seymour,
Texas. They are composed chiefly of red clays and sandstones of
fresh-water or delta origin, perhaps eight hundred feet in total
thickness. Beds of like character and yielding similar fossils are also
known from northern New Mexico on the tributaries of the Chama River.
Their chief characters, as well as restorations of some of the more
noteworthy forms, have already been given.
No vertebrate fossils are known in America from the Upper Permian and
Lower Triassic. Marine limestones of Middle and Upper Triassic age
of Nevada and northern California have yielded numerous remains of
primitive ichthyosaurs, the only known remains of the thalattosaurs,
and a few others of doubtful affinities, all of which have been
described by Dr. Merriam. The Upper Triassic exposures, of considerable
extent, occur between the Pitt River and Squaw Creek in Shasta County,
California. Reptilian remains from the Middle Triassic are so far known
only from the limestones of West Humboldt and New Pass regions of
western and central Nevada.
[Illustration: FIG. 29.—Restoration of _Varanops_, a theromorph reptile
from the Permian of Texas; about four feet long.]
Land reptiles of Middle and Upper Triassic age are known from many
widely separated localities in the United States, but chiefly from
the extensive “red beds” of the Rocky Mountain region. The fossils
from these beds occur for the most part at least in the horizon called
the Shinarump. Its age is usually considered to be Upper Triassic,
but the character of the fossils seems to indicate possibly the
Middle Triassic. Aside from the stereospondylian amphibians, the last
of the Stegocephalia, the vertebrates from this horizon and these
regions are chiefly Phytosauria. A few anomodonts, or what seem to be
anomodonts—the only record of their occurrence outside of Africa—are
known from Wyoming and Utah. And a single specimen from the Wind
River red beds, described by the writer as _Dolichobrachium_, may
represent reptiles allied to the dinosaurs. Phytosaur fossils of this
horizon have been discovered in Utah, the Wind River Mountains, and
near Laramie City in Wyoming; in southwestern Colorado; in western
Texas; and in various places in New Mexico and Arizona. Doubtless when
these fossiliferous beds are more thoroughly explored many new and
interesting reptiles will be discovered.
Phytosaur remains, probably of about the same age as the Rocky Mountain
ones, have long been known from the Triassic of North Carolina.
From somewhat more recent Triassic deposits in Connecticut and
Massachusetts, several skeletons of small carnivorous dinosaurs, and
various parasuchian remains have been described by Marsh, Lull, and
Talbot. And these beds have long been famous in Massachusetts for their
footprints, for the most part originally referred to birds, but now
pretty well known to have been made by dinosaurs and amphibians.
No vertebrate fossils of Lower or even Middle Jurassic age are known
from North America. From the Baptanodon beds of Wyoming, limestones of
about two hundred feet in thickness, four genera of plesiosaurs, the
very peculiar ichthyosaur from which the beds take their name, and a
few bones of an ancient crocodile are known.
Immediately overlying the Baptanodon beds, the Morrison beds, of from
two hundred to four hundred feet in thickness, probably of Uppermost
Jurassic and Lowermost Cretaceous age, have yielded an exceedingly rich
vertebrate fauna, consisting chiefly of dinosaurs. Discovered first in
the vicinity of Morrison, Colorado, in 1877, hundreds of tons of bones
have been collected from these beds for various museums. The dinosaurs
include many genera of all three suborders, varying in size from that
of a cat to some of the largest known land animals. Of other reptiles a
very few jaws of a true rhynchocephalian, a fragment of a wing bone of
a pterodactyl, numerous turtles, and crocodiles, only, are known. The
beds are predominantly black-clay shales, intercalated with sandstones,
and all are of fresh-water origin.
From beds definitely known as Lower Cretaceous (Trinity) in Oklahoma,
a few bones of a sauropod dinosaur are known, and from nearly
corresponding rocks in southern Kansas, plesiosaurs, crocodiles,
turtles, and carnivorous dinosaurs are known from sparse remains.
Doubtless the Potomac beds of Virginia, which have yielded bones of
various dinosaurs, are also of Lower Cretaceous age.
[Illustration: FIG. 30.—Restoration of _Casea_, a theromorph reptile
from the Permian of Texas, about four feet long.]
With the exception of a single vertebra of doubtful affinities and
the cast of a turtle-shell no vertebrate fossils have ever been
discovered in the extensive sandstones of Dakota age, the lowermost
of the Upper Cretaceous. From the next horizon above the Dakota, the
Benton Cretaceous, chiefly marine limestones, at least three genera
of plesiosaurs are known from Kansas, Texas, and Arkansas, with two or
three more from the limestone shales of Wyoming. A few specimens of
armored dinosaurs, two genera of ancient crocodiles, nearly the last of
their kind, some marine turtles, and a few vertebrae of ichthyosaurs,
the last of the order known anywhere in the world, are also known from
the Benton Cretaceous of Wyoming.
Continuous with the Benton limestones above in Kansas are the famous
beds of Niobrara chalk; perhaps no fossil deposits in the world are
more famous. Exposures covering hundreds of square miles in western
Kansas, almost pure chalk, have furnished fossil-hunters during the
past forty years literally thousands of specimens of mosasaurs,
hundreds of pterodactyls, and scores of plesiosaurs and marine turtles,
in addition to the famous birds with teeth and countless fishes of
diverse kinds. Two or three specimens of spoon-billed dinosaurs have
been found in these deposits, but no other reptiles of any kinds. Beds
of like age in Colorado and New Mexico have furnished a few specimens
of mosasaurs.
From the marine beds of Fort Pierre age, next above the Niobrara in the
west, have come some excellent specimens of two genera of mosasaurs,
three or four forms of plesiosaurs, a few pterodactyls, the largest of
all marine turtles, and still fewer specimens of dinosaurs, in Kansas,
South Dakota, Wyoming, and Montana. From deposits of approximately like
age in Mississippi, Alabama, and New Jersey, many incomplete specimens
were found years ago of mosasaurs, plesiosaurs, and turtles, the last
of the amphicoelian crocodiles, the first of the procoelian crocodiles,
and the famous specimen of _Hadrosaurus_ which served for the Hawkins
restoration, the first attempt of its kind.
From the uppermost Cretaceous beds of America, the Lance, Judith
River, or Belly River beds as they are variously called, have come
the remains of a marvelous reptilian fauna. These beds may be grouped
together though not all contemporaneous, and there is dispute about
their age, some excellent paleontologists insisting that the uppermost
are really of Eocene age. From Colorado east of Denver, from eastern
Wyoming, from Montana, and especially from the vicinity of Edmonton
in Canada, as also occasionally in western Texas and New Mexico,
have come many marvelous specimens of dinosaurs, huge bipedal
carnivorous dinosaurs, great spoon-billed aquatic dinosaurs, armored
stegosaurian dinosaurs, and many kinds of the great horned dinosaurs,
the Ceratopsia, so far known only from these beds. Here at the very
close of the Age of Reptiles, at the close of the Age of Dinosaurs,
are found the ultimate specializations of all the chief groups of
dinosaurs except the long-necked quadrupedal dinosaurs which gave up
the ghost in Lower Cretaceous times. Many were provided with horns and
spines, some indeed seemed to have bristled with spines throughout,
a sure sign that they were approaching the end of their career. The
modern type of crocodiles had usurped the ancient forms of the early
Cretaceous, and reached the largest size of their race perhaps, though
but few specimens are known. Here also in these beds we find the
first representatives of lizards and snakes in America, though snakes
have been described from earlier strata, perhaps, in Brazil. Those
archaic, old-fashioned rhynchocephalians described on a later page as
the Choristodera appeared also for the first time in these beds, and
persisted for a little while in the Eocene, in Europe and America. And
with all these there has very recently been described the last of the
plesiosaurs, whose race went out with the dinosaurs at the very close
of the Mesozoic. It is needless to say that the turtles also occur,
for, as a general rule, wherever vertebrate fossils are found, in rocks
of the land or the sea, marine or fresh-water, there will be some bones
of turtles among them.
With the beginning of the Cenozoic the record of the reptiles
becomes relatively scanty in America. In the warm waters of the old
Eocene lakes and rivers of Wyoming lived countless crocodiles, true
crocodiles of modern aspect and of large size. But, as the climate
of North America grew progressively colder, the crocodiles retreated
to the south, till, in the Oligocene, the scanty remains of the last
crocodiles are found in the American Tertiary. On the other hand, as
the open lands appeared toward the close of the Eocene, and in the
Oligocene and Miocene, the land tortoises throve and grew greatly in
size. In the Bad Lands of South Dakota one may see their remains in
almost incredible numbers. And in equally great numbers are these land
tortoises, in shape much like the common box tortoise of today, but
vastly larger, found in the rocks of the late Miocene or early Pliocene
age in western Kansas. And these are the last records of the big
tortoises in North America; their descendants are perhaps yet living in
the Galapagos Islands.
The history of the lizards and snakes, the only other reptiles found
in the Cenozoic rocks of America, is very brief. A few specimens from
the Lower Eocene of Wyoming; a few skinks and amphisbaenas from the
Oligocene Bad Lands of South Dakota, and some bones of a python-like
snake in the early Eocene of Wyoming are about all that we know of the
Squamata in the Tertiary. Doubtless snakes and lizards were just as
abundant then as now, though but few were preserved, for they are and
always have been distinctly terrestrial animals, that only by accident
fell into places where they could be fossilized.
The author has collected reptile bones from nearly all of the horizons
here mentioned and believes that the list is complete.
CHAPTER V
ADAPTATION OF LAND REPTILES TO LIFE IN THE WATER
In the never-ceasing struggle for existence all forms of life upon the
earth, whether consciously or unconsciously, are continuously striving
for improvement; striving to flee from adverse environments, or to
adapt themselves better to those which must be endured; to escape
their enemies, or to find means whereby they may withstand them; to
find more or better food, or to prevent others from despoiling them
of what they have. There is always more or less of unrest, more or
less of discontent, if such terms may be used of the lower organisms.
It sometimes happens with groups of organisms that by reason of
unusual or extraordinary traits they become so perfectly adapted to
their environments, to their surroundings, or so easily adaptable to
changes in their environments, that they remain for long ages securely
protected and little changed. But, as with man himself, improvement is
usually the result of adversity—adversity which stimulates but does not
destroy. And the word improvement, translated into biological language,
means simply specialization, that specialization which adapts the
organism better to its mode of life, which fits it the better to excel
its less ambitious or less capable competitors. No animals or plants
are perfect; if they were, there would be no advancement, no struggle.
If all physical conditions stood still, or remained uniform, perhaps
life would stand still, but conditions never have and never will stand
still, and life must change to meet changed conditions.
Thus it is that which makes life easier, which lessens the dangers
of destruction, which insures the continued prosperity of the race,
is seized upon and utilized by all plants and animals, so far as
possible. As said long ago by Tennyson,[2] the first law of life is
not the preservation of self, but the prosperity of the race. Whatever
the causes may be whereby the offspring are better adapted to conquer
in the struggle for existence, whatever may be the laws governing
changes and specialization, whether heredity, Mendelism, mutation,
natural selection, or Lamarckism, we call the process evolution.
[2] Are God and Nature then at strife,
That Nature lends such evil dreams?
So careful of the type she seems
So careless of the single life.—_In Memoriam_, lv.
To escape from the severe competition of the overcrowding animals
of the sea, some of those creatures we call fishes long ago became
air-breathers and took possession of the unoccupied land. From among
the myriads which were driven into unbreathable water, by accident or
by their enemies, or led there in the search for more easily acquired
or better food, some survived and found that the oxygen of the air was
quite as breathable as that of the water. Steadily their progeny became
better and better adapted to the unusual life until they ceased to be
fishes and became amphibians, from which have arisen in like manner all
the reptiles and birds and mammals that live or have lived upon the
earth.
With more and better powers, developed under better opportunities, not
a few of these descendants have repeatedly sought safety from their
newly acquired enemies of the overcrowded land, or a better supply of
food in the sea; gradually, perhaps incidentally at first, as we shall
see is the case with some lizards today, but later with increased
adaptation to their new surroundings, they become truly sea or water
animals, no longer able to live upon the land. In these changed
conditions and with concomitantly changed habits they never reverted to
the primitive condition of fishes, never became water-breathing animals
again, for that would be actual retrogression, a seeming impossibility
in evolution. Nor indeed does it seem possible that a land creature
after its reversion to water life ever can return to the land again.
A fish through long ages of evolution has become well adapted to its
environments; its shape is the best for speed or varied evolutions in
the water; its teeth and mouth-organs are best suited for the food it
requires. Now it is evident that if animals of very different habits
and form should go back to the water and seek to compete with creatures
already well adapted to their surroundings, they must, so far as
possible, acquire like forms and like habits. And any improvement on
such forms and habits that their higher development permits them to
attain will of course be of advantage in their competitive struggles.
A fish makes most use of its tail fin for propulsion. It follows
that a land animal seeking to compete with it under like conditions
must acquire a tail fin or some other organ which subserves its
purpose as fully. The body fins are of little use to a fish, save for
equilibration, for preserving its position, for stopping quickly, or
for changing the direction of its movements quickly—very different
functions from those of the corresponding organs, the limbs, of higher
vertebrates. There are few better examples of predaceous, fish-eating
fishes than the common gar-pike of our rivers, fishes with a slender
body covered with very smooth scales, a strong tail, a short neck, and
long jaws armed with numerous slender and sharp teeth. Such a fish,
darting into a school of smaller fishes, by quick, sudden changes of
movement, actively opening and closing its jaws, is sure to seize some
of its sought-for prey. In a direct trial of speed with its victims it
would most likely be worsted.
There have been many animals of high and low rank which in the
past and present have gone back from a terrestrial existence to a
life in the water, finding at last a congenial home away from the
shores. Or, perhaps, like the monitor lizards of today, they have
found temporary safety in the water when hard pressed by their land
enemies, and finally found, not only protection, but an abundant
supply of easily obtainable food therein. As in every vocation of
life there have been many failures in such attempts, many partial
successes only. But not a few have found abounding and enduring
success and final prosperity—success that has led possibly to undue
adaptation to surroundings, and to the acquirement of great size,
for that has been the invariable end of water air-breathers of long
duration—specializations which finally prevented them from meeting new
exigencies. It seems to be a law of evolution that no large creatures
can give rise to races of smaller creatures; and as we shall see, the
largest sea-animals have been the final evolution of their respective
races.
There are no better examples of such success today, nor has there been
in all the geological ages, so far as we know, more perfect examples
of the adaptation of air-breathing animals to an aquatic life than the
great whalebone whales. In Eocene times their ancestors were walking
and running land animals; of that there can be not the slightest doubt,
since we cannot conceive, as did the older naturalists, of their direct
descent from the fishes while having all the essential structure of
mammals, i.e., lungs, circulatory system, manner of breeding and
rearing the young, etc. Of the living whales, or Cetacea, there are
now in existence two very distinct types, so different from each other
that some have supposed them to have been evolved from different
types of land mammals. One of these is best exemplified by the great
baleen whale, having a broad, short head and no teeth. It feeds upon
crustaceans chiefly, which are strained from the water by the great
fringe or net of “whalebone.” The other type is seen in the porpoise
or dolphin. These cetaceans have numerous, pointed and recurved teeth,
which they use as did many of the reptiles, hereinafter described, for
the seizure and retention of fishes and other swimming animals. So
great have been the changes in all these cetaceans, in the adaptation
to an aquatic life, that we are almost at a loss to conjecture from
what kinds of land animals they have descended. The great zeuglodont
whales of early Tertiary times have long been thought to be a sort of
connecting link between them and their land ancestors, and it is still
probable that they were. The forms of zeuglodont whales that have been
discovered in Africa within recent years bear so much resemblance
in their skull and teeth to the contemporary carnivores, that many
paleontologists think, with good reason, that they were descended from
them, that is, from the ancestors of all our dogs, cats, weasels,
bears, etc., of modern times. And we have much reason to believe that
future discoveries will bring further and more decisive proof of their
origin before many years have elapsed. The modern Sirenia, the dugongs
and manatees, exclusively aquatic mammals, which feed upon seaweeds at
the bottoms of shallow bays and harbors, or in the mouths of rivers,
are now known, practically with certainty, to be the descendants in
these same African regions of the earliest ancestors of our sheep,
oxen, and horses, known so certainly that they are often classed with
them, or at least with the elephants, which approach them in their
ancestral line even more closely.
A third type of living aquatic air-breathers is seen in the seals,
sea-lions, etc. They are much less highly specialized, however, than
the whales or sirenians, since they are still capable of considerable
freedom upon land, which they recurrently seek for the breeding of
their young. They still retain the primitive covering of hair, lost
almost entirely by the cetaceans and sirenians and functionally
replaced for the conservation of heat by a thick layer of blubber.
Instead of losing the hind legs and developing the tail as a propelling
organ like the whales, the seals encountered precisely the reverse
experience. The hind legs have been developed into most efficient
paddles or sculls, and the tail has been for the most part lost.
They are fish-eaters, it is true, but they do not have the long jaws
possessed by the porpoises and toothed whales.
In the sea-otters, beavers, and even the muskrats, we have examples
of less complete adaptation of land mammals to water life, the most
of them showing the beginnings at least of structural adaptations
similar to those of the seals. From an attentive examination of all
these animals, living as well as extinct, which have attained partial
or complete success as air-breathing water animals, we find certain
laws existing, if we may call them such, which we may discuss a little
in detail. As we have seen in the comparison of the whale with the
seal, the methods of adaptation have not always been the same, and some
recent writers have endeavored to classify aquatic animals under many
groups, to which they have given learned technical names, most of which
will not concern us here in dealing with the reptiles only.
Beginning with the head, we find that all those reptiles and most of
the mammals which have become aquatic fish-eaters have an elongated
skull, or rather an elongated face. The jaws are long and slender, and
the teeth are not only numerous but also sharp and slender, much like
those of the gar-pike, indeed. It is remarkable, too, that in most
such animals the external nostrils are situated, not at the extremity
of the snout, as in all terrestrial mammals and reptiles, but far back
near the eyes. In the whales this position of the nostril enables the
animals to breathe without continuous muscular exertion while floating
on the surface; that is, the nostrils are at the top of the head. In
the sirenians, on the other hand, which live habitually at the bottom
of shallow waters, coming to the surface to breathe only, the nostrils
are situated so that they are the first to emerge, that is, they are
near the front end. The crocodiles, with a more or less elongated face,
as also the Choristodera, described farther on, are exceptions, since
their nostrils are at the extremity of the snout. Both of these types,
however, notwithstanding the elongation of the face, are only partly
aquatic in habit, and in the crocodiles the breathing organs have
undergone a strange modification in accordance with habits peculiarly
their own, as will be explained later on. Whether this recession of the
nostril toward the eyes can be explained in all cases by the peculiar
breathing habits is, however, doubtful. Possibly in some cases, such as
the phytosaurs, described later, the creatures used their long beaks
to probe in the mud while breathing. Possibly the posterior position
has been in some cases rather the result of the elongation of the face,
leaving the nostrils behind in some forms, or carrying them forward in
others. Nevertheless posterior nostrils always indicate more or less
aquatic habits.
In all the earliest reptiles, as we have seen, the neck was short,
like that of their immediate progenitors, the ancient amphibians. The
shoulders were close to the skull, with not more than two vertebrae
that could be called cervical. It happens that most of the earliest
reptiles, as we know them, were more or less amphibious in habit, and
all of them were probably good swimmers; nevertheless in all likelihood
reptiles began their career as a class with a very short neck. The
earliest known distinctly terrestrial reptiles had a moderately long
neck composed of six or seven cervical vertebrae. It may therefore be
assumed with much probability that all later reptiles with a greater
or less number of cervical vertebrae are specialized animals, so far
as the neck is concerned. Most living reptiles have eight cervical
vertebrae; a few have nine, and still fewer have but five. Birds may
have as many as twenty-four, while all mammals, with two or three
exceptions, have the primitive number seven. Among extinct reptiles,
however, there were not a few with more numerous neck vertebrae, some
having the enormous number of seventy-six.
An ordinary fish has apparently no neck whatever, the trunk
being seemingly attached to the head, nearly as in the primitive
amphibians and primitive reptiles. It is evident that a movable neck
of considerable length would not only be of no use to the swiftly
swimming fish, but a positive disadvantage to it. The body is quickly
and easily turned by the powerful tail fin, and a long neck could be
of no use that the tail would not better subserve. It is therefore
of interest to learn that, as a rule, aquatic animals of all kinds
having a powerful propelling tail have also a short neck, acquired
either by the loss of neck vertebrae, or, as in the mammals, by the
shortening and coalescence of the normal number of seven. There are
very few exceptions to this rule of a short neck and a long tail.
Those strange little reptiles of Paleozoic times, the first that we
know that returned to the water, the Proganosauria, have not only a
long, flattened tail, but also an unduly elongated neck of from nine to
twelve vertebrae.
On the other hand, certain unrelated reptiles of the past, the
dolichosaurs, nothosaurs, and plesiosaurs, with a short non-propelling
tail, developed a long neck—sometimes an excessively long one in the
plesiosaurs. The turtles, some of which have attained a high adaptation
to water life, have invariably a short tail and a freely movable,
relatively long neck, a neck which Dr. Hay tells us has increased
in length from the beginning of their race by the simple elongation
of the vertebrae, as in the giraffe, and never by the addition of
vertebrae. We may then account it a rule that swimming animals with
a long neck have a short tail, and those with a short tail have a
long flexible neck. Even in the plesiosaurs there is some variation
of the length of the tail in correlation with the neck. Short-tailed
animals must necessarily propel themselves through the water by the aid
of their legs, especially the hind legs. If one watches an actively
swimming alligator he will observe that the front legs are folded or
collapsed by the side of the body, while the hind legs, much bent,
are used only slightly in propulsion. The animal swims by a marked
sinuous or serpentine movement, like that of a snake upon land,
extending throughout the tail and part of the body, at least. An animal
propelling itself by its limbs could not move sinuously, and use its
legs actively at the same time, and it is probable that the long neck
has been evolved compensatorily.
With this shortening of the neck and sinuosity of movement there is
developed in every case a long trunk as well as a long tail. The trunk
becomes more slender and cylindrical, more like that of a snake, with
an actual increase of the bones composing it, reaching the great
number of forty-three vertebrae in that most sinuous of all water
reptiles with legs, _Pleurosaurus_ of the Protorosauria. And the tail,
primitively having perhaps sixty or seventy vertebrae, may have as
many as one hundred and fifty in the more typical aquatic forms. This
elongation of trunk and tail must be of great advantage to the swimming
reptile, just as the racing scull is a more perfect type of speedy
craft than a flat-bottomed scow. Dr. Woodward has said that the fate of
all fishes, if they continue their evolution long enough, is to become
eel-like.
Not only was the tail greatly elongated in swimming reptiles, but it
was also more or less flattened. In the beginning of water adaptation
the flattening was throughout the tail, as in the living alligators and
crocodiles. As the adaptation to water life became more perfect, the
flattening became more and more restricted to the extremity; that is,
the flattening begins like that of a salamander and in the end becomes
like that of a fish, a terminal fin. And some of the actual stages in
the evolution of the fish-like fin have been observed by Dr. Merriam
in the earlier and more primitive ichthyosaurs of California. In those
animals swimming chiefly in a horizontal direction the tail fin has
become like that of fishes, that is, vertical; but in those animals
which use the tail chiefly for ascending and descending rapidly in the
water the fin is developed in a horizontal position, examples of which
are seen in the flukes of whales and sirenians.
All animals living upon the land require firm articulations between the
different bones of the skeleton, and especially between the vertebrae,
for the support and control of the body. Among aquatic animals there
is a strong tendency toward looseness of joints, with increasing
flexibility. Fishes have the articular processes between the arches
of the vertebrae feebly or not at all developed, and the centra or
bodies of the vertebrae have thick pads of cartilage between them. Firm
union between the vertebrae would restrict freedom of movement, and
firmness is not required when the body is surrounded on all sides by
water of nearly the same specific gravity as the body itself. And it is
doubtless for the same reasons that the articulations of all strictly
aquatic reptiles have for the most part become looser and less firm,
especially those between the different vertebrae.
The same looseness of articulation is also found in the ribs of
aquatic animals. In most animals, and in all those which walk erect,
like the mammals, each rib is firmly attached to the backbone by two
distinct joints, the head and tubercle, with an interval between
them. This double attachment prevents much in-and-out movement of the
ribs and gives a firm support for the attachment of the muscles of
respiration, as well as for those supporting the viscera. This firmness
is unnecessary in animals living always in the water, and the ribs
therefore in all aquatic animals tend to become single-headed and
loose. The lower or capitular articulation has been lost in part, or
almost wholly, in many cetaceans. It has been said that a whale cast up
on land will die of suffocation, not for the lack of air, for it is an
air-breathing animal like ourselves, but because it can no longer use
its respiratory muscles attached to the loosely articulated ribs; it
suffocates because the ribs collapse.
As would be expected, the greatest modifications of structure in the
adaptation of air-breathers to water life are found in the limbs. No
other parts of the body have such different functions in water and on
land as the limbs and fins. The limbs of a dog, or a cat, or a man
are feeble organs for swimming in comparison with the fins of a fish,
and if the land animal must compete with fishes to prey upon them for
food it must acquire like swimming powers. As a matter of fact, the
limbs of all typically aquatic air-breathing animals have lost nearly
all external resemblance to the legs of walking and running animals,
and have become more or less fin-like in function—fin-like in shape
and function, but never fin-like in actual structure. No creature can
go back and begin over again, any more than a man can again become a
child with all its possibilities for improvement and development. If
an animal cannot modify the organs it already possesses so as to adapt
them to new and changed uses by the aid of evolutionary forces it must
fail in the struggle. It can never acquire new material, never get new
fingers and toes, new organs or parts of organs; all its possibilities
lie in the improved and new uses it can make of the material which it
received from its ancestors.
The beginning of aquatic adaptation of the limbs lies in the membranous
webs between the toes of frogs, salamanders, ducks, seal, otters, etc.,
where the feet are used largely or entirely for propulsion through
the water, in the absence of a propelling tail. And this membrane,
in the majority of cases, is the extent of aquatic adaptation in
air-breathing animals. In those animals, however, such as most of the
reptiles described in the following pages, where the tail has developed
as the propelling organ, the limbs lose to a greater or less extent
their propelling function and become merely organs of equilibration
and control. Of the two pairs of fins of fishes it is evident that
the anterior ones have the more important equilibrational function;
the hind ones have a much less important use as guiding organs; as
a matter of fact, in not a few fishes the hind or pelvic fins have
actually migrated forward to supplement the function of the pectoral
fins. It is for these reasons that those animals best adapted of all
for life in the water—the whales and sirenians—have lost the hind legs
completely. In other tail-propelled air-breathers the hind legs have
become progressively smaller and less powerful than the front ones. In
all short-tailed water animals, however, where the legs, and especially
the hind legs, have the important function of propulsion to subserve,
they still retain the large size and firm connections with the body,
examples of which will be seen in the seals, sea-otters, marine
turtles, and plesiosaurs.
Because the legs are no longer needed for the support or propulsion of
the body in long-tailed air-breathers, their connection with the body
becomes less and less firm, long before their entire disappearance. In
animals using the legs for crawling or walking the bones of an arm and
thigh are elongated, and the joints are always well formed, permitting
varied, extensive, and firm movements. Just the reverse is the tendency
in all those animals that propel themselves by the aid of the tail in
the water, since here what is needed is broad, short limbs, not long
and slender ones.
Most reptiles have five digits on each hand or foot; the bones of the
wrist and ankle are well formed, as in mammals, and the digits are
elongate, with a very definite arrangement of the bones composing them,
as already described, never exceeding five in any one finger or toe.
In the paddles of water reptiles, as the limbs are usually called,
the bones of the first segment, that is, the humerus and femur, are
always greatly shortened in those having a propelling tail, and even
in some with a short tail, such as the seals, and in a lesser degree
in the sea-otters. On the other hand, in those animals which use the
legs chiefly for direct propulsion these bones are elongated, as
exemplified by the plesiosaurs and marine turtles. In all save the
seals and their kind, and the otters, whose legs are used rather as
sculls than as oars, the bones of the next segment, the radius and
ulna of the front pair, the tibia and fibula of the hind pair, are
always shortened, and one can tell the stage of aquatic adaptation, as
exemplified, for instance, in the plesiosaurs and ichthyosaurs by the
degree of shortening of these bones. Indeed, the first suggestion in
any crawling animal of water habits is shown in the relative lengths of
the epipodial bones, as these bones are called. Furthermore, cursorial
or terrestrial habits are suggested by the relative size of the smaller
bone of the leg, that on the little-toe side, the fibula. In birds,
pterodactyls, and most running animals, it disappears in part or
wholly. In swimming animals it tends to grow larger than the tibia, as
will be conspicuously seen in the paddle of the mosasaurs.
The bones of the wrist change in two ways: by becoming cartilaginous,
as in whales and salamanders, or by becoming more firmly ossified
and more closely united, as in the plesiosaurs. The digits always
are elongated, often extraordinarily so, either by the elongation of
individual bones or phalanges, or by the development of new bones.
These new bones, when they occur, are new growths, not the reproduction
of the old elements of fishes, and there may be as many as twenty
such new elements or phalanges in a single digit. There is one marked
exception among reptiles to this hyperphalangy, as the increased
number of phalanges is called, and that is the turtles. As we have
seen, in the elongation of the neck among turtles there never has
been an actual increase in the number of vertebrae; so also in the
elongation of the digits the normal number of three in each digit
has never been exceeded, except among the river turtles, where there
are four in the fourth digit—possibly a relic of original conditions
rather than the beginning of hyperphalangy; but the individual bones
have become greatly elongated. In living reptiles, birds and mammals
of the land, the fifth toe is always shorter than the fourth. In the
seals, the sea-otter, and to a less degree in the muskrat, the fifth
toe has become elongated. And the elongation of this toe is the first
and most decisive indication of a webbed foot of strong propelling
power among the aquatic reptiles of the past, as exemplified especially
by the proganosaurs. Finally, in one order of extinct reptiles, the
ichthyosaurs, there has been an actual increase in the number of
digits, in some to as many as nine in each paddle.
In addition to all these modifications of the skeleton, the bones
themselves tend to become softer and more spongy in aquatic animals.
The bones of the whale, as is well known, are very spongy in texture,
and those of the seals and sea-lions contain an unusually large amount
of oily matter. So, too, the bones of the extinct water reptiles—of
many of them at least—were more spongy than those of their land
relatives; and this is due in part perhaps to their lessened use as
muscular supports, in part perhaps to the necessity of a lessened
specific gravity. As a rule sea-animals need to be of the same specific
gravity as the water in which they live, or a little less. The bones
of the living sirenians, the manatees and dugongs, so far from being
light and porous, are unusually dense and solid. The sirenians live
habitually at the bottom of shallow waters, feeding upon vegetable
growths; and doubtless their bottom-feeding habits account for the
solidity of the bones. A whale would float to the top, while a
dugong would sink to the bottom, on the relaxation of all muscular
movement. And we shall see that certain reptiles in the past had in all
probability like bottom-feeding habits, because of the solidity of the
bones of their skeletons.
Many birds and fishes have a peculiar ossification of the usually
tendinous outer covering of the eyeball, called the sclerotic
membrane. These ossifications form a flattened or somewhat projecting
conical bony ring about the pupil of the eye. The individual bones
are flat and more or less imbricated plates, with some motion between
them. Accommodation for vision in reptiles, birds, and fishes is not
the simple process that it is in mammals, where it is controlled
by simple ciliary muscles which compress the lens, causing it to
assume a more spherical or a more flattened form, thus changing the
focus. In reptiles accommodation is effected by the compression of
the eyeball by means of external muscles, elongating it and causing
its front part to expand or project. The imbricated sclerotic plates
permit this expansion and contraction of the eyeball. Under great
internal or external air pressure the cornea, the only unprotected
part, must necessarily change its contour unless some compensatory
force is brought to bear to counterbalance it; and this doubtless was
the function of the sclerotic plates so commonly present in aquatic
reptiles.
Among terrestrial reptiles there are not a few examples of the
ossification of such sclerotic plates, notably among the skink
lizards. Every known form of extinct reptiles of aquatic habit had
them, and even some of the subaquatic dinosaurs, like _Diplodocus_ and
_Trachodon_. One may say with assurance that it is impossible for any
reptile to become thoroughly adapted to aquatic life without acquiring
large and strong sclerotic plates.
Most land reptiles are or were covered by horny scales or bony plates;
the pterodactyls are the only order of terrestrial reptiles with
no such covering of which we have any evidence. Such coverings are
wholly unneeded for animals living in the water. Not only are they
unnecessary, but the increased resistance to the water would be more or
less detrimental to rapid swimming. It is for these reasons doubtless
that bony plates or horny scales disappeared for the most part from the
skin of all truly aquatic reptiles and mammals.
The foregoing are the chief acquired characteristics of aquatic
air-breathing animals and especially aquatic reptiles in adaptation
to their new mode of life. The resemblances, sometimes striking,
thus brought about in animals of very different origin and remote
relationships have often been mistaken for evidences of kinship,
that is, direct inheritance from common ancestors. Such acquired
resemblances in unrelated animals are known as parallel or convergent
evolution. It has often been difficult to distinguish between
convergent evolution and direct evolution, and difficulties still
perplex and trouble the student of natural history in every branch
of life. Not till all such problems are solved can we hope to attain
the true classification of animals and plants. The whales a century
ago were considered merely breathing fishes; the ichthyosaurs until a
quarter of a century ago were supposed to be the direct descendants of
fishes; lizards and crocodiles were grouped together in a single order;
and salamanders were called reptiles not very long ago.
Perhaps the reader will be able from the foregoing to understand
and appreciate better some of the difficulties that confront the
paleontologist in his attempts to solve the problems of past life; to
understand why he sometimes makes mistakes, for he has by no means
yet learned all the permutations of the skeleton in any class of
vertebrates, and is not sure that the laws he accepts are not subject
to modifications and exceptions. If he is truly scientific he hesitates
long in prophesying or conjecturing.
CHAPTER VI
SAUROPTERYGIA
Very scanty are the early human records of those strange reptiles
known as the plesiosaurs. Were one to search through the many works
published during the latter half of the seventeenth century and all
of the eighteenth, devoted to “lapides petrifacti,” “figured stones,”
“reliquia diluvii,” or by whatever other fanciful names fossils were
known, here and there he would probably find descriptions and figures
of bones of these reptiles. It would hardly seem that plesiosaurian
bones could have been overlooked by the curious, so abundant are they
in many places. But there is no such history of the early discovery
of the plesiosaurs as there is of the ichthyosaurs and mosasaurs.
Their birth into human history was very formal and proper, under
the ministrations of a learned doctor of science, the renowned
Conybeare, of whom we shall speak again. It was he, who with De la
Bêche, late Director of the British Geological Survey, described for
the first time, in 1823, one of these reptiles, to which he gave the
name _Plesiosaurus_, meaning “like a lizard.” He distinguished the
plesiosaurs from ichthyosaurs, with which it is possible that they had
previously been confounded, and gave a good description of considerable
material. Cuvier, a little later, gave a more complete description
of the same remains which had served Conybeare and De la Bêche for
their original description, and for the first time made it evident
that fossil plesiosaurs were widely and abundantly distributed over
the earth. The closing sentence of Cuvier’s chapter devoted to the
discussion of these creatures in his _Ossemens Fossiles_ was really
prophetic, not only of the many discoveries of the plesiosaurs yet to
be made, but of all other extinct animals as well: “I doubt not that,
in a few years it may be, I shall be compelled to say that the work
which I have today finished, and to which I have given so much labor is
but the first glimpse of the immense creations of ancient times.”
[Illustration: FIG. 31.—Restoration of _Plesiosaurus guilelmi
imperatoris_ (left figure) and _Thaumatosaurus victor_ (right figure),
Liassic plesiosaurs. (From E. Fraas.)]
In quick succession there followed many other discoveries of
plesiosaurs, not only in England but elsewhere in Europe. The famous
English anatomist and paleontologist, Sir Richard Owen, to whom we
owe, perhaps, more than to anyone else our present knowledge of these
animals, the eccentric Hawkins of England, the learned von Meyer of
Germany, and, in later times, more especially Seeley and Andrews of
England, Fraas of Germany, Bogalobou and Riabanin of Russia, as well
as many others, have brought to light during the past century many and
varied forms of those sea-reptiles. Blaineville in 1835 gave to the
plesiosaurs an ordinal rank under the class Ichthyosauria, and even the
astute Owen in 1839 united them with the ichthyosaurs as a suborder of
his Enaliosauria, or “sea-saurians.” He called them Sauropterygia, or
“reptile-finned,” and these terms, Enaliosauria, Ichthyopterygia, and
Sauropterygia, have long persisted in works on natural history because
of the prestige of Owen’s name. As we shall see later, the plesiosaurs
are really of remote kinship to the ichthyosaurs, and there is no such
natural group as the Enaliosauria. It often takes years to distinguish
between apparent and real relationships among living organisms, and
both of these groups of sea-saurians have had a sorry experience in the
treatment they have received from nomenclators.
Perhaps because of the writings of Dean Buckland in his famous
_Bridgewater Treatise_, in large part a theological disquisition,
though of real scientific merit, the ichthyosaurs and plesiosaurs
early became widely and popularly known, and, even to this day, these
reptiles, together with the dinosaurs, first made known by Rev. Dr.
Mantell, are often supposed to be the most typical and horrid of
monsters. Many and fabulous are the tales that have been told of
them in literature both grave and gay. The preacher adduced them as
evidences of the great world-catastrophe told in biblical history, and
the German student sings of them to the tune of the “Lorelei”:
Es rauscht in Schachtelhalmen, verdächtig leuchtet das Meer;
Da schwimmt mit Thränen in Auge ein Ichthyosaurus einher.
Ihn jammert der Zeiten Verderbniss, denn ein sehr bedenklicher Ton
War neuerlich eingerissen in der Liasformation.
Der Plesiosaurus, der alte, der jubelt in Saus und Braus;
Der Pterodactylus selber flog jungst betrunken nach Haus.
Der Iguanodon, der Lümmel, wird frecher zu jeglicher Frist;
Schon hat er am hellen Tage die Ichthosaura geküsst.
We now know that they were not the monsters of horrid mien that they
were once supposed to be: the largest plesiosaurs, were they living
today, would find unopposable foes in the vicious and cruel crocodiles.
They were relatively stupid and slow, cruel enough to the smaller
creatures, but of limited prowess. But in structure and habits they are
among the most remarkable of all the animals of the past or present.
Although their remains are among the most abundant and widely
distributed of all fossil reptiles, the plesiosaurs as a whole are less
perfectly known than either the ichthyosaurs or the mosasaurs, and it
has been within a comparatively few years only that an approximately
complete knowledge of any form has been obtained. This is partly due
to the fact that the order comprises vastly more kinds, more species,
genera, and families than does any other order of marine reptiles;
partly because their remains, though widely distributed over the earth,
and in rocks of many geological epochs, are seldom found completely
preserved; usually specimens comprise only a few bones or single bones,
and complete skeletons are rare. Were there but few kinds, the many
specimens discovered would mutually supplement each other, finally
completing our knowledge; but the fragments of many kinds only add to
our confusion. Nevertheless, because the plesiosaurs lived so long in
geological history, their remains are found in rocks of many different
kinds, and since it is improbable that any of them had great specific
longevity, it is very probable that all these described species, or
most of them, often made known from single bones, will eventually be
found to be distinct, and that many more will be added to them. It does
not seem improbable that within the next forty or fifty years not less
than a hundred species of plesiosaurs will have been discovered in
North America alone. At the present time perhaps that many have been
described from the whole world.
When Blaineville gave the name Plesiosauria to the aquatic reptiles
described by Conybeare, Cuvier, and others, he had no knowledge of
others of an intermediate kind between them and land reptiles. His
group-term then can be properly applied only to the truly aquatic
forms, and Owen’s name Sauropterygia becomes available in a wider
sense to include all the known types belonging to the order of which
the plesiosaurs form a part. Of this order then there are two clearly
marked divisions or suborders, the Plesiosauria and the Nothosauria,
the former having a complete aquatic adaptation, the latter only
a partial one. While the two suborders are evidently allied, some
authors have suggested that their differences are only familial; others
have thought that they are really orders. We shall see how close the
relationships are.
PLESIOSAURIA
It was Dean Buckland who facetiously likened the plesiosaurs to a snake
threaded through the shell of a turtle, and the simile was not an
inapt one in his day. The vernacular designation of them—long-necked
lizards—conveys the same impression of their chief peculiarity, but the
name is less applicable than it once was, since recent discoveries have
brought to light forms with a relatively short neck.
Though the plesiosaurs are nearly perfectly adapted to an aquatic
life, the adaptation was, in many respects, of a very different kind
from that of the ichthyosaurs—so very different that we have not yet
quite finished conjecturing as to the habits of the living animals. As
already suggested in the popular name, the most striking characteristic
of the typical plesiosaurs, the one which suggested to Buckland his
frequently quoted simile, is the ofttimes enormously long neck,
proportionately longer than that of any other known creatures of the
past or present. In other truly aquatic animals the neck is actually
shortened in the acquirement of a fish-like shape, and the number of
bones composing it reduced. In the Sauropterygia the neck is usually
longer than any truly land animals ever possessed, the longest-necked
forms having as many as seventy-six vertebrae in the cervical region.
The elongation of the neck among mammals is always due to an increase
in the length of the individual bones, never to an increase in the
number from seven, with but a single exception—a South American sloth
which has nine cervical vertebrae. The long neck of birds is due both
to an increase in the length of the individual vertebrae and to an
increase in their number, to as many as twenty-one. But the elongation
of the neck among plesiosaurs was very variable indeed; sometimes it
was ten or twelve times the length of the head, at other times it was
even shorter than the head. And the number of bones composing it was
also extremely variable, scarcely any two species having the same,
the known extremes being seventy-six and thirteen. In _Elasmosaurus
platyurus_, for instance, the longest-necked plesiosaur known, the head
was two feet in length, the neck twenty-three, the body nine, and the
tail about seven; on the other hand, in the shortest-necked plesiosaur
known, _Brachauchenius Lucasi_, the head was two and one-half feet
in length, the neck less than two feet, and the body about five; the
length of the tail is unknown.
[Illustration: FIG. 32.—Skeleton of _Trinacromerum osborni_, a
Cretaceous plesiosaur, as mounted in the University of Kansas Museum.]
Not only was the number of vertebrae so extraordinarily increased in
many plesiosaurs, but in the longest necks the vertebrae themselves, as
in birds, were more or less elongated, especially the posterior ones,
which may be six or seven times the length of the anterior ones. Not
only was the neck of such great length in many plesiosaurs, but it also
tapered very much toward the head.
The vertebrae are always biconcave, but the cavities are shallow,
saucer-like, sometimes almost flat at each end, and very different from
the conical fish-like cavities of ichthyosaurian vertebrae.
[Illustration: FIG. 33.—Restoration of _Elasmosaurus platyurus_, an
Upper Cretaceous plesiosaur.]
Often the vertebrae are short throughout the vertebral column;
sometimes the posterior cervicals and the dorsals are elongated and
very robust. The trunk or body proper was never much elongated in the
plesiosaurs, having only from twenty-five to thirty vertebrae. The
tail was always shorter than the trunk, and it tapered rapidly to the
extremity; in some specimens it has been observed to turn up slightly
near the extremity, as though for the support of a small terminal fin.
[Illustration: FIG. 34.—Cervical vertebrae, from the side and behind,
and dorsal vertebra from in front of _Polycotylus_, a Cretaceous
plesiosaur: _az_, anterior zygapophysis; _pz_, posterior zygapophysis,
_r_, _r_, _r_, cervical ribs; _d_, articulation of dorsal rib.]
The ribs in the cervical region are short, but so locked together
posteriorly as not to permit much lateral motion. They are sometimes
double-headed in the neck, sometimes single-headed, but both heads
when present articulate or are attached to the body of the vertebrae,
distinguishing them at once from those of other animals, except the
ichthyosaurs. In the dorsal region the ribs are attached high on the
arch to the extremity of the stout transverse processes by a single
head, very much as they are in some cetaceans, and quite unlike the
condition in any other known reptile. They end freely below, having
no attachment to a breast bone or other bony parts. Because of their
shape and position as frequently found, the body in life must have been
flattened from above downward, and broad; indeed, this shape is quite
certain because of the very broad expanse of the coracoids, between the
articulations of the front legs.
[Illustration: FIG. 35.—Pectoral girdle of _Trinacromerum_ from above:
_ic_, interclavicle; _cl_, clavicle; _sc_, scapula; _c_, coracoid.]
The shoulder-girdle or pectoral arch is strangely unlike that of any
other reptiles. There is no breast bone, since the breast bone is a
comparatively late development in reptiles, not appearing, probably,
until after the plesiosaurs had begun their existence. Taking the
place of the sternum, the very large and broad coracoidsjoin each
other in the middle, forming a sort of subdermal armor on the under
side of the body in front. In some of the largest plesiosaurs these
two bones measured together about six feet in length by four in
width. Though so very large they are thick only in front between the
articulations of the forelegs. The shoulder-blades are much reduced
in size and are extraordinarily modified. The blade proper, that is,
that part extending backward and upward, is narrow and small, affording
but little surface for the attachment of muscles. On the inner side,
extending toward the middle in front of the coracoids, there is another
projection, often broad and large, to which was attached the clavicles
when present, and often this projection met its mate of the opposite
scapula in the middle in front of the coracoids in a broad union. The
clavicles or collar-bones are small and thin, and sometimes absent;
they also are united in the middle posteriorly with the coracoids when
the scapula did not intervene. And the interclavicle also is sometimes
wanting. Altogether the pectoral bones form a very large, broad, and
concave trough inclosing the whole of the under side of the anterior
part of the body. This extensive surface must have furnished attachment
to stout and strong muscles controlling the downward and inward motion
of the paddles.
There is a well-developed sacrum of three vertebrae for the support
of the pelvis or hip bones. The reason for its persistence in animals
so thoroughly adapted for life in the water will be understood later.
The ilium is slender; it was attached to the sides of the sacrum by
ligaments, only, not forming a firm union, but strong nevertheless.
The pubes and ischia, the other bones of the pelvis on the under side
of the body, like the corresponding bones of the pectoral girdle, were
enormously enlarged, forming great flat, bony plates.
Besides these large bony plates of the shoulder and pelvic girdles,
the short abdominal region was inclosed by numerous series of strong
ventral ribs, that is, overlapping rod-like bones on each side,
connected with a central piece. It will be seen that the whole under
side of the body, from the base of the neck to the base of the tail,
was well protected by bones, rigid and unyielding in front and behind,
flexible for a short space below the abdomen; this surface, however,
was not flat like the under shell of a turtle, but rounded from side to
side.
[Illustration: FIG. 36.—Pelvic girdle from above of _Trinacromerum
osborni_, an Upper Cretaceous plesiosaur: _p_, pubis; _is_, ischium;
_il_, ilium.]
Many of the characteristics of the limbs of the plesiosaurs are
peculiar to themselves; others they had in common with other aquatic
reptiles and mammals. The paddles resemble those of the ichthyosaurs
more nearly than those of any other reptile, and it was doubtless this
superficial resemblance which so long deceived the early anatomists
as to the affinities of the two orders. Unlike all other aquatic
animals, however, the plesiosaurs have the hind limbs nearly or quite
as large as the front ones, and they doubtless were equally effective
in function. The humerus and femur are always elongate, though broad
and massive. In no other aquatic animals, save the marine turtles,
do we find these bones relatively so long and strong; they are very
short in the cetaceans, the sirenians, the ichthyosaurs, mosasaurs,
thalattosaurs, and the marine crocodiles, in front at least. The strong
muscular rugosities of the plesiosaurian bones are very suggestive of
powerful swimming muscles.
[Illustration: FIG. 37.—Pelvic girdle of _Elasmosaurus_: _p_, pubis;
_is_, ischium; _il_, ilium.]
[Illustration: FIG. 38.—Paddles of Plesiosaurs: _A_, right hind
paddle of _Thaumatosaurus_, after Fraas; _B_, right hind paddle of
_Trinacromerum_; _C_, right front paddle of same individual; _f_,
femur; _fb_, fibula; _t_, tibia; _h_, humerus; _r_, radius; _u_, ulna.]
The bones of the forearms and legs, the wrists and ankles are all
polygonal platelets of bones, closely articulating with each other.
The finger and toe bones have a more elongated, hour-glass shape
than those of the ichthyosaurs, resembling more nearly those of the
mosasaurs, indicating a greater flexibility than the ichthyosaurs
possessed. The ichthyosaur paddles must have been quite like the fins
of fishes in function, while doubtless those of the plesiosaurs were
capable of a more varied use, as indeed was required of them. Their
articulation with the trunk was more of a ball-and-socket joint than in
the other reptiles, showing possibility of considerable rotation on the
long axis, and an antero-posterior propelling action. The paddles were
certainly more powerful than those of any other aquatic air-breathing
animals. There were no additional digits, all plesiosaurs having
neither more nor less than five in each hand and foot. Hyperphalangy
was sometimes carried to an excessive degree, some digits of some
species having as many as twenty-four bones, a larger number than has
been observed in any other air-breathing vertebrate.
[Illustration: FIG. 39.—Pectoral girdle (in part) and front paddles
of _Elasmosaurus_ (after Riggs): _sc_, scapula; _h_, humerus; _cor_,
coracoid; _r_, radius; _u_, ulna.]
In Fig. 38 on p. 85 are shown two paddles, the front and hind paddles
of a single individual of a very specialized plesiosaur from the
Upper Cretaceous of Kansas (_Trinacromerum_). The long arm and thigh
bones are followed by remarkably short and broad bones in place of
the elongated forearm and leg bones of the land reptiles. Not only
are these bones much broader than they are long, but there have been
developed additional bones back of them in the same row—new bones
which have no counterpart in any terrestrial reptiles. In the first of
the three figures is shown a hind paddle of one of the earliest known
plesiosaurs, _Thaumatosaurus_, from the lower part of the Jurassic of
Germany. It will be seen here that the tibia and fibula are much more
elongated than in _Trinacromerum_, and much more like the leg bones
of land reptiles. A still more primitive stage in the evolution of
the swimming paddle of the plesiosaurs will be seen in Fig. 48 on p.
99, the possibly ancestral, amphibious nothosaur. Here the tibia and
fibula, while relatively very much shorter than in any land reptile,
still have, together with all the other bones of the leg, a terrestrial
or amphibious type. In Fig. 39 is seen the front paddles of the
long-necked _Elasmosaurus_, which, though one of the latest of all
plesiosaurs in geological history, has the structure of its paddles
somewhat intermediate between that of the earlier _Plesiosaurus_ and
the later _Trinacromerum_.
[Illustration: FIG. 40.—Skull of _Elasmosaurus_ from the side: _pm_,
premaxilla; _m_, maxilla; _po_, postorbital; _j_, jugal.]
The skull of the long-necked plesiosaurs is surprisingly small in
comparison with the remainder of the skeleton, often very snake-like
in shape, though very un-snake-like in structure. The short-necked
plesiosaurs had often a relatively larger skull, in _Pliosaurus_, for
instance, more than five feet long, sometimes rather broad and short,
sometimes remarkably long and slender. The external nostrils were
situated far back, very near the eyes, and were very small. The eyes,
of considerable size, though by no means so large as those of the
ichthyosaurs, were directed laterally, and were provided with a ring of
bony sclerotic plates—rather small and weak ones, however. The quadrate
bones—bones peculiar to the reptiles and birds—to which the lower jaws
are articulated, are, as in the ichthyosaurs and crocodiles, rigidly
fixed and immovable. The lower jaws, always rather slender, are firmly
united in front, sometimes for a long distance, as in the modern
gavials. The teeth of the broad-headed plesiosaurs are long, slender,
pointed, and recurved, of a murderously cruel shape; they are deeply
implanted in sockets, and number from twenty to thirty on each jaw
above and below. There are no teeth on the bones of the palate, such
as the mosasaurs possessed. The slender-jawed, gavial-like plesiosaurs
have more numerous, but smaller teeth. The surface of the skull on each
side behind, for the attachment of the muscles closing the mandibles,
is of great extent; in some this surface is increased by a high, thin
crest in the middle, as in strongly carnivorous animals, all of which
give conclusive evidence of the powerful muscles used in biting and
seizing. There is but one temporal opening on each side, as in the
ichthyosaurs and the mosasaurs, whereas the crocodiles, thalattosaurs,
phytosaurs, etc., have two. The brain cavity of all plesiosaurs is
small, though the cavities of the internal ears, the semicircular
canals at least, are large. The semicircular canals in vertebrates have
little or nothing to do with the function of hearing; they serve rather
for equilibration, for the co-ordination of muscular movement; possibly
we may infer from their large size in the plesiosaurs that they were
not at all clumsy in their movements. There is a large opening for
the pineal body, the so-called eye in the roof of the brain cavity,
though its possession does not necessarily imply the possession of a
functional organ.
[Illustration: FIG. 41.—Skull of _Trinacromerum_ from the side: _ang_,
angular; _d_, dentary; _pm_, premaxilla; _po_, postorbital; _j_, jugal;
_sur_, surangular.]
The Plesiosauria included some of the largest aquatic reptiles that
have ever existed, equaled, perhaps, though not exceeded, by some of
the extinct crocodiles. The largest known are probably those of the
Kansas chalk, or the Jurassic of Wyoming, which probably reached a
length of nearly or quite fifty feet, of which the neck formed about
one-half. Some of them had paddles more than six feet in length. The
head of the largest was about five feet in length, or about the size
of that of the largest known ichthyosaurs and mosasaurs. The smallest
known adult plesiosaurs were nearly ten feet in length. The teeth of
the largest and most carnivorous plesiosaurs sometimes measure four
inches in length.
[Illustration: FIG. 42.—Restoration of _Trinacromerum_, a Cretaceous
plesiosaur; length about ten feet.]
As is the case with both the ichthyosaurs and mosasaurs, skeletons
of plesiosaurs have been discovered with nearly all their bones in
their relative positions, and with impressions of skin and outlines of
body made before decomposition. Though our knowledge of the external
appearance of the plesiosaurs when alive is perhaps not as full as we
could wish, it is sufficient to give us a fairly good conception of
what the animals really were. The skin was smooth and bare, without
scales or plates of any kind, and Dames has described a terminal or
nearly terminal fleshy dilatation of the tail, forming a sort of caudal
fin, which may have aided as a steering apparatus. Mounted skeletons
are preserved in a few museums, notably the British Museum, the
American Museum of New York City, and the museum of the University of
Kansas. Many nearly complete skeletons, however, preserved as they were
found in the matrix, are shown in various museums.
With these, principal facts regarding the structure, size, and external
form of these animals we may venture to draw certain conclusions, or at
least to offer certain conjectures as to their habits in life.
Because of the rigid structure of the jaws, united in front and
incapable of any lateral movement posteriorly, quite as are the jaws
of crocodiles, we are sure that prey of any considerable size could
not have been swallowed whole. The crocodiles tear away portions of
the flesh of their victims by quick, powerful jerks, and it is very
probable that the flat-headed plesiosaurs tore their food apart in
the same manner. In these kinds the teeth are much larger and more
irregular in size than are those of the long-snouted plesiosaurs, and
their use was certainly as much for tearing as for seizing. There are
the same differences between the size of the head and the size of the
teeth among the various plesiosaurs that there are among the modern
crocodiles and gavials. While the crocodiles seize and destroy even
larger prey, drowning and tearing their victims to pieces, the gavials
are more exclusively fish-eating, for which their small, sharp, and
more numerous teeth especially fit them. Their food, of small size,
is swallowed entire, and they are comparatively harmless, so far as
animals of considerable size are concerned.
The long neck, the thickset body, and short, stout tail are not at
all what we should expect to find in quick-swimming animals. We may
therefore assume that the motions of the plesiosaurs through the
water were more turtle-like than fish-like. The tail, even though
provided with a terminal, fin-like dilatation, was of little use in
the propulsion of the body, since the range of its movements was
restricted; it possibly served in a measure as a steering organ, a
rudder. The large, freely movable paddles must have been effective
organs of locomotion, and this function accounts for the relatively
large size of the posterior pair, and the firm union of the pelvis with
the vertebral column through the sacrum. With the hind limbs used as
oar-like organs, a firmer union with the skeleton was required than
the soft yielding flesh would permit. At the same time this union
was ligamentous only, not bony and unyielding, since the limbs were
never used to support the body upon the ground; and it is of interest
to observe that the ilia are directed, not upward and forward, but
upward and backward to the sternum, precisely the position that would
be expected with the force or thrust coming from behind, and not
below the yielding ligaments. Were the tail longer and more powerful,
the hind limbs would have been smaller and weaker, of use chiefly in
equilibration, involving the loss of any connection with the vertebral
column and the disappearance of the sacrum. It is of interest, finally,
to observe that many of the slender-jawed plesiosaurs had a relatively
short neck; they were doubtless more distinctively fish-eating in
habit, and possessed greater speed. That the limbs of plesiosaurs
were powerful propelling organs is also conclusively proved by their
structure. Quite unlike all those animals whose locomotion in the water
is chiefly effected by the tail, the humeri and femora, the upper
arm and thigh bones were elongated, and not shortened. They form the
rigid and stout handles of oars whose blades are the thinner, flexible
forearm, wrist, and fingers, or the corresponding foreleg, ankle,
and toes. No other purely aquatic reptiles, save the turtles, which
likewise are of the oar-propelled type, have elongated arm and thigh
bones.
Textbook illustrations of the plesiosaurs usually depict the necks,
like those of the swans, freely curved, and a popular scientific
article in one of our chief magazines a few years ago depicted one
of them with the neck coiled like the body of a snake. One noted
paleontologist, indeed, not many years ago described the plesiosaurs as
resting on the bottom in shallow waters with the neck uplifted above
the surface viewing the waterscape! And when we consider the fact that
some species of the elasmosaurs had a neck not less than twenty feet in
length, such a flexible use of it would not seem improbable. But the
plesiosaurs did not and could not use the neck in such ways. They swam
with the neck and head, however long, directed in front, and freedom
of movement was restricted almost wholly to the anterior part. The
posterior part of the neck was thick and heavy, and could not have been
moved upward or downward to any considerable extent and not very much
laterally. From all of which it seems evident that the plesiosaurs
caught their prey by downward and lateral motions of their neck, rather
than by quick swimming.
[Illustration: FIG. 43.—Gastroliths and bones of an undetermined
plesiosaur from the Lower Cretaceous of Kansas.]
About thirty years ago, the late Professor Seeley, a well-known English
paleontologist who devoted much attention to the study of these
reptiles, found with the remains of a medium-sized plesiosaur nearly a
peck of smoothly polished, rounded, and siliceous pebbles. He believed
that their occurrence with the skeleton was not accidental, but that
they had been intentionally swallowed by the animal when alive, and
formed at its death a part of its stomach contents. Even earlier than
this the same habit had been noticed. Nearly at the same time that
Seeley mentioned the peculiar discovery he had made the present writer
found several specimens of plesiosaurs in the chalk of western Kansas
with which similar pebbles were associated, an account of which was
given soon afterward by the late Professor Mudge. Since then numerous
like discoveries have made it certain that the plesiosaurs usually,
if not always, swallowed such pebbles in considerable quantities, for
what purpose we do not yet feel sure; one can only hazard a guess. The
small size of the pebbles, or gastroliths, as they have been called,
a half-inch or less in diameter, found with skeletons of large size,
indicate much more complete digestion of the hard parts of their food
than is the case with many other reptiles; no solid substance of size
could have passed out of the plesiosaur stomach, and such is the case
with the modern crocodiles, which have a like habit of swallowing
pebbles. That the plesiosaurs picked up these siliceous pebbles,
sometimes weighing a half-pound, accidentally with their food is highly
improbable; they surely had something to do with their food habits. It
is not at all unreasonable to suppose that the plesiosaurs, because of
their comparative sluggishness, fed upon anything of an animal nature,
whether living or dead, which came in their way; that carrion, squids,
crustaceans, and fishes were all equally acceptable; they were probably
largely scavengers of the old oceans. Barnum Brown found among the
stomach contents of a plesiosaur fragments of fish and pterodactyl
bones, and cephalopod shells. Gallinaceous birds, most of which have
the same pebble-swallowing habit, have a thick-walled muscular stomach
or gizzard, in which the pebbles serve as an aid in the trituration of
food. Modern crocodiles, with the same pebble-swallowing habit, have a
thick-walled muscular stomach, gizzard-like, though of course not as
large as in birds; and the same habit has been noted by Des Longchamps
in the ancient teleosaur crocodiles.
It is hardly possible yet to decide whether or not the plesiosaurs
were denizens of the open oceans for the most part, far from land.
That many of them were rovers is quite certain. With the skeleton of
a large plesiosaur found some years ago in western Kansas, there were
many siliceous pebbles which could have come only from the shores
of the old Cretaceous seas about the Black Hills, hundreds of miles
distant. Some of the pebbles are red quartzite, quite identical with
that of the bowlders brought to Kansas millions of years later by the
glacial drift from outcroppings near the northern line of Iowa. The
bones of plesiosaurs are often found in deposits believed to be of
deep-water origin. But they are also found in Kansas associated with
the remains of small turtles, flying reptiles, and birds which could
only have lived near the shores. Indeed, their remains have often
been found with those of strictly fresh-water animals which had been
brought down by the floods to the seas. Their wide but rather sparse
distribution in all kinds of marine sediments would rather indicate
that they were at home far out in the tempestuous ocean or near the
shores in protected bays, though probably they preferred the shallow
water littoral regions. One conclusion is quite justified: they were
not gregarious, as were the ichthyosaurs.
It is not certain that the plesiosaurs were viviparous, though there
are good reasons for the belief that they were. Remains of two embryos
were found years ago in England associated in such a way that it is
reasonable to suppose they were unhatched young, though embryos have
never yet been found associated with skeletons of adults, as have those
of ichthyosaurs in numerous instances. Bones of young, often quite
young, plesiosaurs, are frequently found in shallow water deposits, and
if the young were actually born alive they must have swum freely in the
open waters while yet of very tender age. Rather singularly, however,
the remains of these young plesiosaurs always occur as isolated bones.
In geological range the plesiosaurs were very persistent, extending
through nearly all the Mesozoic. They began their career as fully
evolved plesiosaurs, so far as we now know, near the close of the
Triassic period, and reached their culmination in the Upper Cretaceous,
but survived to the close of that period. In the beginning of their
career they were associated with the marine crocodiles and the
ichthyosaurs, but outlived them to find companions and probably
enemies in the huge and voracious mosasaurs of the later Cretaceous
times. At no time do they appear to have been especially numerous, nor
does it seem probable that they were ever a dominant type of marine
vertebrate life, though their remains occur everywhere that marine
deposits of the Jura and Cretaceous are known. Indeed, it may be said
with almost certainty that rocks of these ages and of that character
everywhere in the world contain fossil plesiosaurs. Their bones have
been made known from Europe, Asia, Africa, Australia, and North and
South America. From North America thirty or more species have been
described from New Jersey, Alabama, Mississippi, Texas, Arkansas,
Kansas, Nebraska, Colorado, New Mexico, Wyoming, North and South
Dakota, California, etc.
The cause of their final extinction no one knows, nor can we
conjecture much about it with assurance. That climatic conditions
became unfavorable for them is highly improbable, considering
their cosmopolitan habits; they were not discriminating in their
environments. After successfully withstanding their fiercest foes, the
ichthyosaurs, crocodiles, and mosasaurs, and large carnivorous fishes,
it does not seem probable that they would succumb to lesser enemies,
though it may be that they were finally attacked successfully, not
in the fulness of their strength as adults, but while young, by more
insidious enemies. More probably after their long life of millions of
years they had grown old, as everything grows old, and had become so
fixed and unplastic in their structure and habits that even slight
causes were at last their undoing. When we shall have bridged over
that still imperfectly known transition period between the great Age
of Reptiles and the greater Age of Mammals we shall have learned
more definitely some of the causes of the extraordinary revolution
in vertebrate life that then occurred. The plesiosaurs went out
with nearly all of their kind, the mosasaurs, the pterodactyls, the
dinosaurs; and, so far as we now know, their places in the sea, land,
and air were not immediately taken by any other creatures.
[Illustration: FIG. 44.—_Nothosaurus_; restoration after E. Fraas;
landscape by Dorothy Williston.]
NOTHOSAURIA
A few years after the discovery of the plesiosaurs by Conybeare, the
remains of animals of allied kinds were found in the Triassic rocks of
Bavaria. At first they were supposed to be those of true plesiosaurs,
and even the astute Cuvier was not very clear about them. Cuvier was
the first to call attention to them, expressing the opinion that
some of the fossils were of previously unknown animals allied to the
crocodiles, lizards, and plesiosaurs. It was von Meyer, however, who
first introduced a nothosaur to the scientific world under the name
_Conchiosaurus_. A year later Count George of Münster described other
forms under the name _Nothosaurus_, meaning “false lizard.” Count von
Münster was a most zealous collector of the fossils of the Triassic
deposits of Bavaria, amassing, after thirty years of active and
enthusiastic labor, a very large amount of material, which, at his
death, was purchased by the King of Bavaria and placed in the hands of
von Meyer for study. Von Meyer was to Germany what Owen was to England,
a man of deep learning, having an extensive knowledge of comparative
anatomy, and being thorough and critical in his work. His descriptions
and illustrations of these rich collections made by von Münster are
masterpieces of scientific thoroughness. He recognized in _Nothosaurus_
and other allied forms from the Bavarian Triassic a distinct group of
semiaquatic reptiles allied to the plesiosaurs, and his conclusions
have never been gainsaid. In more recent years additional remains
of these animals from Bavaria and other places in Europe have been
described, but none are known from other parts of the earth, or from
other than Triassic rocks. Altogether about ten genera and about twice
as many species have been described, probably all belonging in one
family, and all by common consent now classified with the Sauropterygia.
[Illustration: FIG. 45.—Head and neck of _Nothosaurus_; photograph of
specimen in the Senckenberg Museum, from Dr. Dreverman.]
[Illustration: FIG. 46.—Pectoral girdle of _Nothosaurus_, from
photograph by E. Fraas: _icl_, interclavicle; _cl_, clavicle; _sc_,
scapula; _cor_, coracoid.]
[Illustration: FIG. 47.—Pelvic bones of _Nothosaurus_: _il_, ilium;
_ac_, acetabulum; _p_, pubis; _is_, ischium. (After Andrews.)]
The Nothosauria were much smaller reptiles than the plesiosaurs, none
of them perhaps exceeding the size of the smallest known plesiosaurs.
They were semiaquatic in habit, with many curious resemblances to
other semiaquatic reptiles of a later time known, as the dolichosaurs.
The neck is more or less elongated, having about twenty vertebrae in
the longest-necked forms; the body is moderately long, and broad,
and the tail is relatively short. The vertebrae and ribs are quite
like those of the plesiosaurs, that is, the vertebrae are gently
concave at each end, and the dorsal ribs are attached by a single
head to the transverse process high up on the arch; the cervical ribs
are double-headed, precisely like those of the older plesiosaurs,
one of the characters which insistently proves the relationships of
the two groups. The bones of the shoulders (Fig. 46) also have many
resemblances to the extraordinary ones of the plesiosaurs, though
they are much less specialized. There was no sternum; the coracoids
are large, though very much smaller than those of the plesiosaurs.
The collar-bones are large and strong, joining each other in front
of the coracoids and firmly united with the shoulder-blades at the
outer extremity. Four vertebrae are united to form a sacrum, and their
union with the hip bones (Fig. 47) was much firmer than was the
case with the plesiosaurs. The limbs are elongated, but it will be
observed in the figures (Fig. 48) that the radius and ulna, tibia and
fibula, that is, the bones of the forearm and of the leg proper, are
relatively very short as compared with the humerus and femur, a sure
indication of the beginning of aquatic habits. The toes and fingers
were doubtless webbed, and there was no increase in the numbers of
bones in the digits, so conspicuous in the plesiosaurs. The external
nostrils are large, but are not situated so far back near the eyes as
in the plesiosaurs. There is a large pineal opening in the top of the
skull, as in the plesiosaurs, but no sclerotic or bony plates have
been observed in the eyes. They had ventral ribs like those of the
plesiosaurs.
[Illustration: FIG. 48.—Legs of _Lariosaurus balsami_, an Upper
Triassic nothosaur: _h_, humerus; _r_, radius; _u_, ulna; _i_,
intermedium; _ue_, ulnare; _f_, femur; _fi_, fibula; _t_, tibia; _a_,
astragalus; _c_, calcaneum. (After Abel.)]
No impressions of scales or bony plates have ever been found with the
remains of the nothosaurs, and it is the belief that the skin was
bare. A good idea of their general appearance will be gained from the
accompanying restoration adapted from that of Professor Fraas (Fig. 44)
and the restoration of the less highly specialized _Lariosaurus_, made
from a very complete skeleton in the Frankfort museum (Fig. 49).
It has been thought that these nothosaurs, so intermediate in structure
between the true plesiosaurs and land reptiles, were the actual
ancestors, but this is rather doubtful. It is probable that they were
only very closely akin to the real ancestors, since in some ways they
had become specialized too much, and, as we have already explained,
highly specialized characters or organs can never go back to their
earlier condition. The nothosaurs do prove beyond all possibility of
doubt that the plesiosaurs were at least the descendants of animals
closely allied to them, so closely, indeed, that it is doubtful whether
we could distinguish external differences were all of them actually
living at the present time.
[Illustration: FIG. 49.—_Lariosaurus balsami_.]
We have repeatedly seen that all aquatic animals have some or all the
bones of the limbs shortened, and it is of interest to observe that the
early plesiosaurs had longer forearm and foreleg bones than the later
ones, just as we have seen was the case with the early ichthyosaurs.
It would seem probable that all the early plesiosaurs had long necks,
though some of the late ones in Cretaceous times had relatively short
necks, shorter even than the known nothosaurs possessed.
The nothosaurs doubtless lived about the shores of the ancient seas,
spending much of their time in the water, leaving it perhaps when hard
pressed by their enemies, as do some modern reptiles, or to rear their
young. The teeth of the nothosaurs are long and slender in front,
shorter behind. The animals must therefore have been carnivorous in
habit, feeding probably upon such fishes as they could catch, and the
various invertebrates which live in shallow water. The structure of the
jaws and their attachments are quite as in the plesiosaurs, proving
that they could not have swallowed large objects; but the skull is
broader and flatter than that of most plesiosaurs, indicating habits
not unlike those of the modern alligators and crocodiles.
Some time we shall doubtless find remains of nothosaurs or nearly
allied animals elsewhere than in Europe, but probably not from later
deposits than the Triassic. So far as we now know, their geological
range and geographical distribution were much restricted; they
evidently wholly died out shortly after the plesiosaurs appeared.
CHAPTER VII
ANOMODONTIA
LYSTROSAURUS
Over a large area of South Africa, chiefly along the Orange River and
its tributaries, there is an extensive series of deposits many hundreds
of feet in thickness, usually called the Karoo beds, which, for more
than fifty years, have been widely famous among scientific men for the
many and remarkable vertebrate fossils which they have yielded. These
deposits seem to represent the whole of the vast interval of time from
the Carboniferous to the Jurassic, that is, the whole of the Permian
and Triassic, though not many fossils have been found in the lowermost
strata. Among the fossils of the lower strata are those of the strange
creatures described in the following pages as _Mesosaurus_. From the
deposits representing the Upper Permian and the Triassic the fossils
that have been obtained are both abundant and diverse. Unfortunately,
however, of the scores of forms that have been discovered few are known
completely, and still fewer are known sufficiently well to enable us to
picture the living animals.
From the Upper Permian Karoo rocks two orders of reptiles have been
recognized, the Cotylosauria, represented by more specialized forms
than those from the Lower Permian of North America; and the order or
group called by Broom the Therapsida. While the forms of this latter
group have certain definite structural relationships with each other,
they show so great a diversity among themselves that, when they shall
be better known, it will be found necessary perhaps to separate them
into several distinct orders.
At least five groups of the Therapsida are now recognized by Broom,
the Dromasauria, Dinocephalia, Anomodontia, Therocephalia, and
Theriodontia. Of all these the members of the last-mentioned group
have attracted the greatest interest among geologists and naturalists,
because of their intimate relationships to the mammals—so intimate,
indeed, that they seem almost to bridge over the interval between the
two classes. From higher Karoo beds primitive representatives of the
more crocodilian types have been discovered, forms which seem to be the
beginning of that order described on later pages as the Parasuchia.
It would lead us too far astray to mention even, let alone describe,
the many forms of reptiles that have been discovered in the Karoo beds;
nor indeed is it possible for anyone who has not attentively studied
their remains to get a very clear conception of many of them, so
incompletely have they been made known.
Doubtless from among all these diverse forms there have been not a few
which sought wider opportunities in the water, but, if so, we have
as yet very little knowledge of them. One form only, so far as the
writer is aware, has been credited with aquatic habits, a remarkable
reptile belonging to the group originally called by Sir Richard Owen,
the Anomodontia, a word meaning “lawless teeth,” and to the genus
_Lystrosaurus_, also described by the same noted paleontologist.
A restoration of the skeleton of _Lystrosaurus_ has recently been
published by Watson. This restoration the writer has reproduced in
the present pages, though he has taken the liberty of making some
minor changes, to accord better with what he believes must have been
the position of the shoulder-blades and the hind legs. And he would
also suggest that the tail in life did not turn down so much at its
extremity as depicted by Watson.
Both Broom and Watson believe that this animal was a powerful swimmer,
and thoroughly aquatic in habit. To the present writer, however, this
does not seem so evident. He is rather inclined to believe that the
creature was chiefly terrestrial in habit, living probably in marshy
regions, and perhaps seeking its food in shallow waters and in the mud.
Aside from the position of the nostrils, which it will be observed are
rather close to the eyes, a position so characteristic of many swimming
reptiles and mammals, there is but little indication of aquatic
adaptations elsewhere in the skeleton.
[Illustration: FIG. 50.—Skeleton of _Lystrosaurus_, as restored by
Watson, slightly modified.]
The skull is of most extraordinary form. The face is turned downward,
leaving the nostrils high up, in front of the eyes. The jaws were
doubtless covered with a horny shield, like that of the turtles, having
a cutting edge. There is a single pair of elongated canine teeth,
possibly a sexual character. The lower jaws are heavy and stout, and
Watson has said that the animal doubtless had the ability to open its
mouth very widely. The quadrate, the bone with which the lower jaws
articulate, is firmly fixed to the skull, and there is a single opening
on the side of the skull posteriorly, a character common to all the
Therapsida.
The vertebrae are stout, and they have stout spines. The tail is
remarkably short, stout, and stumpy; it could have been of no use
whatever in the water for propulsion or even for steering. The front
legs are short and stout; the forearm bones are short, suggesting
either swimming or digging habits, and the foot is short and broad.
The pelvis or hip bones are massive and were very firmly connected
with the backbone by the aid of six vertebrae, a very unusual number
in reptiles. The hind legs, as figured, show no indications whatever
of aquatic adaptation, unless possibly the very slight shortening of
the shin may be so construed. Watson believes that the bones of the
pelvis, indicate, aside from its strong union with the backbones,
strong swimming powers, but of this again the present writer is very
skeptical. The very strong ischia and the flatness of the pelvis are
both characters found among American Permian reptiles, which do not
show otherwise the slightest indications of water habits.
If then _Lystrosaurus_ was a powerful swimmer, as has been maintained,
it is very evident that the hind legs must have been used as the seals
or sea-otters use them, to propel and to guide; but they in nowise
resemble the legs of these swimming mammals. It seems altogether
more reasonable to suppose that _Lystrosaurus_ lived in the marshes,
feeding upon vegetable food obtained by aid of its strong jaws
and tusks—if the tusks were possessed by both sexes; and that the
position of the nostrils may be ascribed to causes like those which
brought about their recession in the Phytosauria, and not to strictly
aquatic habits. Possibly the animal had habits somewhat similar to
those of the hippopotamus; that it was an expert swimmer appears,
to the present writer, improbable. The powerful front legs may be
indicative of digging habits; the animal may have used them as an aid
to its powerful jaws and tusks in uprooting marsh and water plants.
However, _Lystrosaurus_, whatever may have been its habits, was a
curious reptile. It was about three feet in length, massive in all its
structure, and doubtless of slow and sluggish gait.
CHAPTER VIII
ICHTHYOSAURIA
Early in the eighteenth century a curious work in the Latin language
was published by a famous physician and naturalist—a professor in the
University of Altorf by the name of Scheuchzer—entitled _Querulae
Piscium_, or “Complaints of the Fishes.” The work was illustrated by
many expensively engraved figures of various fossil remains, including
one of some vertebrae which the author referred to as “the accursed
race destroyed by the flood”! The history of the finding of these
famous bones is recorded by Cuvier as follows:
Scheuchzer, while walking one day with his friend Langhans
in the vicinity of Altorf, a village and university of
Nuremburg, went to the vicinity of the gallows to make some
researches. Langhans, who had entered the inclosure of
the gallows, found a piece of limestone containing eight
dorsal vertebrae, of a black color and shining. Seized,
says Scheuchzer, with a panic terror, Langhans threw the
fragment of limestone beyond the wall of the inclosure, and
Scheuchzer, picking it up, preserved two of the vertebrae
which he believed to be human, and which he figured in his
book, _Piscium Querulae_.
About the same time another observer by the name of Baier discovered
other and similar vertebrae in the vicinity of Altorf which he
described and figured as those of a fish; and there was much earnest
contention between Scheuchzer and Baier, as also between their friends,
as to their supposed nature. Scheuchzer’s figure was often cited as
indubitable evidence of the destruction of mankind by a universal
flood, and it was not until nearly a century later that Cuvier showed
that the bones were really those of a marine reptile.
[Illustration: FIG. 51.—Restoration of _Ichthyosaurus_ with young, by
Charles R. Knight. (By permission of the American Museum of Natural
History.)]
It must be recollected, in extenuation of so extraordinary a blunder
on the part of so learned a man as was Scheuchzer, who, as a physician
and professor, one would think ought to have been able to distinguish
between vertebrae so different as are those of an ichthyosaur and a
man, that, during all of the eighteenth century and well into the
nineteenth, the belief was prevalent that all fossils were the relics
of animals and plants that had perished in the great biblical flood.
The science of geology was yet in its infancy, and there was no known
record, other than the biblical one, of any great inundation of the
earth’s surface which might account for the remains of sea-animals
in rocks remote from the seas. This belief, so long held by even
the wisest and most learned of scholars, so long welcomed by the
theologians as proof of the literal accuracy of the Bible, was one
of which Scheuchzer was quite convinced. His _Piscium Querulae_ was
largely a fantastic discussion of the supposed great world-catastrophe,
the Noachian Deluge, by which the fishes had been destroyed and long
imprisoned in the rocks through no sin of their own.
It was the same author who, in a subsequent work, described and figured
the fossil skeleton of a large salamander which he believed to be that
of a child destroyed in the flood, and which he called “Homo diluvii
testis.” In this specimen, which was discovered in the Tertiary rocks
of Oeningen, and which is still preserved among the historically as
well as scientifically famous fossils of the museum at Haarlem under
the name _Andrias Scheuchzeri_, Scheuchzer thought that he detected,
not only the skeleton of a child, but even its brain, liver, muscles,
etc.! His engraving of this “Witness of the Flood,” the “sorrowful
skeleton of an old sinner drowned in the Flood,” as also that of
the ichthyosaur vertebrae of Altorf, were afterward printed in the
famous “Copper Bible” as positive proof of the literal accuracy of the
biblical record.
Earlier than the publication of these figures by Baier and Scheuchzer,
at the very close of the seventeenth century, a Welsh naturalist
by the name of Lluyd, in a large and beautifully illustrated work,
figured—perhaps for the first time—remains of ichthyosaurs, which he
believed to be those of fishes. But Lluyd accounted for these and all
other fossil remains by a very different theory from that of Scheuchzer
and the theologians—a theory which at one time had many adherents among
scholars. He believed that the spawn of fishes or the eggs of other
creatures had been carried up from the seas and lands in moist vapors
into the clouds, whence they had descended in rain, penetrating the
earth to give origin to the fossils; in other words, he believed that
all fossils grew in the earth from germs of the living animals that
inhabited the land and seas. Certainly the old philosophers were hard
driven to make facts agree with theories!
Remains of ichthyosaurs, abundant as they were and are in many deposits
in England and Germany, attracted very little attention from the
naturalists of the eighteenth century after the time of Scheuchzer and
Baier, and nothing more was written about them until 1814, when Sir
Everard Home, an English comparative anatomist, in an extensive series
of large and finely illustrated, though rather discursive, works,
described and figured a number of good specimens. To the animal the
remains of which he rather vaguely and imperfectly described, he gave
in 1819 the name _Proteosaurus_, in the belief that it was allied to
the living _Proteus_, a salamander.
In 1821 the curator of mineralogy of the British Museum—Koenig by
name—after a more critical study of other remains, reached the
conclusion that these animals were intermediate between the fishes
and the reptiles, and gave to them the generic name _Ichthyosaurus_,
meaning fish-reptile, a name by which the chief forms have ever since
been known. Within the next few years many specimens of ichthyosaurs
were carefully and fully described by Conybeare, Cuvier, Owen, and
others of England, France, and Germany, making very clear all the
more important details of their skeletal structure. Blaineville, in
1835, thought that the ichthyosaurs constituted a distinct class of
vertebrates equivalent to all other reptiles, the birds, and the
mammals, which he called Ichthyosauria, the first appearance in
literature of the name by which the order is properly known. Five years
later, however, the famous English anatomist and paleontologist, the
late Sir Richard Owen, united the ichthyosaurs with the plesiosaurs as
a single order of reptiles, to which he gave the name Enaliosauria,
meaning sea-reptiles, a name which has long been current in textbooks
and general works on natural history. Moreover, Owen rather arbitrarily
changed Blaineville’s name Ichthyosauria to Ichthyopterygia, a name
which is often, though incorrectly, used to designate this order of
reptiles. These briefly given and perhaps dry details will make clear
how necessary is that rule of priority upon which naturalists so often
insist. When anyone may change the names of organisms at will there
will be no stability and no uniformity, because there is no one to
decide, and the prestige of a great name, like that of Owen, will carry
authority till someone else with greater authority appears. Whether or
not the name _Proteosaurus_, first given to any member of this order,
should take precedence over the later _Ichthyosaurus_ is still in
doubt, since Home gave no specific name to his species, and the very
particular purists of modern times have decided that a genus is not
named unless the species is also! We moderns sometimes are inclined to
impose very stringent conditions upon the older naturalists; let us
hope that we shall be treated more leniently by the future naturalists!
It will lead us too far astray to follow in detail the history of
the further discoveries of the ichthyosaurs during the early part
of the nineteenth century. It may briefly be said, only, that no
other group of extinct backboned animals excited more interest among
scientific men. One incident will suffice. More than sixty years ago,
an interesting deduction as to the living form of the ichthyosaurs was
made by Sir Richard Owen. He observed that many of the known skeletons,
as they were found in their rocky matrix, had a remarkable dislocation
of the vertebrae at a certain place near the end of the tail, and,
although such an appendage was quite unknown in other reptiles either
living or extinct, concluded that the living animals had a terminal,
horizontal, fleshy fin, very much like that of the whales and
sirenians. Sure enough, discoveries made forty years later disclosed
impressions in the rocks, not only of a large caudal fin, but also of
a dorsal fin, as well as outlines of the flesh-covered paddles. The
dislocation of the vertebrae at the place where the fleshy fin joined
the more slender tail was due to the action of currents of water, or
simple gravitation, upon a thin vertical fin and not, as Owen supposed,
to the twisting of the terminal part as it fell to a horizontal
position after partial decomposition of the soft parts.
About twenty-five years ago, Professor E. Fraas, the present director
of the Stuttgart Museum, described and figured very fully, not only
specimens showing impressions of the fins and paddles, but also others
of well-preserved and very complete skeletons of different species
of ichthyosaurs from the Jurassic deposits of Würtemberg, in which
remains of these animals occur in great profusion. His researches,
and those of several authors since then, supplementing and confirming
or disproving those of the many observers made during the preceding
seventy years, have finally determined almost perfectly the complete
structure of the more typical ichthyosaurs, enabling us to infer not a
little as to their habits and distribution in the old Jurassic oceans.
Within the past few years the discoveries of Professor J. C. Merriam of
California have likewise added greatly to our knowledge of the earlier
ichthyosaurs. It may now truthfully be said that of no group of extinct
reptiles do we have a more complete and satisfactory knowledge than of
the ichthyosaurs.
[Illustration: FIG. 52.—_Ichthyosaurus quadricissus._ Photograph of
specimen in Senckenberg museum, from Dr. Dreverman.]
Nevertheless we have yet very much more to learn about the order
Ichthyosauria as a whole—whence they came and how they originated;
what their nearest kin were among other reptiles; and especially, more
about the connecting links between them and terrestrial reptiles. They
have, as an order, so isolated a position, are so widely separated from
all other reptiles in structure, that they have long been a puzzle to
paleontologists. Like the whales and other cetaceans among mammals,
we know the ichthyosaurs well in the plenitude of their power and the
fulness of their development, but have yet only an imperfect knowledge
of their earlier history, and none whatever of their earliest. However,
as will be seen farther on, the recent discoveries by Merriam have
shed much light on some of the stages of their evolution. So nearly
perfectly were all the later ichthyosaurs adapted to their life in the
water that it was believed by nearly all paleontologists until about
a score of years ago that they had descended directly from fishes.
But this belief has been quite abandoned by all, not only because the
recent discoveries of the earlier ichthyosaurs have demonstrated a
positive increase in the aquatic adaptations of the later forms, but
also because a double origin of any type of animal life is quite out of
accord with all known facts and principles of paleontology. It is quite
possible for animals, in becoming adapted to peculiar environmental
and food conditions, to acquire certain resemblances to other animals,
but quite impossible for them to acquire their actual structure. The
ichthyosaurs are true reptiles, and all reptiles must have had a common
origin.
[Illustration: FIG. 53.—_Baptanodon (Ophthalmosaurus)._ Skull from
the side, from above, and from below (after Gilmore): _ang_, angular;
_bs_, basisphenoid; _d_, dentary; _fr_, frontal; _j_, jugal; _la_,
lacrimal; _mx_, maxilla; _na_, nasal; _oc_, occipital condyle; _p_,
palatine; _pa_, parietal; _pm_, premaxilla; _po_, postorbital; _ps_,
parasphenoid; _pt_, pterygoid; _pf_, prefrontal; _sa_, surangular;
_sp_, splenial; _sq_, squamosal; _st_, supratemporal; _q_, quadrate;
_qj_, quadratojugal.]
[Illustration: FIG. 54.—Occiput of _Baptanodon (Ophthalmosaurus)_:
_pa_, parietal; _soc_, supraoccipital; _sq_, squamosal; _exoc_,
exoccipital; _op.o_, paroccipital; _sta_, stapes; _st_, supratemporal;
_qu_, quadrate; _qj_, quadratojugal; _pt_, pterygoid; _bs_,
basisphenoid; _sag_, surangular; _ag_, angular; _art_, articular;
_pra_, prearticular. (After Gilmore.)]
We are sometimes in doubt, however, as to whether characters resembling
those of other animals are really acquired as adaptations to peculiar
environments, that is, parallel, convergent, or homo-plastic
characters, or whether they are due to heredity from remote ancestors.
The reptilian characters of the ichthyosaurs, however, are so emphatic
that they can only be ascribed to heredity. Ichthyosaurs are as truly
reptilian as crocodiles or snakes, notwithstanding their fish-like form
and habits. The ichthyosaur ancestors were once truly land reptiles—of
that we are as sure as we well can be. Some have thought that those
ancestors were the primitive Rhynchocephalia, but most are now
convinced that they were among the most primitive of reptiles, a branch
probably from the cotylosaurs or cotylosaurian ancestors. Probably of
all the extinct forms that we know the Proganosauria come the nearest;
indeed it is not impossible that they may have been the actual forbears
of the ichthyosaurs.
The ichthyosaurs varied in length from two to thirty feet, but the
different species, especially all the later ones, resembled each other
pretty closely in shape; the beak was more slender in some than in
others, and the shapes of the fins and paddles varied not a little, as
we shall see. The jaws were long and slender, provided with numerous
rather small but sharp and recurved teeth, especially well fitted for
the seizure and retention of slippery prey. The teeth were inserted,
not in separate sockets, as are those of the crocodiles and many other
reptiles, but in long, deep grooves, and were easily lost, indeed so
easily lost that one late American form was originally described as
edentulous, and it was not till a number of years had elapsed that
the teeth were found. The nostrils were small, and situated far back
on the sides of the face, near the eyes. The eyes were very large,
not only in proportion to the size of the skull, but, in the largest
species, actually attaining in some, perhaps, the size of a human head.
The eyeball was surrounded in front by an extraordinarily large and
strong ring of ossifications in the sclerotic membrane, giving not only
protection to the eye under the varying pressure of the water, but
also greater control over vision. The neck was very short, so short,
in fact, that no construction was visible in the living animal between
the head and body; it was capable of only slight movement. The trunk
was elongated and relatively slender, sometimes with more than fifty
vertebrae in it. The tail also was long and flattened, ending in all
the later species in a large fleshy fin, resembling the caudal fin of
many fishes in shape and doubtless also in function. There was also a
large dorsal fin, supported by hardened or calcified sinews, in shape
like the dorsal fin of most fishes and many cetaceans. This character
is absolutely unique among reptiles, so far as is known, and was one of
the extreme specializations of water life. The hind limbs were smaller,
often much smaller than the fore ones, and both were quite fin-like in
life, or rather flipper-like, though not at all fin-like in structure.
The skin was smooth and bare. In brief, to quote Fraas’s words:.
The general aspect of the ichthyosaurs was very
dolphin-like. The body was everywhere naked and probably
dark in color. The head was produced in front into a
long, slender snout, and was closely joined to the body
posteriorly without indications of a neck. The body itself
was cylindrical, expanded in front by the large thorax and
abdomen, but rapidly diminishing into the long, slender, and
strong tail. Close behind the head were the front paddles,
which in some species were broad and shovel-like, in others
elongated and pointed. The hind paddles were smaller than
the front ones, sometimes greatly reduced in size, their
function replaced by that of the very broad tail.
From the foregoing descriptions and the restoration shown in Fig. 51,
we see how very fish-like, or rather dolphin-like, these animals were
in the external form—so fish-like that the name _Ichthyosaurus_ is not
misleading, though Koenig gave it in the mistaken belief that they were
really allied to the fishes. When to these external features certain
other fish-like details of the skeleton are added, we do not wonder
that the early observers were so long in doubt about them. A more
careful examination of the skeleton will, however, disclose so many
truly reptilian characters that their external appearance and habits
lose all significance.
The vertebrae are deeply biconcave and fish-like, it is true, but a
consideration of the reasons therefor will convince us that any other
kind of vertebrae would be more remarkable. At the time when the
ichthyosaurs must have originated, at the time when the first known
ichthyosaurs appeared in geological history indeed, all reptiles had
biconcave vertebrae, and for the most part at least deeply biconcave
ones. The vertebrae remained fish-like throughout all their history,
perpetuating their type until most other reptiles had developed a
firmer one, because such vertebrae were best adapted for the quick,
pliant movements of the spinal column so necessary for the well-being
of the animals in the water. In the modern dolphins, animals in shape,
size, and habits most wonderfully allied to what these old reptiles
must have been, the small, flat-ended vertebrae are widely separated by
disks of flexible cartilage.
Not only were their vertebrae fish-like in form, but there are other
characters in the spinal column of a primitive or generalized nature.
As in all aquatic animals, the articulating processes between the
vertebrae are either weak or wanting in the posterior part of the
column. And they were not only small, but were situated, in many,
high up, very remarkably resembling the peculiar arrangement of the
articulations in the dolphins.
There is no sacrum, that is, there were no united vertebrae posteriorly
for the attachment and support of the pelvis, as no such support was
needed. In only one other group of aquatic reptiles was the sacrum
lost, though it has wholly disappeared in the cetaceans and sirenians
among mammals. The chevron bones of the tail, usually bony arches on
the under side of the tail for the protection of the blood-vessels, in
crawling reptiles, were very imperfectly developed in the later forms,
though normal in shape in the early ones. The ribs are numerous, long,
and slender, very much resembling those of the fish-eating dolphins.
They usually had, however, two attachments to the body of the vertebra
and none to the arch, differing in this respect from all other animals.
Of the shoulder bones, the scapula or shoulder-blade, as usual among
water animals, is short and broad. In the place of a sternum the
coracoids joined each other broadly in the middle, just as they did
in the oldest known land reptiles. And there were clavicles and an
interclavicle. Below the abdomen behind were numerous slender bones
called ventral ribs. The pelvis is very weak, and was suspended
below the spinal column in the fleshy walls of the abdomen. The hind
legs were so small that little support was necessary for them, and,
because they were not used either for the support of the body or for
propulsion, they did not require a firm union with the skeleton.
Doubtless had the ichthyosaurs continued to the present time, they
would have lost entirely the hind legs, as have the cetaceans.
[Illustration: FIG. 55.—Pectoral girdle of _Baptanodon
(Ophthalmosaurus)_, an American Upper Jurassic ichthyosaur. (After
Gilmore.)]
It is in the limbs that most extraordinary differences from all other
animals are seen. So great are these differences that it has been a
puzzle to naturalists to understand how they could have arisen. In no
other animals above the fishes, that is, in no other reptiles, in no
amphibians, birds, or mammals, are there ever more than five fingers or
five toes, the number with which air-breathing animals began. Fingers
and toes may be lost and often are lost in all groups of life, until
a single one in each limb may remain, as in the domestic horse. An
increase of fingers and toes, however, seems to be an impossibility
in evolution, and doubtless of real fingers and toes it is an equal
impossibility. All naturalists are now agreed that a specialized
character can never revert to a generalized condition, or rather to a
generalized structure, that an organ once functionally lost can never
be regained by descendants. A character once lost is lost forever;
horses of the future can never have more than one finger or one toe in
each limb.
[Illustration: FIG. 56.—Front paddle of _Ophthalmosaurus_ (after
Andrews): _h_, humerus; _r_, radius; _u_, ulna; _p_, pisiform; _re_,
radiale; _int_, intermedium; _ue_, ulnare.]
[Illustration: FIG. 57.—Front paddle of _Merriamia_, a Triassic
ichthyosaur. (After Merriam.) Explanations as in Fig. 56.]
And there was an increase in the ichthyosaurs, in some not only of
the number of digits in each limb, but in all of the number of bones
in each digit, a character found also in the unrelated mosasaurs and
plesiosaurs. This increase in finger and toe bones, or hyperphalangy
as it is called, is one of the most peculiar of all the adaptations to
water life, changing the feet and hands from the ordinary walking type
to the fish-like swimming type. The bones beyond the humerus and femur
in the ichthyosaurs were so increased in number and so changed in form
and relations that they bear little resemblance to the corresponding
bones of other reptiles. They are merely polygonal platelets of bone,
articulating on all sides and fitting closely together, permitting
flexibility, but not much else.
It is now believed that the increase, not only of additional
digits, sometimes to as many as ten in each hand and foot, but
of the finger and toe bones as well, was the result of a sort of
vegetative reproduction. The margins and ends of the flippers were
doubtless hardened by cartilage or fibrous material, and because of
the action of the limbs this cartilagenous material broke up into
nodules each of which took on ossification finally. Among the whales,
where hyperphalangy also occurs, though to a less extent, it has
been thought that the increase in number has been due simply to the
ossification of the parts of each bone normally present, that is, to
the epiphyses, which became separated from the shaft of each bone. But
this explanation will hardly suffice for the fingers and toes of the
plesiosaurs and ichthyosaurs, for there are altogether too many such
ossifications; and besides, the bones in these animals, as in most
reptiles, did not have epiphyses, or terminal separate ossifications of
the bones of the skeleton.
It will be observed from the figures that the arm and thigh bones
of _Ichthyosaurus_ are very much shortened—a striking adaptation to
water life, so conspicuously seen in the modern whales and dolphins
as well as in the mosasaurs, thalattosaurs, etc. So characteristic
indeed is this shortening that, were every other bone of the skeleton
of an ichthyosaur unknown save the humerus or femur, it would be quite
certain from these alone that the animal was thoroughly aquatic in
habit.
About sixty years ago a rather aberrant form of ichthyosaur, now
known as _Mixosaurus_, was discovered in rocks of Triassic age, that
is, of much greater age than any ichthyosaurs previously found, in
which not only the forearm but also the lower leg bones were longer,
resembling more the corresponding bones of land animals. It was from
the examination of specimens in 1887 of these mixosaurs that the
late Professor Baur became convinced that the ichthyosaurs were the
descendants of land reptiles, and not directly of the fishes as they
were universally thought to have been at that time. As Professor Baur
very pertinently said, if the ichthyosaurs were descended from the
fishes directly, the earliest forms should be more nearly like the
fishes than the later ones, whereas just the opposite was the real
fact. The arguments which he gave in support of his contention were so
convincing that they found immediate acceptance among all naturalists.
Fortunately within the past fifteen years many other remains of early
ichthyosaurs from the Triassic rocks of California have been brought
to light by Professor Merriam, remains which throw a flood of light
upon the early, though not the earliest, history of these strange
reptiles. He has recognized among the forms he has discovered, not
only new species, but several new genera, and perhaps new families
of ichthyosaurs. His studies have demonstrated so well the stages of
evolution between the early ichthyosaurs and the later ones in their
progressive adaptation to water life that it will be of interest to
summarize them here.
In the early ichthyosaurs locomotion was largely by the aid of the
limbs; in the later ones almost exclusively by the aid of the caudal
fin. In the former the paddles were larger and the bones longer, more
like those of land animals; in the latter they were relatively smaller
and shorter, and more fin-like. In the digits of the early forms the
finger and toe bones were more elongated and fewer in number. The hind
limbs were nearly as large as the front ones in the Triassic, often
very much smaller in the later ichthyosaurs; and the increased number
of digits occurs only in the later forms.
In the Triassic ichthyosaurs, all classed in the family Mixosauridae,
the pelvis was larger and more firmly connected with the body than in
the later forms.
The skull of the early forms was relatively shorter, as compared with
the trunk, the jaws shorter as compared with the head, the eyes were
relatively small, the teeth in some less numerous, and set in distinct
sockets like those of land reptiles; the vertebrae were relatively
longer and less fish-like, and their articulations more like those of
land reptiles.
The distal part of the tail was not bent downward so sharply, that
is, the terminal fin was smaller, or the tail may have been simply
flattened near its end and not really fin-like. The scapula was longer
and less fan-like in shape.
And all these are remarkable evidences of an increased adaptation
to water life in the more recent ichthyosaurs over the older ones.
Were someone now so fortunate as to find ichthyosaurs in late Permian
rocks, we should doubtless have the nearly complete chain between the
most highly specialized type of water reptiles and their terrestrial
ancestors.
[Illustration: FIG. 58.—Caudal fin of _Ichthyosaurus_, after Baur (left
figure); caudal fin of _Mixosaurus_, after Wiman (right figure).]
From the structure of the skeleton alone the early observers were
justified in inferring much concerning the shape and habits of the
living ichthyosaurs. Later discoveries have added so many definite
facts that, at the present time, we know more about their habits
than we do of any other extinct reptiles. In various places in
England and Germany, especially in Würtemberg, the remains of
ichthyosaurs are found in extraordinary abundance and perfection, not
only whole skeletons lying in the positions which they had assumed
after the decomposition of their bodies, but also often the actual
remains, carbonized, of the skin, muscles, and ligaments, as well as
delicate impressions of external parts. Many of these skeletons are
obtained from the numerous stone quarries, where they are a sort of
“by-product,” the sums received for them adding not a little to the
income of the quarrymen. So many are obtained in this and other ways
that specimens of ichthyosaurs are perhaps more frequently seen in the
museums of the world than those of any other extinct backboned animal.
Fairly complete skeletons may now be purchased of dealers in such
things for from fifty to seventy-five dollars. As may be supposed, the
best and most complete collections of these fossil remains are those of
the British Museum in London and the museum in Stuttgart. From a study
of those of the last-mentioned museum Professor Fraas has learned many
interesting facts and reached many interesting conclusions regarding
the life-habits of the ichthyosaurs. In the accompanying figure (Fig.
59) is shown a photographic reproduction of a very complete specimen,
in which not only is the outline of the whole body shown, but also much
of the carbonized remains of the muscles and skin has been detected.
[Illustration: FIG. 59.—_Ichthyosaurus quadricissus._ (From a
photograph from B. Hauff, Paleontologisches Atelier, Holzmaden.)]
The attachment of the paddles to the body was broad antero-posteriorly,
proving conclusively that they could not have been much used in
propulsion, either in the water or upon land, since such use would
require a fore-and-aft movement, and a consequent twisting or rotation
of the whole arm or leg, which, because of the broad attachment, must
have been very difficult, if not impossible.
Microscopic examination of the remains of skin preserved disclosed
an abundance of dark pigment, indicating, Professor Fraas believes,
that the skin was dark colored above. Doubtless, also, the under
side, as in nearly all swimming animals of the present time, was of
a lighter color, because such coloration rendered the animals much
less conspicuous in the water when seen either from above or below.
That the skin was bare is proved by many impressions or molds of it
that have been discovered in the rocks, in which many fine creases are
seen, but nothing suggesting scales or bony plates, save on the front
edge of the paddles, where impressions of overlapping scales have been
observed. This is an interesting fact, bearing witness that their land
ancestors had been covered everywhere with scales, much like those
of existing lizards and other reptiles. Scales or bony plates were
not only useless to the ichthyosaurs in the water, since they could
afford no protection, but would have been detrimental in increasing the
resistance in swimming.
That the ichthyosaurs were predaceous animals is of course evident
from their teeth, adapted for the seizure and retention of slippery
prey, but not for tearing or comminuting. The fossilized remains of
food found between the ribs of some specimens, in the place where the
stomach was, together with fossil excrement, called coprolites, usually
attributed to these animals, prove that they fed largely upon fishes,
squids, belemnites, and probably other invertebrates. One ichthyosaur
specimen preserved in the Stuttgart Museum has preserved in its stomach
contents a mass composed of the remains of more than two hundred
belemnites.
Most interesting of all is the fact that, not very rarely, embryonic
skeletons of ichthyosaurs have been found associated with the remains
of adult animals, in such positions that they must have been inclosed
within the body cavity at the death of the animals. As many as seven
such embryonic skeletons have been observed with a single specimen. At
first it was supposed that these skeletons were of small ichthyosaurs
which had been swallowed whole as food, since it is not at all likely
that these predaceous reptiles were discriminative in their choice
of food when hungry. It is not improbable that in some cases this is
the true explanation of the smaller skeletons within the larger ones,
but it cannot be true of all, since wherever the small skeletons are
identifiable they have been found to belong to the same species as the
adult, and it would be absurd to suppose an ichthyosaur bent upon its
prey would be at all likely to select as many as seven young animals,
all of the same size and all of its own species. Furthermore, some
of these young skeletons have been found in such positions as would
indicate that they were inclosed within their egg-covering at the time
of their death. Some of these embryos measure as much as twenty inches
in length.
Because the ichthyosaurs were born alive, and because so many of their
skeletons are found with their various parts in orderly relation
to each other, it is inferred with much probability that they were
inhabitants, in large part at least, if not exclusively, of the open
and deeper oceans. Had they been oviparous they must necessarily have
laid their eggs upon the beaches, since no reptiles of the present
time lay eggs in the water, and we have no other indications that
the reptiles of the past have ever done so. And such habits would
necessitate the periodical return to land. Had they been denizens of
shallow waters, like the mosasaurs and plesiosaurs for the most part,
their skeletons must surely have been disturbed by the currents and
tides, as also by predaceous fishes, breaking up or displacing them or
carrying away their bones. In shallow waters, also, the decomposing
bodies would have been more liable to despoliation by the many
scavengers of the seas.
The ichthyosaurs must have been quite helpless upon land, their limbs
being of little more use for locomotion than are the fins of fishes.
Breathing air as they did, they were of course not suffocated when
exposed, unless, as is the case with the whales, the feeble attachment
of the ribs prevented the action of the respiratory muscles. If
accidentally thrown upon the beaches, they doubtless were able to
return to their home element more easily than the fishes can, by
flopping, wriggling, and turning. As we have seen, the food consisted
in part, perhaps the larger part, of small invertebrates, and because
the bones of the lower jaws were closely united, permitting little or
none of that expansion so characteristic of the snakes, all their prey
must have been of relatively small size. In habit the ichthyosaurs were
doubtless, like the dolphins and gavials, inoffensive and harmless,
so far as animals of larger size were concerned. The abundance of
their remains often found in restricted localities, while deposits of
like age and character not far distant may be almost free from them,
suggests that in all probability the ichthyosaurs, or the later ones at
least, were more or less gregarious in habit as are the sea-mammals.
They probably lived in schools, as do the porpoises, each species
keeping to its restricted locality and not wandering far.
The ichthyosaurs began their existence, so far as we now know, about
the middle of Triassic times and continued to near the middle of Upper
Cretaceous, when they disappeared forever from geological history.
As we have seen, however, the earliest forms that we know were true
ichthyosaurs in all respects, though more primitive than the later
ones, indicating a long previous existence of which we yet have no
knowledge. Their remains have been found widely distributed in Triassic
rocks of Europe, Spitsbergen, Australia, and North America. During the
Jurassic period they lived in great numbers and variety throughout
the region that is now Europe. In North America the only marine rocks
of this period that we know of have yielded numerous remains. These
American ichthyosaurs were, however, among the most specialized of all
ichthyosaurs—the culmination of their development. They were originally
named _Sauranodon_ in the belief that they were toothless, but in
recent years their teeth, small and numerous, have been discovered. And
the genus seems also to be identical with one previously named from the
Jurassic of Europe called _Ophthalmosaurus_. The last known remains
of ichthyosaurs have recently been found in the Benton Cretaceous of
Wyoming. Scanty remains of ichthyosaurs are also known from Australia
and New Zealand. Why the ichthyosaurs should have gone out of existence
before the plesiosaurs and mosasaurs did, one cannot say; possibly
their stock had grown old and feeble.
CHAPTER IX
PROGANOSAURIA
MESOSAURUS
There is some doubt whether those little creatures of Paleozoic times,
to which some years ago the late Professor Baur gave the ordinal
name Proganosauria, are really entitled to so much distinction among
reptiles. The question of their rank has been much disputed for the
past twenty years without any positive conclusion. Nor were they wholly
aquatic in habit, though they did possess many aquatic adaptations.
That they were skilful and fleet swimmers, and capable of rapid
evolutions in the water is quite certain, and, as the oldest known
water reptiles, they are of more than passing interest.
[Illustration: FIG. 60.—_Mesosaurus_; life restoration, after McGregor,
the posture of hind leg slightly modified.]
But two genera and three or four species of the group are known, and
of them, even, our knowledge in some respects is not as complete as
one could desire. The first description of any member of the group
was by the late Professor Gervais of Paris in 1867. He had only the
anterior part of a single skeleton, from the Karoo beds of South
Africa, to which he gave the name _Mesosaurus_, a rather meaningless
term signifying “middle” or “intermediate” saurian. Nothing more
was learned about any form till 1885, when the late Professor Cope
described a specimen from the supposed Carboniferous of Brazil, which
he believed to be closely related to _Mesosaurus_, though he had
only a very imperfect specimen. He called it _Stereosternum_, also a
meaningless term, since none of the animals has a “solid sternum,”
nor any sternum at all, in fact! A few years later, in 1888 and 1892,
the late Professor Seeley of England studied a number of specimens of
_Mesosaurus_, adding not a little to our knowledge of the animals. More
recently Dr. Woodward of England and Professor Osborn of America have
given us still further information concerning them, and within the past
few years Dr. McGregor of Columbia University has figured and described
excellent specimens of a new species from Brazil, which he calls
_Mesosaurus brasiliensis_. Not only were Dr. McGregor’s discoveries
of great interest as settling many doubtful points in their structure,
but they were still more so from the fact that he found his species
so nearly like that from Africa that he placed it in the same genus.
Since the proganosaurs were purely fresh-water or terrestrial animals,
one can only wonder how they crossed from Africa to America, or, what
is more probable, how they migrated from America to Africa, across the
broad Atlantic Ocean, so long ago. The geologists tell us that the
Atlantic and Pacific, in the main, have always been oceans since the
beginning of terrestrial life upon the earth. Possibly the tribe of
proganosaurs migrated by the very circuitous route of Europe and North
America, or Asia and the Northwest; but that is very improbable, since
nothing whatever resembling them has ever been found in the Northern
Hemisphere, and it is quite certain that in the many thousands of
years it must have taken them to travel from southern Africa to South
America many of the reptiles must have perished on the way and left
their remains in the rocks. The only conclusion that seems probable is
that there was a direct land communication in those olden times between
Africa, or at least India, and South America across what is now the
Atlantic Ocean. Of course this route will be very difficult to prove,
since we can never get to the bottom of the ocean to hunt for fossil
proganosaurs. Were this peculiar distribution of the proganosaurs an
isolated example, one might perhaps ascribe our lack of knowledge of
any fossil proganosaurs in the Northern Hemisphere to the meagerness of
the fossil records, but there are many other examples of similar import
among other early animals.
[Illustration: FIG. 61.—_Mesosaurus_; restoration of skeleton. (After
McGregor)]
The age of the South American proganosaurs is now believed to be lower
or lowermost Permian, like that of the African _Mesosaurus_; possibly,
however, the age first described to _Stereosternum_ (Carboniferous) may
be correct.
The known skeletons are all small, none exceeding a few feet in length.
The skull, as shown in the figure by Dr. McGregor, is elongate, and
its teeth are extraordinarily so, and very slender. The external
nostrils are situated close to the eyes; and no sclerotic bones have
been discovered. There are small teeth in the bones of the palate.
The neck is elongate, composed of ten or twelve vertebrae. The trunk
also is long and slender, and the tail is not only long, but also
much flattened or compressed. All these are very characteristic of
water life. The limbs, however, show a much less complete adaptation
for swimming—not much more so in fact than do those of the living
Crocodilia. The upper arm and the thigh bones are relatively long,
while those of the forearm and the leg are shorter than among
terrestrial reptiles, the first indication of swimming habits to appear
in crawling animals. The digits are not much elongated, and they
have no additional finger bones, save perhaps in a lately discovered
form in Africa, in which Dr. Broom reports supernumerary bones in
the fifth or “little” toe.[3] The fingers and toes have only blunt
terminal bones, that is, they were not distinctly clawed, and they
were probably connected with each other by a membrane, as in a frog’s
foot. This webbing of the feet is probable, not only because of the
positions in which the bones have been found, but also because of the
great length of the “little” toe, which is the longest in the foot, a
character quite abnormal for a land reptile and quite characteristic
of certain aquatic mammals, like the seals and sea-otters. There is
a strong sacrum of two vertebrae, however, the pelvis and hind legs
being connected with the spinal column firmly, clearly proving that,
like the crocodiles, the proganosaurs had by no means lost their land
proclivities.
[3] An additional phalange has also been observed in the fifth toe of a
South American species.
Their vertebrae, as would be expected in such old reptiles, are quite
primitive in structure, that is, they are deeply concave in each end,
probably being perforated for the remains of the notochord. The pelvis
also is of the old-fashioned type, that is, without an opening or
vacuity between the bones below. The shoulder bones are old fashioned
too. The shoulder-blade, especially, shows a decided adaptation to
water life in its short, fan-like shape, very much like those of the
mosasaurs, ichthyosaurs, whales, etc. Just why swimming animals should
have short and broad shoulder-blades has not yet been explained, but
doubtless they afforded better attachment for those muscles used more
especially in swimming. The ribs are remarkably flat and heavy, and
were not very firmly attached to the vertebrae. Heavy ribs are unusual
among free swimming animals, but do occur in the modern sirenians,
which live on the bottoms of shallow bays, etc., feeding upon plants.
We may perhaps infer from this peculiar structure of the ribs that
the proganosaurs lived more on the bottoms of shallow waters, feeding
upon such fishes or invertebrates as they could capture, coming to the
surface to breathe from time to time. Possibly they sought the shores
for safety from their enemies, as do the Galapagos lizards, figured
on p. 142; and doubtless they laid and hatched their eggs on land.
A character which suggests that the proganosaurs lived only in the
shallow waters is the elongated neck, reminding one of those two other
groups of swimming reptiles, the dolichosaur lizards and the nothosaurs
of the Sauropterygia, the only known reptiles besides the plesiosaurs
having an abnormal number of neck bones. Still more suggestive of
shallow, fresh-water habits is the absence of eye bones, as in the
modern crocodiles.
The long snout, with the long and slender teeth, and the position
of the external nostrils far back near the eyes, together with the
flattened and long tail and the webbed feet, are sufficient proof of
expert swimming habits. The legs still functioned more or less for the
support and propulsion of the body on the land, and they probably were
only of slight service in the water. The alligator swims sinuously
with its front legs collapsed and extended by the side of the body;
its hind legs are used more as propellers, with the knee flexed and
the feet turned outward and expanded. The legs of the proganosaurs
doubtless were used in the same way, as shown in the restoration, which
has been modified from the original of Dr. McGregor in accordance with
this probable use of the legs.
There seems to be an incongruity between the posterior nostrils and the
heavy flat ribs, the former suggesting free swimming and diving habits,
the latter shallow water and bottom habits. Possibly the position of
the nostrils has been the result of the great elongation of the face in
front of the nostrils; and we know that their posterior position in the
phytosaurs (Figs. 95 and 96) has not been due to swimming habits only.
Nothing has been discovered to indicate the nature of the external
covering of the body. Possibly, even probably, the skin was more or
less covered by horny scales or plates, though it may have been quite
bare, as in the salamanders.
To which other reptiles the proganosaurs are nearest related has long
been a subject of dispute, and still is. The more probable view,
however, is that they were a very early branch of the most primitive
stock of reptiles, the Cotylosauria, one that soon perished, leaving
no descendants, unless possibly the ichthyosaurs were their progeny.
Some writers have thought that they were the early ancestral stock of
the plesiosaurs, and they are often classified with the Sauropterygia.
Still others have believed that they were an early side-branch of the
great group of Rhynchocephalia. And this doubt has been chiefly due
to our imperfect knowledge of the bones of the cranium. As has been
explained, very much stress in the classification of reptiles has been
laid by students on the possession of one, two, or no openings on the
side of the skull back of the eyes. And this part of the skull of the
Proganosauria has not yet been satisfactorily made out. Dr. McGregor
thought that there are two openings in the temporal region, allying the
group with the Rhynchocephalia. Dr. Huene is more positive that there
is but one, like that of the ichthyosaurs. In this state of indecision,
the proganosaurs may be dignified by giving them an ordinal position by
themselves.
CHAPTER X
PROTOROSAURIA
PROTOROSAURUS
The genus _Protorosaurus_ is of peculiar interest, as one of the first,
if not the first, known fossil reptiles, described by Spener as long
ago as 1710 as a crocodile, from fragmentary remains found in 1706 in
the Permian deposits of Thuringia. Numerous other skeletons or parts
of skeletons attracted the attention of naturalists of the eighteenth
century, but were very imperfectly described. No name was given to the
animal represented by the various specimens until 1840, when Herman
von Meyer restudied all the known material and described it under the
name _Protorosaurus speneri_. The position of the genus among reptiles
always has been and yet is uncertain, for the reason that the structure
of the skull, and especially the structure of the temporal region, has
never been satisfactorily determined. Seeley, in 1887, described more
fully the original specimen of Spener, now preserved in the museum of
the College of Surgeons of London, and because of certain peculiarities
which it showed proposed for its reception the order Protorosauria.
He thought that he detected an upper temporal vacuity, like that of
lizards, but was very uncertain about the structure of the lower
part of the temporal region. The writer, who has examined this type
specimen, must admit that the structure of the region here is very
doubtful. Under the general assumption, however, that all old reptiles
must be related to _Sphenodon_, the Protorosauria have generally been
classified as a suborder of the Rhynchocephalia. It is merely another
instance of the proclivity we all have to propose hypotheses, and then,
speedily forgetting that they are hypotheses, to accept them as facts.
_Protorosaurus_ was long supposed to be an aquatic reptile, but we now
know that it was a strictly terrestrial one, probably with climbing
habits; and the genus concerns us only by reason of its possible
relationships to distinctly aquatic reptiles of a later age.
[Illustration: FIG. 62.—Life restoration of _Araeoscelis_.]
A few years ago the writer described a very slender, lizard-like
reptile about two feet in length from the Permian of Texas under
the name _Araeoscelis_, so named because of its slender legs. The
structure of both the skull and the skeleton of this reptile is now
quite satisfactorily known, so well known indeed that the accompanying
restoration (Fig. 62) has little that is conjectural about it, at
least so far as the form is concerned. The skull has a single, upper
temporal opening, quite like that of lizards, but the quadrate is
not loose below. And this is really what we should expect in the
ancestral lizards; and everything else of the skeleton, except perhaps
one character, is what would be expected. That one character is the
elongation of the cervical vertebrae, which are about twice the length
of the dorsal vertebrae following them. The cervical ribs are very
slender bones, articulating by a single head with the centrum only. In
these and other characters, so far as they are known, _Araeoscelis_
seems to agree with _Protorosaurus_, and both have very hollow bones.
[Illustration: FIG. 63.—Skeleton of _Pleurosaurus_. (After Lortet)]
PLEUROSAURUS
We may for the present be justified in maintaining the order
Protorosauria for those reptiles having a single, typically upper
temporal opening on each side, with a fixed quadrate, not including
the ichthyosaurs. It is not improbable, however, that when more is
known of the ancestors of the lizards, the whole group will find its
most natural place among the Squamata. This definition will include
a peculiar aquatic reptile that has been known for many years, but
which has been wrongly classed in the same family as _Sphenodon_, on
the purely gratuitous assumption that it has two temporal openings
on each side; we now know that it has but one. This reptile, known
scientifically as _Pleurosaurus_, was described originally by H. von
Meyer in 1843, but we are indebted to M. Lortet for a more precise
knowledge of the animal, and for the figure (Fig. 63) which is here
given of the skeleton. Not a few excellent skeletons are preserved in
the museums at Lyons and Munich. The specimen here figured, as actually
preserved, measures about three feet in length; a part of the tail is
missing, which is known from other specimens to have been remarkably
long.
The figures show clearly some of the remarkable aquatic adaptations of
the animal, especially the short neck, the very long and narrow body,
and the extraordinarily long and flattened tail. The head is elongate
triangular in shape, resembling very much that of the mosasaurs; and
the external nostrils are likewise situated remotely from the end
of the snout, as in the mosasaurs. The extremity of the snout has a
beak-like projection. The teeth are much longer, more pointed, and more
recurved than is the case with most land reptiles, indicating their use
for the capture and retention of slippery, quick-moving prey.
The single-headed ribs are short, proving that the body was slender
and doubtless cylindrical, more like that of a snake. The tail was
not only enormously elongated, but it was also compressed into a flat
and effective propelling organ in the water. This flattening of the
tail is apparent from the skeleton, with its elongated chevrons below
and spines above, and it is also proved by the fortunate preservation
of the extremity of the tail of one specimen, showing not only the
impressions of the scales in the matrix, but also the outlines that the
soft parts had in life. To quote from Lortet, in translation: “The tail
was covered wholly with small scales, regularly hexagonal in shape,
shining and nacreous, larger on the under side than above. The upper
border of the tail was surmounted by a broad crest, extending to its
extremity, and composed of large, oval scales.” The body doubtless was
wholly covered with scales, though it is not probable that the caudal
crest continued along the back.
[Illustration: FIG. 64.—Life restoration of _Pleurosaurus_.]
The limbs begin to show an aquatic adaptation, though not very
pronounced. They are much shorter and smaller than are those of
land-crawling reptiles; and the bones of the second series, that is,
the radius and ulna, tibia and fibula, are relatively short, the
beginning of adaptation to water habits. It is very probable that the
feet were webbed, though the fifth digit, as usual, is shorter than
the fourth. Doubtless on land the creature moved about in a serpentine
way, for it could not have progressed very rapidly by the aid of its
legs alone. The hind legs are longer than the front legs, and they were
connected firmly with the body by means of a sacrum. The number of
vertebrae in the neck is only five. The number of dorsal vertebrae is
forty-three, a larger number than is known in any other air-breathing
vertebrate with legs.
Upon the whole, these lizard-like, almost snake-like pleurosaurians
present some very curious adaptations to water life. In water they were
doubtless speedy, swimming in serpentine undulations, with the small
legs for the most part folded against the body and only of occasional
use. Doubtless, too, had the pleurosaurs lived longer in geological
history, they would have become quite snake-or eel-like, just as have
some modern salamanders.
In all probability the pleurosaurs lived habitually in fresh-water,
perhaps visiting the shores for refuge, or for the hatching of their
young. That they were not on the way toward a terrestrial snake-like
body is evident from the flattened tail, and especially the crest of
scales above; the tail was like that of the sea-snakes of the present
time. _Pleurosaurus_, then, affords the solitary instance among
reptiles of aquatic adaptation by the diminution of both front and hind
extremities and the acquisition of a snake-like body and snake-like
habits.
CHAPTER XI
SQUAMATA
The order Squamata, so called because of the dermal covering of
overlapping horny scales, comprises the great majority of living
reptiles. Although the scaly covering is characteristic of nearly all
the members of the order, the most essential differences distinguishing
them from other reptiles are, as usual, found in the skeleton, and
especially in the skull. The quadrate bone, that to which the lower
jaw is articulated on each side, is not wedged in immovably between
other bones of the skull, as in all other reptiles, but is, instead,
freely articulated with the cranium in such a way that its lower end
moves both backward and forward, as well as inward and outward. This
freedom of movement has in the past been thought to be due to the
loss of a lower temporal arch, a bony bar connecting the lower end
of the quadrate with the hind end of the upper jaw, which is very
characteristic, for instance, of the Rhynchocephalia. Indeed, because
of the many primitive characters which the lizards possess, it has
generally been supposed that the order was an early branch of the
rhynchocephalian stem. But we are now quite sure that the lizards
are as primitive as the Rhynchocephalia, and that their origin, as
an independent branch of the reptilian stem, goes quite as far if
not farther back—quite sure that the ancestors of the lizards never
had a lower temporal arcade and two temporal vacuities, but that
the looseness of the quadrate bone has been due to the gradual loss
of a bone which covered the whole side of the skull until only the
upper part of it was left. In other words, the ancestral skull of
the Squamata must have been like that of _Araeoscelis_, more fully
described under the Protorosauria, a group than which there is perhaps
none more closely allied to the Squamata.
The bones of the roof of the mouth of the Squamata—that is, of the
palate—are narrow and long, and are not closely articulated, as in
most other reptiles; they often bear teeth, a primitive character. The
teeth of all living lizards and snakes are not inserted in sockets,
as are those of the crocodiles, but are co-ossified to the margins or
sides of the jaws or the bones of the palate. But this is probably not
a primitive character; doubtless the teeth of the early lizards were
inserted in sockets like those of most other reptiles. The shoulder
bones are absent in many and vestigial in some others. When present
and fully developed, they comprise the shoulder-blades or scapulae,
a single coracoid on each side, the clavicles, and an interclavicle.
The vertebrae, except in some lizards, are procoelous, that is, with
the body concave in front and convex behind, a peculiar structure
that was developed only in crawling animals. In addition to the usual
articulations for the union of the vertebrae there are also, in some
of the lizards and mosasaurs and all of the snakes, additional ones
called the zygosphene and zygantrum, which will be best understood
by reference to Fig. 12, p. 28. But little less characteristic than
the loose articulation of the lower jaws, so unique in this order of
reptiles, is the manner of attachment of the ribs. They are always
single-headed, articulating only with the body or lower part of the
vertebra. The single-headed ribs of the plesiosaurs articulate with
a projection on each side of the arch of the vertebra; those of the
turtles to the space between the adjacent vertebrae; nearly all other
reptiles have double-headed ribs, articulating in various ways. This
character, it is seen, though apparently a simple one, immediately
distinguishes a lizard or a snake from all other animals, except the
thalattosaurs and protorosaurs.
There is much difference of opinion among naturalists as to the proper
classification of the different groups of this order of reptiles.
Usually it is divided into four suborders, the Lacertilia or lizards;
the Dolichosauria or long-necked lizards of the past; the Mosasauria,
or extinct swimming lizards; and the Serpentes or Ophidia, the snakes.
It matters very little which classification one accepts so long as it
is remembered that the first three groups are closely related to each
other.
LIZARDS
Popularly a lizard is any four-legged reptile covered with scales,
but such a definition is not strictly correct, since some lizards are
legless and some other four-legged reptiles are covered with horny
scales, notably the tuatera or _Sphenodon_ of New Zealand, a reptile
long classed with lizards, but now known to belong to quite a different
order. Bearing in mind those characters given as characters of the
order, it will be necessary to mention only those distinguishing the
lizards from the snakes.
[Illustration: FIG. 65.—Iguana. (By permission of the New York
Zoölogical Society)]
It is true that the great majority of lizards have four legs, while the
snakes are always functionally legless, but there are some lizards,
like the glass snakes and the amphisbaenas, or slow lizards, which
are quite legless and there are some snakes which have small but
functionless hind legs. As usual, more important differences are found
in the skull. The brain-case in all snakes is surrounded on all sides
by bone, for the better protection of the brain, with the head resting
quite prone on the ground. The brain of the lizards, for the most part,
is protected on the sides and in front by a simple membrane. Nearly all
lizards have movable eyelids, while snakes do not; snakes have a single
lung, and a protrusible tongue, which very few lizards possess; and
the lower jaws in front are united in the snakes by a ligament only.
Notwithstanding these differences, the snakes and lizards are closely
related animals, and must have come from a common ancestry; among all
reptiles the known geological history of the snakes is shortest.
Lizards, on the other hand, have a very high antiquity, beginning,
as we now know, at least as long ago as early Triassic times. They
still have many primitive characters in their structure and are the
least advanced type of reptiles now living, with the exception of the
tuatera. Their remains are seldom found in the rocks, probably because
they have always been so strictly terrestrial in habit, for the most
part seldom frequenting even the vicinity of the water. The true
lizards now living number about eighteen hundred kinds, classified into
about twenty families, divided among four chief groups, of which the
chameleons, the amphisbaenas, our common lizards, and the monitors are
representatives.
Most living lizards are inhabitants of warm climates, though some
extend rather far north in the temperate zone. With the exception of
New Zealand, and the polar and subpolar regions, lizards are found
in all parts of the world. The great majority live only in high and
dry places, though some are denizens of low and marshy places, a few
even not being averse to the water. They are, for the most part, spry
in their movements, some little ones scarcely six inches in length
taxing a vigorous man’s speed to capture; and many are expert climbers
of cliffs, trees, and even the ceilings of residences. Some, the
remarkable little flying dragons of Ceylon, have an extraordinary
development of the skin on the sides of the body, supported by the
expanded ribs, forming a sort of parachute whereby the creatures
can sail considerable distances through the air. Nearly all are
carnivorous, feeding upon small mammals, birds, other reptiles, frogs,
and insects; a few only are herbivorous, such as the iguanas, which are
often used for human food. Nearly all lizards are oviparous, laying
from two to thirty eggs. In size the great majority are small, less
than a foot in length; but some, such as the monitors and iguanas,
reach a length of from four to six feet, or even more, and certain
extinct monitors of India are known to have attained a length of thirty
feet. They are, for the most part, slender, graceful, prettily marked,
and quite inoffensive creatures. A few are short, flat, or stumpy in
shape, such as the so-called horned toad. One or two species only, the
“Gila monsters,” are reputed to be venomous.
[Illustration: FIG. 66.—_Amblyrhynchus cristatus_, the Galapagos
sea-lizard. (From Brehm)]
There is but a single species of lizard now living which is in any true
sense aquatic in habit, the well-known sea-lizard of the Galapagos
Islands, scientifically known as _Amblyrhynchus cristatus_. It is a
large lizard, with a short rounded head, a flat tail, and webbed feet.
Its specific name is derived from the erect fringed crest along its
back and tail. Its habits are best given in Darwin’s words:
It is extremely common on all the islands throughout the
group, and lives exclusively on the rocky sea-beaches,
being never found, at least I never saw one, even ten yards
inshore. It is a hideous looking creature, of a dirty black
color, stupid and sluggish in its movements. The usual
length of a full grown one is about a yard, but there are
some even four feet in length; a large one weighed twenty
pounds. The tails are flattened sideways, and all four
feet are partially webbed. They are occasionally seen some
hundred yards from the shore swimming about. When in the
water this lizard swims with perfect ease and quickness, by
a serpentine movement of the body and flattened tail—the
legs being motionless and closely collapsed to the sides.
A seaman on board sank one, with a heavy weight attached
to it, thinking thus to kill it directly; but when an
hour afterward he drew up the line it was quite active.
Their limbs and strong claws are admirably adapted for
crawling over the rugged and fissured masses of lava, which
everywhere forms the coast. The nature of this lizard’s food
(seaweed) as well as the structure of the tail and feet,
and the fact of its having been seen voluntarily swimming
out at sea, absolutely proves its aquatic habits; yet there
is in this respect one strange anomaly, namely, that when
frightened it will not leave the island. Hence it is easy
to drive these lizards down to any little point overhanging
the sea, where they will sooner allow a person to catch
hold of their tails than jump into the water. They do not
seem to have any notion of biting; but when much frightened
they squirt a drop of fluid from each nostril. I threw one
several times as far as I could into a deep pool left by
the retreating tide, but it invariably returned by a direct
line to the spot where I stood. It swam near the bottom,
with a very graceful and rapid movement, and occasionally
aided itself over the uneven ground with its feet. As soon
as it arrived near the edge, but still being under water,
it tried to conceal itself under the tufts of seaweed, or
it entered some crevice. I several times caught the same
lizard by driving it to a point, and, though possessed of
such perfect powers of diving and swimming, nothing could
induce it to enter the water; and as often as I threw it
in it returned in the manner described above. Perhaps this
singular piece of apparent stupidity may be accounted for by
the circumstance that this reptile has no enemies whatever
on shore, whereas at sea it must often fall a prey to the
numerous sharks.
These lizards are of much interest as indicating one of the ways in
which true land reptiles have become aquatic in their habits. Tempted
by the abundance of food growing in shallow water a little beyond their
reach, the reptiles ventured farther and farther to obtain it. The tail
gradually became a propelling organ, though the lizard still retained
in large measure its land habits and land feet, because of the dangers
from its water enemies. It is not at all improbable that, in course
of time, were these Galapagos lizards left unmolested, they would
become fleeter swimmers by the development of a terminal caudal fin
and paddle-like legs, thus competing with their aquatic enemies and no
longer needing recourse to the land for protection. They also serve to
indicate that long-tailed aquatic reptiles never used their legs to an
appreciable extent as organs of propulsion in the water.
[Illustration: FIG. 67.—_Varanus_, Australian monitor lizard. (By
permission of the New York Zoölogical Society.)]
_Flat-headed lizards._—Among the living lizards there is one group,
called the monitors, which have so many characters peculiar to
themselves that they seem rightfully entitled to an isolated place
among the lizards of the present time. The group includes about
thirty species, all belonging in the one genus _Varanus_, and all
living in India, Africa, and Australia. In size, some of the species
of _Varanus_ are the largest of all terrestrial lizards known in the
past or present; in other ways also they have reached the maximum of
specialization among lizards. The head is pointed, broad, and flat,
and the body and tail are long. They have nine vertebrae in the neck,
a larger number than is to be found in any other terrestrial lizard.
Unlike other lizards they have a protrusible tongue like that of the
snakes. All are carnivorous in habit, feeding upon small backboned
animals, insects, and especially upon eggs, which they crush between
their teeth while holding them aloft. Most species live wholly upon the
land, and some are arboreal. Others, especially those of the Nile, live
about water and are excellent swimmers. The terrestrial species have
a round tail and small external nostrils, but the water species have
the tail much flattened, and the nostrils have large cavities, which,
when closed under water, are said to serve as reservoirs of air for
respiration. Of one of these swimming species Annandale writes:
_Varanus salvator_ is common in Lower Siam where it is
equally at home on land, in water, and among the branches of
trees. The eggs are laid in hollow tree trunks. When in the
water the lizard swims beneath the surface, the legs being
closely applied to the sides, and the tail functioning both
as oar and rudder.
These lizards take to the water to escape from their land enemies and
not for food, a habit also known among certain other lizards, and
one precisely the reverse of that of the Galapagos lizards. It would
seem very probable that animals of such carnivorous habits as are the
monitors might easily learn to capture water animals for food and
thus eventually become aquatic in habit. This inclination toward, and
partial adaptation to, water habits in the monitors is of much interest
because in all probability the instinct is one of long inheritance from
those remote ancestors which gave origin to the truly aquatic members
of the order. Though the known geological history of the monitors does
not extend far back, they are so intimately allied in their anatomical
structure to the aquatic and semiaquatic lizards of Cretaceous times
that there could seem to be no doubt of the common ancestry.
_Dolichosaurs._—About fifty years ago Professor Owen, the famous
English paleontologist, described a peculiar semiaquatic lizard
from the Cretaceous rocks of England to which he gave the name
_Dolichosaurus_, in allusion to the slender form of the body. Just what
relations these slender lizards have to modern lizards has long been
a problem; some have thought that they were their progenitors, but
there are very good reasons for doubting this. No modern lizards, save
the monitors, have more than eight vertebrae in the neck, while these
dolichosaurs had as many as seventeen, a remarkable specialization for
aquatic life that could hardly have been lost by their descendants.
For this reason the dolichosaurs have usually been considered as
representing a distinct suborder. But they have many resemblances
otherwise to the monitors. They were semiaquatic in habit, and never
more than six feet in length. They are yet imperfectly known, and no
restoration of any form has hitherto been attempted. Their peculiar
interest lies in the elongation of the neck, quite like that of the
wholly unrelated nothosaurs and proganosaurs, which have been described
in the foregoing pages. Doubtless similar habits in each had like
results, but just what these habits were in the slender lizards we do
not yet know.
_Aigialosaurs._—Within recent years a number of other lizards have
been made known from the Lower Cretaceous rocks of Dalmatia which
present most remarkable intermediate characters between the monitors,
dolichosaurs, and the mosasaurs, the famous sea-lizards of Upper
Cretaceous age. Some of these lizards had twelve or thirteen vertebrae
in the neck, while others had but seven—an unusually short neck
characteristic of the mosasaurs. These latter kinds, belonging to two
or three genera, are included in a distinct group. They were long and
slender, the head long and pointed. The teeth, conical and sharp, were
attached in shallow pits, quite as in the mosasaurs. The lower jaws
had a hinge just back of the teeth, as in the mosasaurs, of which the
only trace in modern lizards is found among the monitors. Still more
remarkable, though perhaps not so easily appreciated, is the shape of
the quadrate bone, with a broad flaring rim for the ear cavity, quite
unlike that of land lizards, but quite like that of the mosasaurs. In
fact, the very peculiar skull is almost identical with that of the true
sea-lizards. The body and tail also resemble those of the mosasaurs
more than those of the monitors, but there is a firm attachment of the
pelvis to the backbone, and the legs are long and lizard-like, though
not as long as those of land lizards. The feet were webbed in life, and
the toes have no claws, conclusively demonstrating their water habits.
The vertebrae indeed have the same peculiar articulations, called
zygosphenes, as in most of the mosasaurs. The largest aigialosaurs were
about six feet in length, that is, of about the size of the smallest
known mosasaurs.
[Illustration: FIG. 68.—_Clidastes_, an American mosasaur. Life
restoration.]
We have then in the aigialosaurs nearly every known intermediate
character that we could wish for in a connecting link between the
mosasaurs and the monitors, lizards that were equally at home on land
or in the water, and there can be scarcely a doubt that they were
either the direct ancestors or closely akin to the direct ancestors of
the strictly marine mosasaurs; and scarcely a doubt that they were the
descendants of the actual forbears of the modern monitors, which, as we
have seen, have acquired partial aquatic habits in escaping from their
enemies. The dolichosaurs we can now understand were a side branch
from these semiaquatic aigialosaurs which, specializing in another
direction, quickly came to grief, perhaps in competition with their
more agile and skilful short-necked kin.
Taking all these facts into consideration it seems best to unite
the monitors, dolichosaurs, and aigialosaurs into one group of the
Lacertilia, the Platynota, intermediate in place between the true land
lizards and the truly aquatic mosasaurs.
MOSASAURS
At St. Pietersberg, a small mountain in the vicinity of Maestricht,
Holland, there are immense subterranean stone quarries, which have
been worked for more than a thousand years. The stone quarried from
them is a sandy limestone of Upper Cretaceous age containing many
well-preserved remains of extinct animals that have long been sought
by collectors of fossils. In 1776 Major Drouin—an officer of a near-by
garrison, one of much military importance in those days—secured from
one of these quarries some bones of an extinct reptile, which, though
of interest, afforded but little information concerning the structure
and affinities of the animal to which they had once belonged. In 1780
a very perfect skull, in excellent preservation, of the same kind of
an animal was obtained from the same quarry by Dr. Hofmann, an army
surgeon of the same garrison, whose interest in such things had been
incited by Major Drouin’s collections. This specimen, so renowned in
science, has had a remarkable and eventful human history, in part
related by St. Faujas de Fond, a French commissary of the “Army of the
North,” and one of the participants:
In one of the great galleries or subterranean quarries in
which the Cretaceous stone of St. Pieter’s Mount is worked,
about five hundred paces from the entrance, and ninety
feet below the surface, the quarrymen exposed part of the
skull of a large animal in a block of stone which they were
engaged in quarrying. On discovering it they suspended
their work and went to inform Dr. Hofmann, surgeon to the
forces at Maestricht, who for some years had been collecting
the fossils from the quarry, remunerating the workmen
liberally for the discovery and preservation of them. Dr.
Hofmann, arriving at the spot, saw with extreme pleasure
the indication of a magnificent specimen; he directed the
operations of the men, so that they worked out the block
without injury to the fossil, and he then, by degrees,
cleared away the yielding matrix and exposed the
extraordinary jaws and teeth, which have since been the
subject of so many drawings, descriptions, and discussions.
This fine specimen which Dr. Hofmann had transported with
so much satisfaction to his collection, soon became,
however, a source of much chagrin to him. Dr. Goddin, one
of the canons of Maestricht, who owned the surface of the
soil beneath which was the quarry whence the fossil was
obtained, when the fame of the fossil reached his ears,
pleaded certain feudal rights in support of his claim to
it. Hofmann resisted and the canon went to law. The whole
chapter supported their reverend brother, and the decree
ultimately went against the poor surgeon, who lost both the
specimen and his money, for he was made to pay the costs of
the action. The canon, leaving all remorse to the judges
who pronounced the iniquitous sentence, became the happy
and contented possessor of this unique example of its kind.
[Translation by Leidy.]
But the canon was ultimately despoiled of his ill-gotten treasure. At
the siege of Maestricht in 1795, the famous skull to which Hofmann
had devoted so much anxious thought and labor, fell into the hands of
the French and was carried off as one of the spoils of war. So widely
celebrated had the specimen become during the fifteen years which had
elapsed since its discovery, through the writings of several noted
scientific men, that the French general commanded his artillerists
to spare the house in which it was known to be. The canon, however,
shrewdly suspecting that such an unexpected and extraordinary mark of
favor was not for his own sake but rather for the sake of the famous
fossil, had it removed and carefully hidden in a house in the city.
After the capitulation of Maestricht the eagerly sought-for fossil was
not to be found, and the offer of a reward of six hundred bottles of
wine, so the story goes, was made for its recovery. So tempting was
the offer that, ere long, it was brought in triumph to the house of
St. Faujas de Fond, by a half-dozen grenadiers, whence it was later
transferred to Paris, where it now is.
We may well sympathize with Dr. Hofmann in the loss of his cherished
specimen, since, had it not been for his zeal, money, and labor,
it would never have escaped the usual fate of such things—complete
destruction. But we must remember that St. Faujas de Fond, the recorder
of this history, was a Frenchman, and somewhat interested in robbing
the reverend canon of it; possibly there is another side of the story
which has never been told.
After peace was declared, one has regretfully to add that the canon,
not Dr. Hofmann, was reimbursed for it, or so it is said. Cuvier rather
naïvely says that it was ceded to the Garden of Plants of Paris,
perhaps in the way that many other things are ceded to the conqueror
in time of war. The specimen is really a good one, even when compared
with many found in recent years, and there is little wonder that the
cupidity of St. Fond was incited by it. Casts of it are now or have
been in nearly every noted museum of the world, and pictures of it
illustrated nearly every textbook of geology published during the
first three-quarters of the past century. It had been the subject of
considerable controversy even before it came into the hands of Cuvier.
Peter Camper figured and described the skull as that of a whale or
“breathing fish”; while St. Fond himself later called it a crocodile.
Crocodile or alligator skeletons were rare in those days, and St. Fond
made a special trip to the British Museum to study one. But it was
really Adrian Camper, a son of Peter Camper, who deserves the credit,
so often wrongly ascribed to Cuvier, for the recognition of the true
nature of the fossil. He insisted that the animal was a lizard allied
to the living monitors, an opinion which it will be seen has finally
been proved to be correct within very recent years.
In 1808 this famous skull, and all other known remains of a similar
nature, came under the observation of Cuvier, the renowned French
naturalist and paleontologist, who confirmed the views of Adrian
Camper. He fully described and figured all the known parts of the
skeleton that had later come to light, calling the animal the great
lizard of the Meuse, the river near which Hofmann’s specimen was found.
Conybeare, a well-known paleontologist of England, some years later
formally christened it _Mosasaurus_, a transliteration of Cuvier’s
phrase, from the Latin _Mosa_, for Meuse, and _saurus_, a lizard. For
more than half a century Cuvier’s figure of the skull of the original
specimen appeared in works on geology over the name _Mosasaurus
hofmanni_, or _Mosasaurus camperi_. One could wish that the former name
for the species might prevail, in recognition of the zealous doctor who
was so shabbily treated in his possession of the specimen.
For some years the few specimens discovered by Drouin and Hofmann were
all that were known of the mosasaurs. A few others of related forms
were discovered in England, and some were reported from New Jersey by
early explorers, but there was little published about the mosasaurs
till 1843, when Dr. August Goldfuss, a noted German paleontologist,
described and beautifully figured an excellent specimen from the
United States. This specimen also had a rather eventful history.
It was discovered early in the fourth decade by Major O’Fallen, an
Indian agent, near the Great Bend of the Missouri River, whence it
was transported by him to St. Louis and placed in his garden as a
curiosity. It happened that Prince Maximilian of Wied, the famous
naturalist, in his travels through the United States, saw the specimen
and secured it, taking it to Germany on his return. He presented it
to the Museum of Haarlem where Goldfuss saw and described it. Rather
oddly, this specimen was of a species closely allied to the original
one of Maestricht, a species which has since only rarely been found.
It was called _Mosasaurus maximiliani_ by Goldfuss, though some time
previously, it has since been found, some fragments of the same
species were described by Harlan, an American author, under the name
_Ichthyosaurus missouriensis_. Goldfuss’ paper was strangely overlooked
by subsequent writers, and it was not till the discovery of numerous
remains of mosasaurs by Leidy, Cope, and Marsh in the chalk of western
Kansas, nearly thirty years later, that much was added to the world’s
knowledge of these strange reptiles.
[Illustration: FIG. 69.—Skeleton of _Plalecarpus_ as mounted in the
Paleontological Exhibit. Walker Geological Museum, University of
Chicago.]
Perhaps nowhere in the world are the fossil remains of marine animals
more abundantly and better preserved than in these famous chalk
deposits of Kansas. The exposures are of great extent—hundreds of
square miles—and the fossil treasures they contain seem inexhaustible.
Long-continued explorations by collectors have brought to light
thousands of specimens of these swimming lizards, some of them of
extraordinary completeness and perfect preservation, so complete and
so perfect that there is scarcely anything concerning the mosasaurs
which one might hope to learn from their fossil remains that has not
been yielded up by these many specimens. The complete structure and
relations of all parts of the skeleton, impressions of the bodies made
in the soft sediments before decomposition had occurred, the character
of their food, the nature of the skin covering, and even some of the
color markings of the living animals have all been determined with
certainty. Not only from Kansas, but also from many other parts of the
world, have remains of these animals been discovered, until now it may
truthfully be said that no other group of extinct reptiles is better
represented by known fossil remains than the mosasaurs. From England,
Belgium, Russia, and France in Europe; from New Jersey, Georgia,
Alabama, Mississippi, Texas, New Mexico, Colorado, Kansas, Nebraska,
the Dakotas, Wyoming, and other places in the United States; from New
Zealand and South America they have been obtained in greater or less
abundance and perfection.
[Illustration: FIG. 70.—Tooth of _Tylosaurus_.]
Their geological history is relatively brief, notwithstanding their
wide distribution over the earth in such great numbers and diversity.
The earliest are known from near the beginning of the Upper Cretaceous
of New Zealand, whence it is believed by some that they migrated to
other parts of the world, appearing in North America some time later.
They reached their culmination in size, numbers, and variety very soon,
and then disappeared forever before the close of Cretaceous time. The
largest complete specimen of a mosasaur known measures a little more
than thirty feet in length, but incomplete skeletons of others indicate
a maximum length of about forty feet. The skulls of the largest species
are about five feet long. The smallest known adult skeletons are
scarcely eight feet in length. There are now known at the present time
seven or eight genera of three distinct types, all belonging to one
family, the Mosasauridae, including about twenty-five known species.
While a few of the genera are widely distributed over the earth, the
species are all of restricted range, indicating, perhaps, non-migratory
habits.
The adaptation of the mosasaurs to an aquatic life was very complete,
though perhaps not so complete as was that of the ichthyosaurs. The
skull is flattened, narrow, and more or less elongate, but large in
proportion to the remainder of the skeleton—nearly one-sixth of the
entire length; that relative size doubtless is indicative of very
predaceous and pugnacious habits. The teeth in the typical forms are
numerous, strong, and sharp, conical in shape, and recurved. Not
only are there numerous teeth in both the upper and lower jaws, but
there are also two rows of strong teeth implanted in the back part of
the palate, upon bones called pterygoids, the use of which will be
understood later. The teeth were inserted on large, tumid, bony bases,
rather loosely attached in shallow pits or alveoli, unlike the teeth of
all modern lizards. Such a mode of attachment of the teeth doubtless
had some relation to the habits of the animals concerning which we
are not quite clear. They were easily dislodged, and, in consequence,
of very unequal size, some full grown, some small, and others just
appearing above the surface of the gums in the living animals. The
frequent loss of teeth and their constant and easy replacement by new
ones is a peculiarity of predaceous reptiles, thereby insuring their
best functional use.
The external nostrils, of large size, were situated at a considerable
distance back of the end of the snout, but not nearly so far back or so
near the eyes as were the nostrils of the ichthyosaurs, plesiosaurs,
and phytosaurs. Their size and position suggest a use like that of the
modern aquatic monitors, as mentioned on a preceding page. The eyes
were of moderate size, those of the less purely aquatic forms being
directed more laterally than those of species of more distinctly diving
habits. They were protected by a stout ring of bony plates, as were the
eyes of all truly aquatic reptiles of the past. The ears, also, in most
if not all mosasaurs, had a thick cartilaginous ear-drum in place of a
simple membrane, evidently, as Dollo has shown, for better protection
under undue pressure of the water in deep diving.
[Illustration: FIG. 71.—_Clidastes_, inner side of right mandible:
_ang_, angular; _art_, articular; _cor_, coronoid; _pa_, prearticular;
_sur_, surangular.]
As in all other lizards, the bones with which the lower jaws
articulate, the quadrates, were loosely attached at the upper end,
permitting great freedom of movement in all directions, more even than
the land lizards have. The lower jaws were long and powerful, armed
with a single row of teeth on each side, from sixteen to eighteen
in number. Just back of the teeth, a little beyond the middle, each
mandible has a remarkable joint, quite unknown in land lizards, though
a trace of it is found in the monitors, permitting much movement
between the front and back parts, both laterally and vertically, though
chiefly in the former direction. Furthermore, as in land snakes but not
as in land lizards, the front ends of the two sides of the jaws were
somewhat loosely attached to each other by ligaments. This looseness of
the two sides of the jaws, not only in front but also behind, together
with the joint in each, was of the greatest use in swallowing prey, as
will be explained farther on.
[Illustration: FIG. 72.—Skulls of mosasaurs. Upper figure, _Clidastes_,
from the side; middle figure, _Platecarpus_, from below; lower figure,
_Tylosaurus_, from above: _an_, angular; _bs_, basisphenoid; _c_,
coronoid; _ep_, epipterygoid; _fr_, frontal; _j_, jugal; _l_, lacrimal;
_m_, maxilla; _na_, nasal; _oc_, occipital condyle; _pa_, parietal,
palatine; _pm_, premaxilla; _pf_, prefrontal; _pt_, pterygoid; _po_,
postorbital; _q_, quadrate; _sp_, splenial; _sq_, squamosal; _tr_,
transverse; _v_, vomer.]
As in most other aquatic reptiles, the neck was short and strong, the
vertebrae being less in number than in most other lizards. The trunk
was long and slender, more especially so in the surface-swimming kinds,
with from twenty-two to thirty-four vertebrae. The tail was long, no
longer than the tail of some land lizards, but more powerful, and
broader and flatter. It was expanded or dilated more or less toward the
free end, that is, with the beginning of a terminal caudal fin, such
as the more specialized ichthyosaurs and crocodiles possessed. The
vertebrae were procoelous, that is, concave in front and convex behind,
like those of most modern lizards and all modern snakes and crocodiles,
but quite unlike the biconcave vertebrae of all other aquatic reptiles.
This kind of articulation of the backbones gave greater firmness and
strength to the spinal column, but decreased the flexibility, and its
possession by these animals was doubtless due to their descent from
land lizards which had already acquired it. The loss of flexibility,
however, was partly compensated by the loss of the additional
articulating surfaces of the tail.
[Illustration: FIG. 73.—_Platecarpus_; occipital view of skull: _bo_,
basioccipital; _eo_, exoccipital; _pf_, postfrontal; _st_, stapes;
_pt_, pterygoid; _q_, quadrate.]
As in all other aquatic reptiles, it is in the limbs that the most
striking characteristics of these water lizards or “sea-serpents” are
found. The legs were so completely adapted to an aquatic mode of living
that the animals must have been practically helpless upon land, able
perhaps to move about in a serpentine way when accidentally stranded
upon the beaches, but probably never seeking the land voluntarily. The
front limbs, like those of all other swimming animals having a powerful
propelling tail, were larger than the hind ones, though not very much
so. The bones of the first two segments, that is, the arm, forearm,
and thigh and leg bones, were all short and broad, resembling those of
the ichthyosaurs more than those of any other reptiles, save perhaps
the thalattosaurs, discussed below. The articular surfaces of all the
limb bones, as in other aquatic animals, were restricted in extent,
indicating limited motion between the joints, though doubtless having
great flexibility. In the most specialized types, such as _Tylosaurus_,
the wrist and ankle bones were almost wholly cartilaginous, just as
they are in the water salamanders, and in whales and porpoises. This
tendency of the ends of long bones, the wrists and ankles as well as
other bones of the skeleton, to become more cartilaginous, or less
well ossified, in animals purely aquatic in habit is a marked one. So
much is this the case that paleontologists always suspect water habits
in reptiles showing it, even though but few parts of the skeleton are
known.
[Illustration: FIG. 74.—_Clidastes_; left front paddle: _c_, coracoid;
_h_, humerus; _r_, radius; _sc_, scapula; _u_, ulna.]
[Illustration: FIG. 75.—_Tylosaurus_; left front paddle: _c_, coracoid;
_sc_, scapula; _h_, humerus; _r_, radius; _u_, ulna.]
Increase in the number of bones of the digits is a more or less
conspicuous characteristic of all mosasaurs. In those forms in which
the wrists and ankle bones had become cartilaginous in great part, as
many as eleven phalanges have been observed in the longest toes, though
in other forms, those with more completely ossified wrists and ankles,
only two or three additional bones have been developed in the longest
fingers and toes by aquatic habits. The pliability and flexibility of
the fingers and toes were certainly very great, but they could not
possibly have been flexed or bent so as to grasp or seize anything;
and of course all vestiges of claws had disappeared. Many specimens
have been found with all the bones of the limbs, that is, the “paddle
bones,” in the positions they occupied when the animals died. Figures
of three such specimens, made from photographs or careful drawings by
the writer, are shown herewith (Figs. 74-76). In several such specimens
very clear impressions of the smooth membranes between the fingers have
been observed, and in one specimen preserved in the collections of the
University of Kansas the outline of the fleshy parts connecting the
paddle with the body has been preserved.
[Illustration: FIG. 76.—_Platecarpus_; right front paddle: _h_,
humerus; _r_, radius; _u_, ulna.]
It will be seen by comparison of the figures of the mosasaur paddles
with those of the ichthyosaurs and plesiosaurs that there was a wide
difference in their structure, though all have the characteristic
shortening of the limb bones and increase in the numbers of the
finger and toebones, that is hyperphalangy. It is probable that these
differences mean a more powerful and varied use of the limbs in the
mosasaurs. It is certain that the mosasaurs were much more predaceous
and pugnacious in their habits than were any other truly aquatic
backboned air-breathing animals of the past or present. They were the
“land sharks” of the ancient seas, and probably the only ones among
water reptiles that would be dangerous and offensive to man, were they
all living today.
For a long time it was thought that the mosasaurs had no breast
bone, and that, in consequence, the front part of the thorax was
expansible. Under this assumption the mosasaurs would have been much
more snake-like in habit than they really were. The loose construction
of the jaws doubtless permitted the swallowing of prey of considerable
size, and the inference was that they habitually preyed upon animals of
large size. A snake will often swallow a frog of larger diameter than
its own body, the flexible jaws and loosely connected ribs permitting
it to pass to the abdominal cavity. But the unyielding ring formed by
the anterior ribs connected with the breast bone in the mosasaurs, as
in other lizards, conclusively proves that large animals could not
have been swallowed whole by the mosasaurs. In several instances the
fossilized stomach contents, composed chiefly or wholly of fishes, have
been found between the ribs of mosasaurs, and in none were the fishes
more than two or three feet in length, though the reptiles were from
sixteen to twenty feet long. Possibly the largest mosasaurs, those
thirty or thirty-five feet in length, might have captured and swallowed
fishes six or seven feet long, but in all probability their usual prey
was of smaller relative size.
[Illustration: FIG. 77.—_Platecarpus_; pelvis, from below: _p_, pubis;
_il_, ilium; _is_, ischium.]
The very loose construction of the pelvic bones, those to which
the hind legs are articulated, is an evidence of more complete
adaptation to water life than was or is the case with any other water
air-breathers except the ichthyosaurs and cetaceans. The sacrum
had entirely lost its function as a support to the pelvis and had
disappeared, that is, the vertebrae composing it had become quite
like the adjacent ones, by the loss of the ribs connecting them with
the ilium. The small pelvis was suspended loosely in the walls of the
abdomen, or at the most was feebly connected with a single vertebra
by ligaments. It was entirely useless as a support for the legs.
The mosasaurs could not possibly have raised their bodies from the
ground while on land. It is well known that the land lizards and the
crocodiles raise their bodies free from the ground while running or
walking; none drags its body over the surface.
[Illustration: FIG. 78.—Photograph of carbonized remains of scales of
_Tylosaurus_.]
In several instances complete or nearly complete skeletons of mosasaurs
have been discovered with the different bones nearly all in the
positions and relations they had after the decomposition of the flesh,
together with the carbonized remains of the skin and impressions of
the investing scales and membranes. The nature of the body covering is
therefore known with certainty from nearly all parts of the body. The
body everywhere, save on the membrane between the fingers and toes, and
perhaps on the top of the skull, was covered with small overlapping
scales, very much like those of the monitors. These scales, however,
were small and smooth in comparison with the size of the animals, those
of a mosasaur twenty feet in length being almost precisely the size of
those of a monitor six feet long. The top of the skull seems to have
been covered with horny plates, as in most lizards. In one instance
parallel dark bars, obliquely placed, and of narrow width, formed by
carbonized pigment, were observed by the writer. As has been stated,
in some instances fish bones and fish scales have been observed among
the fossilized stomach contents, and it is quite certain that the
food of these creatures must have been composed chiefly of fishes,
though of course it is not improbable that other small vertebrates,
birds, pterodactyls, the young of plesiosaurs, and possibly small
mammals, may occasionally have formed a part of their diet. That the
mosasaurs were very pugnacious in life is conclusively proved by the
many mutilations of their bones that have been observed, mutilations
received during life and partly or wholly healed at the time of death.
Bones of all vertebrates are repaired after injury by the growth of
more or less spongy osseous material about the injured part, forming a
sort of natural splint. This material is more or less entirely removed
by absorption when it is no longer required for the support of the
broken ends. Many such injured bones of the mosasaurs have been found;
sometimes the bones of the hands and feet have grown together, and not
infrequently the vertebrae have been found united by these osseous
splints; occasionally even the skull itself, especially the jaws,
attest extensive ante-mortem injuries. In a single instance the writer
has observed the loss of a part of the tail, where it probably had been
bitten off. It may be mentioned, however, that the bones of the tail
had no such “breaking points” in the mosasaurs as have those of many
land lizards, whereby a part or all may be lost as a result of even a
trivial injury, and then regrown. Such a condition in an organ relied
upon entirely for propulsion would have been immediately fatal to the
existence of the mosasaurs. The large jaws and teeth are in themselves
sufficient evidence of the fiercely carnivorous propensities of the
mosasaurs. The constant renewal of the sharply pointed teeth, thereby
preventing deterioration by use or accident, preserved, even in the
oldest animals, the effectiveness of the youthful structure.
We may now understand how the mosasaurs seized and swallowed their
prey. Living constantly in the water, away from all firm objects, with
small, short limbs quite incapable of holding struggling prey, and
the body not sufficiently serpentine to hold it in its folds after
the manner of snakes, the mosasaurs would have found it difficult or
impossible to swallow fishes of even moderate size, were their jaws of
the same construction as are those of the land lizards. If they preyed
upon small animals only, or if they tore their prey to pieces after the
manner of the alligators, there would have been no especial difficulty
in deglutition. But it is certain that the animals which the mosasaurs
devoured were not always small, and they must have been swallowed
whole, since their teeth were not adapted, like those of the alligators
and true crocodiles, for the rending of bodies. One who has watched a
snake swallow a frog or another snake will appreciate the difficulties
against which the mosasaurs contended in swallowing fishes a fifth or a
sixth of the length of their own bodies. The ordinary snake, no matter
where or how it seizes its prey, invariably swallows it head first. Its
mandibles are even more loosely united in front than were those of the
mosasaurs, and while there is no joint in the snake’s mandibles such
as there is in the mosasaurs’, the loose union of the various bones of
the mandibles serves the same purpose. The frog or lizard, while firmly
held by the slender teeth, is slowly moved sideways by the alternate
lateral action of the jaws till the head is reached. Many non-poisonous
snakes, if they find it impracticable or impossible to reverse the
position of their prey in this way, wrap the folds of their body about
it, holding it firmly while they release their mouth-hold and seize
it by the head. An amusing instance of these habits came under the
observation of the writer not long ago, in Texas. A large “blue racer”
(_Bascanion_), six feet four inches in length, caught an unusually
large bullfrog by one hind leg, but in almost less time than it takes
to relate, the head of the frog had entered the snake’s gullet and the
mouth was closed over it, notwithstanding the vigorous muscular and
vocal protests on the part of the frog. Wishing to secure the skull
of the snake for his collection, the writer seized an ax standing
conveniently by and cut the snake cleanly in two. The peristaltic
action of the deglutitional muscles carried the frog slowly on about
two feet farther to the ax-made orifice, from which it emerged, and,
after a few croakings against such unkind usage, calmly hopped off into
the near-by pool of water! Many poisonous snakes release their prey
after killing it; other snakes may force their prey down the throat by
pressing it against the ground.
Even small fishes could not possibly have been swallowed by the
mosasaurs in any other way than head first, since the backwardly
projecting, and often long, spines would have rendered any other
procedure impossible. Even after the head had entered the gullet,
deglutition could have been effectively completed only by the aid of
some mechanism whereby the fish could have been pulled or pushed back
into the constricting fauces. The strong teeth of the upper jaws and
palate held firmly the struggling prey, while the loosely united jaws,
bending laterally at the joint back of the middle, either alternately,
or more probably in unison, steadily forced it far enough back to be
seized by the muscles of the fauces.
The shape of the mosasaurs, though slender, does not suggest
extraordinary speed in the water; doubtless most of the fishes that
lived in the seas with them could swim faster than they. Their prey
was captured, for the most part at least, by sudden and quick lateral
movements, for which their powerful and flexible paddles admirably
adapted them.
It is a rather remarkable fact that, among the thousands of specimens
of mosasaurs which have been collected during the past forty years in
both Europe and North America, there never has been found one of a
very young animal. Of almost all other animals occurring abundantly
as fossils some specimens are sure to be discovered of young and even
embryonic individuals. It is quite certain that all such voracious
monsters as were the mosasaurs did not die of old age. Some specimens,
it is true, have been found that were evidently not full grown animals,
but the observed differences in the size of the fossil bones are not
great. All are of adult or nearly adult animals. If the mosasaurs were
oviparous, as were the ichthyosaurs, and probably the plesiosaurs,
and as are some living land lizards, the apparently entire absence of
embryonic bones associated with often nearly complete skeletons of the
mosasaurs is inexplicable; certainly _some_ mosasaurs must have died
a short time before the birth of their young. But embryos have never
been discovered, though numerous skeletons inclosing fossilized stomach
contents have been found. From this fact it would seem very probable
that the mosasaurs were oviparous, as are most other lizards. But this,
after all, may be a hasty inference.
No known reptiles lay their eggs in the water. Perhaps there is some
reason why the eggs of reptiles and birds, so different from those of
fishes and amphibians, cannot hatch in water; and there is no good
reason for supposing that the mosasaurs were exceptions to this rule.
Unless carefully hidden or protected by the parent, the eggs or very
young of the mosasaurs would have been subject to many and grave
dangers. Fish eggs are usually small and produced in great numbers,
thousands often being extruded from a single female. Among so many
there is a greater probability that at least two will hatch and survive
to maturity, reproducing their kind. It is unreasonable to suppose
that the lizards of the past were more prolific of eggs than are their
relatives now living; nor is it possible that their eggs could have
been as small as are those of most fishes. Modern lizards seldom lay
more than twenty-five or thirty eggs at a time; even the turtles, with
their greater vicissitudes, seldom produce more than one hundred. The
eggs of the mosasaurs were certainly large and few in number, and the
young animals must have begun breathing air immediately after escaping
from the shells. If the mosasaurs were oviparous they must have laid
their eggs upon the shores and beaches, as do the sea-turtles and the
Crocodilia. Nor is it at all probable that the female mosasaurs gave
even that protection to their eggs or young that the crocodiles and
turtles give. The young mosasaurs, perhaps reaching a foot in length,
must have been left entirely to their own devices and their own fate at
the very earliest stages of their independent careers.
The waters in which the mosasaurs abounded swarmed with many kinds of
predaceous fishes, to say nothing of the hordes of their own kinds,
all carnivorous in the highest degree, to all of which the tender
saurians must have been choice food. Possibly the shallow waters of the
bays and estuaries may have afforded protection to the newly hatched
reptiles. It would seem probable that the female mosasaurs went up
the rivers for a shorter or longer distance to lay their eggs or give
birth to their young, and that the young reptiles remained in such
relatively protected places until of a sufficient size to cope with
the fierce enemies of the open seas. We know practically nothing of
the inhabitants of the lakes and rivers during all the time in which
the mosasaurs existed; and this perhaps is the real reason why we have
never yet found a specimen of a young mosasaur.
[Illustration: FIG. 79.—Head of _Tylosaurus_.]
We have seen that many skeletons of ichthyosaurs are found entire, and
but little disturbed in position, suggesting, if not proving, that the
animals as a rule lived and died far out in the deep seas, away from
the disturbing effects of currents of water on their decaying bodies.
Among the thousands collected, the great majority of the specimens
of mosasaurs consist of a few bones or a part of the skeleton only.
Moreover, nearly all specimens show the disturbing effects of currents
of water; and the bones are usually associated with those of turtles,
birds, and flying reptiles, which probably did not often venture far
from the shores; all of which goes to prove that the mosasaurs in
general lived in the comparatively shallow waters of the seas, and
not far from the shores. That some were excellent divers, descending
probably many fathoms deep in the water, is certain, because of the
extraordinary protective structures of the eyes and ears.
But the various kinds of mosasaurs differed not a little in their
habits. Some, like _Mosasaurus_ and _Clidastes_, were doubtless chiefly
surface swimmers, as is evidenced by their better ossified bones,
firmer articulation, and the presence of the additional zygosphenal
articulations of the vertebrae, wanting in other forms, as also by the
structure of their paddles. They had a relatively long body and short
tail, the tail having a more pronounced distal expansion than in the
case of other forms, and the eyes looking laterally, not at all upward.
The feet, as shown in Fig. 74, were broad and short, with most of the
wrist and ankle bones well ossified, and with but few extra bones in
the digits. _Tylosaurus_ (Fig. 79), on the other hand, had a more
slender skull, the nostrils were situated farther back from the tip of
the snout, the tail was longer and more powerful, and the feet were
very highly specialized (Fig. 75). The wrist and the ankle were almost
wholly cartilaginous, the fifth finger and fifth toe were much longer,
and the number of phalanges was greatly increased. Moreover, the bones
of the skeleton are more spongy, the joints are more cartilaginous,
and the ears were better protected by a heavy coat of cartilage. In
most of these respects the genus _Platecarpus_ was intermediate between
_Clidastes_ and _Tylosaurus_ (Fig. 76).
Like nearly all other lizards, the mosasaurs had a pineal opening
in the skull, but it is not at all probable that they possessed a
functional pineal eye.
Many and varied have been the opinions of scientific men regarding
the relationships of these animals, as has been intimated. They were
thought to be a kind of whale or breathing fish by Peter Camper;
crocodiles, by St. Fond; and aquatic lizards, by Adrian Camper and
Cuvier. The late Professor Cope thought they were more nearly related
to the snakes than to the lizards, and that they might even have been
the ancestral stock from which the snakes have descended. Because
of this belief he gave to them the name Pythonomorpha, meaning
python-like, and this name, really the first ever applied to them, is
yet often used instead of Mosasauria. A more complete knowledge of
the mosasaurs, however, and especially the recent discoveries of the
semiaquatic connecting links, called the aigialosaurs and described
on a preceding page, have set at rest all doubt as to their real
affinities. They are real lizards, differing less from the living
monitor land lizards than do the monitors from some other land lizards,
especially the amphisbaenas and chameleons. And to Adrian Camper is
due the credit for the recognition of their real relationship, though
it required more than a century to prove that he was right.
[Illustration: FIG. 80.—_Globidens alabamensis._ Part of mandible, with
teeth. (From Gilmore.)]
Very recently, and since the foregoing was written, a remarkable new
type of mosasaurs has been discovered in Alabama and Europe. Only
fragmentary jaws, a few vertebrae, and some skull bones are known,
so that it is impossible yet to decide how closely the new form is
related to the true mosasaurs, but so far as the evidence goes the
only distinguishable character is the teeth. These, instead of being
elongated and pointed, are nearly spherical, as shown in Fig. 80. Such
teeth could have been used only for crushing shell-fish, and not at all
for the seizure and retention of slippery fishes. The genus, which was
called _Globidens_ by its discoverer, Mr. Gilmore, includes two known
species, from Alabama and Europe, the latter recently described by
Dollo. It has been suggested that this peculiar kind of dentition was a
more primitive or intermediate one, a kind that the first mosasaurs had
before they became fully adapted to the water; but this is doubtful,
since _Globidens_ comes from late Cretaceous, and must be one of the
later types. If _Globidens_ is a true mosasaur, and it seems to be one,
its life-habits must have been remarkably different from those that
have long been known. Possibly when the limbs and more of the skull are
found, _Globidens_ will prove to be of a distinctive type.
SNAKES
The chief differences between snakes and lizards have already been
given and need not be repeated, save very briefly. Snakes are always
functionally legless, though some have vestiges of the hind pair; the
brain-case is wholly bony; the upper temporal bar is wanting; the
lower jaws are united in front by ligaments only, like those of the
mosasaurs; the vertebrae are greatly increased in number, and always
have the additional zygosphenal articulations like those of _Clidastes_
and _Mosasaurus_ and some lizards; there is but one lung, and the eyes
are always without free eyelids. But these characters are really not
very important, since every one of them is found in the lizards or
mosasaurs, except the complete ossification of the brain-case, and even
this is partly ossified in the mosasaurs. It is rather the presence of
all these characters which distinguishes a snake from a lizard.
The number of living snakes is nearly as great as that of the living
lizards, and their distribution over the earth is very similar. Snakes
are for the most part strictly terrestrial in habit. Some live more
or less among trees, and some live in the water, though with but few
exceptions all are fully capable of rapid progression upon land. They
are almost invariably carnivorous in habit, swallowing their prey
whole, and usually alive, as has been described. Some poison their prey
or crush it to death before swallowing it. Some feed upon eggs which
are swallowed whole and then crushed in their stomachs by projecting
bones from the under side of the vertebrae developed for that purpose.
In size snakes vary from a few inches in length to twenty-five or more
feet, no known extinct forms being larger than the living anacondas
and boas. In geological history the earliest remains known date from
the latter part of the Cretaceous, and it is quite probable that they
have a briefer history than that of the lizards of which they are the
descendants. Venomous serpents are known only from comparatively recent
geological times, and it is probable that venomosity is the latest and
final specialization of importance in the reptilian class.
[Illustration: FIG. 81.—_Hydrus bicolor_; sea-snake. (From Brehm)]
Of strictly aquatic snakes there is no known geological history, and it
is improbable that there is any such history. There are a few snakes
now living—very venomous ones, allied to the deadly cobras—which
have become so completely adapted to life in the water that they are
unable to exist or even move about on land. These are the well-known
sea-snakes of the Indian Ocean and adjacent waters. Perhaps the most
highly specialized and typical of these is the black-banded sea-snake,
_Distina cyanocincta_, which reaches a length of four or five feet,
and is a rapid and excellent swimmer. From the figure (Fig. 81) it is
seen that the body is very much flattened from side to side, and lacks
or has but a few vestiges of the transverse scales on the under side
so characteristic of all other snakes, and which enable them to move
about on land. So helpless are these snakes on land that it is said
sailors will handle them carelessly, because of their inability to bite
while out of water, though the bite is very venomous. They never come
on land for any purpose whatever, and their young, unlike those of
most other snakes, are born alive. There are a number of species of
these sea-snakes, though comparatively little is known of their habits.
They are of especial interest as another example of the ways in which
air-breathing land vertebrates have become adapted to water life. The
adaptation, however, was simple, for nearly all snakes swim freely in
water by undulatory movements; it would require not much change to
convert an ordinary water snake into one like these sea-snakes.
CHAPTER XII
THALATTOSAURIA
Millions of years before the first appearance of the mosasaurs in
geological history, another group of reptiles showing many curious
resemblances to them attempted a rather precarious existence in the
water. Its members survived long enough to acquire many structural
adaptations to a water life, long enough to become diversely modified,
but not long enough, apparently, to wander far from their birthplace,
not long enough to attain that security from their enemies and more
ambitious competitors, the early ichthyosaurs and plesiosaurs, to
insure them a long existence. They were only a partial success as water
reptiles.
It has been only within a few years that we have had any knowledge
whatever of them, and that knowledge is still very incomplete, too
incomplete to justify any attempt to picture them as living animals,
even though we take the liberties that some of our illustrators
of extinct animals feel warranted in assuming. The first known
specimens of these “sea-reptiles”—for that is the meaning of the word
Thalattosauria—were discovered and described by Professor J. C. Merriam
less than ten years ago, and all our knowledge of these animals is due
to the same author, who has studied attentively the known specimens,
all of which are preserved in the museum of the University of
California. The first discovered fragmentary specimens were confounded
with those of early ichthyosaurs, from the Upper Triassic rocks of
northern California with which they were associated. No specimen has
yet been found that is even approximately complete; some parts of the
skeleton are not yet known, even from fragmentary remains, and not till
other and more complete specimens have been found will it be possible
to determine the real form of the living animals or to decide what
their nearest relationships with other reptiles were. Professor Merriam
thinks that they were related most closely with the Rhynchocephalia
(p. 176) of which the _Sphenodon_, or tuatera, of New Zealand is the
only living representative, but whose direct genealogical history runs
back nearly or quite to the time in which the thalattosaurs lived. On
the other hand, there are so many resemblances to the mosasaurs shown
in the remains that have been discovered, that it is possible the
thalattosaurs were only a short-lived branch of the primitive lizards,
which we also know were in existence at the time when the thalattosaurs
lived. However, even though they resembled the mosasaurs, there could
have been no direct genealogical relationships between them, for it is
quite certain that the thalattosaurs very soon went out of existence,
leaving no descendants. But it matters little which were the land
forbears of the thalattosaurs; they present such distinct adaptations
to water life—characters all their own—that their ancestral kinship may
well be left to the future researches of the curious paleontologist.
For the present, at least, they may well be placed in an order
of reptiles all their own, as Professor Merriam has proposed—the
Thalattosauria.
[Illustration: FIG. 82.—Skull of _Thalattosaurus_. (After Merriam)]
No thalattosaurs were large animals. If they had the same proportions
between the lengths of head, body, and tail as the mosasaurs, none
exceeded seven feet in length, and they may have been even shorter,
though probably not much. The figure of the skull, as restored by
Professor Merriam, shows many striking aquatic adaptations, in the
elongated, pointed muzzle, in the large external nostrils, situated
far back toward the eyes, and in the well-ossified ring of bones
surrounding the eyeball. There is a parietal opening in the roof of
the skull, as in the modern lizards and tuatera; but it is not known
for certainty whether there were two openings on each side in the
roof of the skull, as in the modern tuatera. While this character may
seem trivial, it is really one of the most important in the reptilian
anatomy in determining the relationship and classification of reptiles.
The teeth are conical and pointed in the front end of the upper and
lower jaws, but farther back they are rounded, rugose, and obtuse, and
could have been used only for crushing hard objects, like mollusks,
crustaceans, etc. (Fig. 82). And not only was there a row of such teeth
on each jaw (only partly seen in the figure), but similar teeth covered
a large part of the palate. And the lower jaws, it is seen, are rather
massive.
The vertebrae were, of course, of the more primitive kind, that is,
with the ends concave, both in front and behind. It would have been
strange indeed were they of any other kind, since reptiles with
ball-and-socket joints to the vertebrae, that is, concave on one end
and convex on the other, as in nearly all living reptiles, did not come
into existence till long after the thalattosaurs had disappeared from
geological history; and it is also a curious fact that such vertebrae
appear to have originated only among animals crawling on land, so that
they would not have been a character acquired by the thalattosaurs
after descending into the water. It will be seen from the figure of
a dorsal vertebra that the rib was attached by a single articular
surface, almost exclusively to the body of the vertebra, quite like
those of all lizards, snakes, and mosasaurs, and unlike those of
other reptiles. This too may seem to be a trivial character to prove
relationships with the lizards, but it is a curious fact that no two
animals having different kinds of ribs are closely related to each
other. Possibly, however, this looser mode of attachment of the ribs
in the thalattosaurs was one of their peculiar adaptations to a water
life, and may not have been derived from their land ancestors.
[Illustration: FIG. 83.—Dorsal vertebra of _Thalattosaurus_. (After
Merriam.)]
[Illustration: FIG. 84.—_Thalattosaurus_: bones of front extremity:
_s_, scapula; _c_, coracoid; _h_, humerus; _r_, radius; _u_, ulna.
(After Merriam.)]
Of the limbs, only a few bones are known, but these are very
instructive. The arm bones, as shown in Fig. 84, are strikingly like
those of the mosasaurs, as will be seen by comparing the figure on p.
157. The humerus is a little more elongated than that of the mosasaurs,
more nearly like the mosasaurian femur. The shoulder-blade and the
coracoid are imperfectly ossified, as is seen from the figure—another
characteristic of aquatic life. What the fingers and toes were like
cannot be said; probably they were bound together by membrane, forming
swimming paddles similar to those of the mosasaurs. Some of the bones
referred to the pelvis are known, but it is not known whether they are
united to the spinal column by a sacrum, as in land animals. Nor is
anything certainly known of the hind leg or much of the tail. Since the
front legs show marked aquatic adaptations, it is altogether certain
that the hind legs will be found to be modified more or less, though
not so much modified as the front legs, because, as we have seen, the
front legs are always more specialized in aquatic animals than the
hind ones, even as the hind legs are more specialized than the front
ones in land animals. Possibly the hind legs will be found to be more
like those of the Thalattosuchia, as shown on p. 212, that is, partly
terrestrial in character. Doubtless the tail was long and flattened,
possibly with a terminal fin-like dilation, though this is less
probable.
As regards the habits and food of the thalattosaurs, no better summary
can be given than that of Professor Merriam, in his own words:
The remains of thalattosaurs are known only in purely marine
deposits containing little or no material of terrestrial
origin. They are associated with a fauna consisting of
numerous forms, both vertebrate and invertebrate, which
are not known to have existed away from marine areas. In
the structure of the skeleton we find the abbreviated and
broadened proximal segments of the limbs, the slender snout
with prehensile terminal teeth, and the median superior
nostrils, indicating a purely aquatic type. There can
scarcely be room for doubt that the thalattosaurs as a group
were typical marine forms. The larger and more specialized
species comprised in the genus _Thalattosaurus_ were
strictly natatory. They may have visited the shore, but,
like the plesiosaurs, were better fitted for swimming
than for crawling. Of the smaller _Nectosaurus_
we unfortunately do not know the limbs. They may have
been considerably less specialized, and the animal
to a correspondingly greater degree a shore-dweller.
_Nectosaurus_ is, however, found in the same deposits
with other forms and appears to be as common as the others;
so that it is safe to consider it as having passed the
greater part of its life away from the shore.
From what we know of the vertebral column of
_Thalattosaurus_ it appears that the animal had
a relatively short neck and a long dorsal region, the
proportions being nearly those in the vertebral column of
some mosasaurs. Only the anterior portion of the caudal
region is known. The slender, rounded neural spines with
well-developed articulating processes seen here are not
such as commonly appear in forms with a highly specialized
sculling tail, and it is hardly probable that a caudal fin
of large size was developed.
The anterior limbs evidently formed paddles of moderate
size. The posterior pair may have been larger, in
compensation for lack of a strong sculling tail. It is,
however, possible, that as in _Geosaurus_ (of the
thalattosuchian crocodiles) the hind limbs were not
typically natatory, and the distal end of the tail was
vertically expanded.
No specimens have yet been found which are well enough
preserved to show any remains of the stomach contents,
and we have no definite evidence concerning the food of
the thalattosaurs, more than is furnished by the general
structure of the animal. The character of the paddles, the
form of the skull, and the presence of slender prehensile
teeth in the terminal portions of the jaws would indicate
that they fed in part upon some swiftly moving prey which
was caught by a quick snap of the jaws, deglutition being
assisted by the curved teeth of the pterygoids. The heavy
vomerine and posterior mandibular teeth may have been used
for crushing the light shells of ammonites, which existed in
vast numbers in the same seas.
CHAPTER XIII
RHYNCHOCEPHALIA
In some of the small islands near the northeast coast of New Zealand
certain small and peculiar, lizard-like reptiles, known as tuateras,
have long been known. For many years they were supposed, even by
scientific men, to be real lizards, so much do they resemble in
external appearances and in habits the lizards of other parts of the
earth. It was early observed, however, that they presented certain
remarkable internal differences from the real lizards or Lacertilia,
though it was not till about twenty-five years ago that the importance
of these differences was recognized by the late Professor Cope, who
separated them into a distinct order quite co-ordinate with the
lizards, crocodiles, and turtles. These little reptiles, seldom
reaching a length of two feet, have now become so scarce that the New
Zealand government protects them by law from unnecessary destruction;
nevertheless it will probably be only a short time before they become
extinct, the end of a long genealogical line. No other living reptiles
have retained more of the old-fashioned or primitive characters than
this _Sphenodon_ or _Hatteria_, as the animal is called, and because
of them it is of peculiar interest to zoölogists, and especially
paleontologists.
The differences of these beaked lizards from the true lizards are
especially noticeable in the skull, and more especially in the
arrangement of the bones which give articulation to the lower jaws
(Fig. 8). In the lizards and snakes the quadrate bone is loosely
articulated at its upper end with the cranium, and has no inferior
bar or arch connecting its lower end with the jugal and the back part
of the upper jaw. _Sphenodon_, on the contrary, has the quadrate
bone firmly fixed to its adjacent bones at both ends, and is quite
immovable. The vertebrae are biconcave like those of all early
reptiles, not concavo-convex as are the vertebrae of most other living
reptiles. The intercentra or hypocentra, little wedge-shaped bones
between the centra below, are more persistent in _Sphenodon_ than
in any other living land animals except the gecko lizards. Upon the
whole the tuatera is the most old-fashioned of living reptiles, and in
consequence it has nearly lost out in competition with new things.
[Illustration: FIG. 85.—_Sphenodon punctatum_, or tuatera. (From
specimen in the Yale University museum.)]
With these living tuateras we have nothing further to do, since they
are land animals, living about the beaches of the New Zealand islands,
and only occasionally venturing into the water, hiding from their
enemies in the holes in the rocks. But, from some of their antecedents,
from some of their direct forbears perhaps, there have gone off at
different times various branches, whose descendants wandered into
foreign lands or into foreign places, and lived and flourished for a
brief time and then became extinct. Some of these went down into the
water and became more or less aquatic in habit; some, indeed, changed
their forms and habits so greatly that they are often, perhaps rightly,
segregated into different orders. Whether or not they should be called
Rhynchocephalia matters little, however. It is merely a matter of
opinion as to how great the changes should be in order to entitle the
offspring to a genealogical tree all its own. Of these branches there
are two, whose relationships seem to be definite, the Choristodera and
Thalattosauria, though there is more doubt about the latter than the
former. A third group, that included _Pleurosaurus_, seems, from more
recent discoveries, to belong to a different line of descent and has
been described under the Protorosauria.
In the direct line of ancestry there is no known form that was
distinctly aquatic. The oldest known of these, perhaps, is that shown
in Fig. 86, _Sapheosaurus_ from the Jurassic of Solenhofen. Its
resemblance to the modern tuatera is great, and doubtless its habits
were very similar, though its rather long tail and rather short neck
possibly indicate subaquatic habits.
[Illustration: FIG. 86.—_Sapheosaurus_, an Upper Jurassic
rhynchocephalian. (After Lortet.)]
CHORISTODERA
Among the many reptiles of the past which have sought a more congenial
or a safer home in the water few have had a more interesting history,
or a briefer one, than those to which the late Professor Cope gave
the name Choristodera in 1876. Many students of repute consider the
group an order, others a suborder of the Rhynchocephalia. The group,
whether order or suborder, are interesting because of their long and
devious migrations from western North America to Europe, or vice versa,
through rivers and ponds; interesting also because of the persistence
of certain old-fashioned traits that clung to them long after their
disappearance in other animals. Perhaps these traits were among the
causes of their merely moderate success as animals of the water, traits
that led to their early dissolution. Like the proganosaurs, which they
must have resembled in external appearance not a little, they wandered
from their birthplace in the western continent, to perish in the
eastern; and like them their span of existence was short.
Their history among mankind, too, is brief. The first known specimens,
from western North America, were described by Professor Cope in 1876,
under the name _Champsosaurus_. In the following year Professor
Gervais of Paris made known another form from Rheims, which he called
_Simoedosaurus_, so closely allied to the American that even yet
they have not been sharply distinguished. Some years later these
European specimens were more fully described by the well-known Belgian
paleontologist, Dr. Dollo, but it has been only within the past few
years that our knowledge of the animals has been made at all complete
by the discovery and description of several excellent skeletons of
_Champsosaurus_ by Mr. Barnum Brown of New York.
[Illustration: FIG. 87.—_Champsosaurus_; skeleton, as mounted in
American Museum. (Brown.)]
These semiaquatic reptiles never grew very large—not more than four
or five feet in length; nor did they ever succeed in becoming fully
at home in the water, certainly no more so than our modern alligators
and crocodiles. They remained to the end of their comparatively brief
existence essentially land animals, probably seeking their food in the
water but fleeing to the land for protection and for the breeding of
their young. Their chief water adaptations are seen in the elongate
face and flattened swimming tail. Their legs remained essentially
terrestrial, and could have been of but little use in the water for
propulsion; the feet even were doubtfully webbed, or if so, not more
than are those of the alligator. Singularly, like the proganosaurs,
their ribs were heavy and stout, also suggesting bottom-crawling
habits, like those of the living Galapagos lizards. The skull was
lightly built, and the face was long and slender, like that of the
gavials and proganosaurs; but, like those of the former and unlike
those of the latter, the nostrils were situated at the extreme tip.
The hind legs were firmly attached to the body by the sacrum; and no
sclerotic bones of the eyes have been discovered. The neck was neither
unusually long nor unusually short. The body was probably covered with
horny scales.
[Illustration: FIG. 88.—Restoration of _Champsosaurus_.]
[Illustration: FIG. 89.—_Champsosaurus_; skull from above. (After
Brown.)]
To the student of paleontology these animals are of interest because of
the retention of several primitive traits which had long disappeared in
other known reptiles. While the vertebrae had ceased to be perforated
by the notochord, as in the early reptiles, they were still shallowly
biconcave. The first bone of the neck, the atlas, had changed but
little from that of their very ancient forbears of Permian times, and
the bones of the palate still retained numerous teeth scattered over
it, like those of the same Paleozoic ancestors. Most primitive and
old fashioned of all was the pelvis, which was so unlike that of all
known contemporary or later reptiles that, were a paleontologist to
see it without knowing whence it came, he would be almost sure to say
that it belonged to a Paleozoic, or at least a Triassic, reptile, and
not only to an early reptile but a very primitive one at that. This
peculiarity consists in the absence of any opening between the ischium
and pubis, which is characteristic of every living vertebrate with
legs. And these and other old-fashioned characters could not possibly
have been new developments; they must have existed in all the ancestors
of the Choristodera from Paleozoic to early Tertiary times, though not
a single other reptile is known to have possessed them, for the greater
part of this time. Perhaps when Asia and northwestern America have
been more thoroughly explored for vertebrate fossils, some of their
ancestors which perished on their great migration from the western to
the eastern continent in late Cretaceous times will be discovered.
[Illustration: FIG. 90.—Pectoral girdle of _Champsosaurus_. (After
Brown.)]
[Illustration: FIG. 91.—_Champsosaurus_; pelvic bones. (After Brown.)]
The choristoderans began their existence, so far as is now known, in
North America in late Cretaceous times and died out in both Europe
and North America in early Tertiary times. That is, they were one
of the few branches of reptilian life which not only witnessed the
extinction of the great dinosaurs and plesiosaurs, but the advent also
of the early placental mammals. They lived millions of years after the
proganosaurs became extinct, and, similar as they are in form, there
is no relation between them. Moreover, in all probability they did not
migrate to the eastern continent over the same route.
The structure of the head and teeth of the Choristodera clearly
indicates a fish-eating habit, or at least a diet of soft-bodied, free
swimming invertebrates. The legs and ribs, as also the armor of ventral
ribs, like those of the plesiosaurs, point very insistently toward a
bottom-crawling habit while in the water.
CHAPTER XIV
PARASUCHIA
The first known specimen of the order of reptiles now generally
known as the Parasuchia was found in Würtemberg, Germany, in 1826
and very briefly and inadequately described[4] two years later by
Professor George Jaeger. The specimen was a sorry one, and was sadly
misinterpreted by Jaeger. It consisted chiefly of casts of the alveoli
or sockets of a number of teeth, more or less connected by corroded or
decomposed portions of the jaws. He recognized the casts as teeth of
a peculiar reptile, but mistook the roots for crowns, and, naturally
concluding that such obtuse teeth would be of use only for the
mastication of vegetable food—about the last kind of food to which the
phytosaurs were addicted—called the animal _Phytosaurus_, meaning plant
saurian. Because of differences he observed in the shapes of the teeth
he thought that they belonged to two distinct species, which he called
_cylindricodon_ and _cubicodon_; but the differences were due simply to
the different positions they held in the jaws.
[4] “The author showed drawings and some specimens of two hitherto
unknown reptiles from the white, coarse-grained sandstone, of which one
in the form of the skull resembles the gavial, but is distinguished by
the cylindrical form of the lateral teeth of the jaws; he therefore
calls it provisionally _cylindricodon_, and a second species or genus,
of which, however, so far only fragments of the jaws have been found,
because of the four-cornered form of the teeth, _cubicodon_, while at
the same time for the genus or family, to which the remains of these
animals have belonged, he proposes the name _Phytosaurus_, since the
teeth seem to be more adapted to a vegetable diet, even though they
have not been worn away, as in _Iguanodon_.”—_Isis_ (1828), p. 441
(translation).
Fourteen years later Hermann von Meyer, the renowned German
paleontologist, described and figured other remains of the same or an
allied reptile under the name _Belodon plieningeri_. In subsequent
papers during the next twenty-three years von Meyer very fully
described and beautifully illustrated the skulls and other remains
of this and other species, all of which he referred to the genus
_Belodon_, the name by which for many years the animals were generally
known in scientific literature. Von Meyer thought that he recognized in
_Belodon kapfii_, the species most often figured in textbooks, the same
animal that Jaeger had previously described.
Von Meyer was not at all certain about the relationships of his
_Belodon_, though he recognized its affinity with the crocodiles. It
was Huxley who, in a famous paper on the evolution of the crocodiles,
published in 1875, united _Belodon_ and another genus from the Trias
of Scotland, which he called _Stagonolepis_, with the Crocodilia as
representatives of the suborder Parasuchia, one of the three into which
he divided the order. Huxley admitted that the relationships between
the Parasuchia and the Mesosuchia or Eusuchia, the other suborders
which he proposed, were not as intimate as those between the latter
two, which were separated solely on the structure of the palate and
vertebrae, as has been explained in chap. xv. As early as 1869 the
late Professor Cope recognized certain forms which had been previously
described from Carolina as belonging to the group, calling them
_Belodon_, but it was not until 1896 that E. Fraas separated _Belodon
planirostris_ of von Meyer as a member of a distinct genus, to which he
gave the name _Mystriosuchus_.
Here, as a part of the order Crocodilia, the phytosaurs remained
till within very recent years, though there have been some mild
protests against the association, especially by Marsh, Zittel, and
Baur. The famous English paleontologist, Richard Owen, located the
“Belodontia,” as the phytosaurs were often called, in his order
Thecodontia, based chiefly upon the manner of the insertion of the
teeth in sockets. But this has long since been shown to have little
value in the classification of reptiles. Various authors have written
about the phytosaurs in later years, notably Cope, Fraas, Huene, and
Jaekel, but it was J. H. McGregor who first definitely separated the
phytosaurs into a distinct order, in a careful revision of the American
forms. He called the order Parasuchia, after Huxley, dividing into
two suborders, the Phytosauria, after Jaeger, and the Aetosauria, a
group which, for lack of a better place, had previously been classed
with the Crocodilia, either as a member of the Parasuchia or as an
independent suborder by Zittel (the Pseudosuchia). More recently
Huene has shown that certain African reptiles from the Lower Trias
had certain very definite characters entitling them to an independent
position, for which he proposed the order Pelycosimia. Upon the whole,
however, these characters seem to be primitive parasuchian, and the
group may provisionally be placed in the order Parasuchia, as a third
suborder, the Pelycosimia. The order Parasuchia, then, until we know
much more about the latter two groups, may be conveniently divided into
three suborders, the Phytosauria, Aetosauria or Pseudosuchia, and the
Pelycosimia, all of Triassic age.
McGregor was quite right in retaining for the suborder the name
Phytosauria, suggested by Jaeger in 1828, inappropriate as the word is
etymologically, but was hardly justified in substituting the generic
name _Phytosaurus_ for the long and well-known _Belodon_, because it
is quite impossible to say that Jaeger’s very fragmentary specimen
upon which he based the genus _Phytosaurus_ really is the same as
_Belodon_. Professor Fraas very kindly showed the writer the original
type-specimen of Jaeger, now preserved in the Stuttgart Museum,
and both are agreed that it is impossible to prove the identity of
_Belodon_ and _Phytosaurus_ from the very fragmentary and imperfect
specimen. It is quite as probable, for instance, that _Phytosaurus_ and
_Mystriosuchus_ are identical as that _Phytosaurus_ and _Belodon_ are.
Unfortunately, this is not the only case in vertebrate paleontology
where the fragmentary specimens to which names have been given are
inadequate to determine the species, or genus, or even the family
to which they belong; there have been very many such instances. The
pioneers in paleontology were often justified in naming small and
obscure fragments of bones, or single bones. One would be justified
even yet in giving a name to an indeterminable fragment of a bird bone
from the Triassic formation, because the discovery of a bird of any
kind in that formation would be very important for science, even if its
precise kind might never be recognized from the specimen. Nevertheless,
the custom is a very reprehensible one when indiscriminately followed.
For these reasons the writer disagrees with McGregor in substituting
the inappropriate name _Phytosaurus_ for _Belodon_, the name by which
the most typical forms were so long known.
The Aetosauria, which have long been known from a marvelous specimen
found in Würtemberg many years ago and described by the elder Fraas,
need not detain us long. They were relatively small reptiles about
two feet long, almost completely incased in a bony armor, and purely
terrestrial in habit. The skull even yet is not perfectly known, and
it is possible that when it is the group may have to be dissociated
from the phytosaurs. The nostrils were not posterior, and the skull is
short. Other specimens of the same group have been described from the
Upper Triassic rocks of Massachusetts.
The Pelycosimia of Huene are very interesting as showing apparently
primitive forms with which the true phytosaurs may have been intimately
related ancestrally. They, too, have a rather short skull with the
nostrils in front, and were not at all aquatic in habit. Not much is
known about the single genus that is located in the group, aside from
the skull and a few limb bones.
PHYTOSAURIA
The Phytosauria, so far as known, were all reptiles of considerable
size, greatly resembling the crocodiles, and especially the gavials
in form and habit, but differing very greatly in having the external
nostrils situated far back near the eyes; in having no false palate
so characteristic of the Crocodilia; in having a more primitive
shoulder-girdle, consisting of a short coracoid, interclavicle, and
clavicles; and in having the ordinary type of pelvis, that is, with
the pubis entering into the acetabular articulation for the femur.
They were all, like the crocodiles, covered more or less by a bony
armor; there are two openings on each side of the temporal region;
there is no pineal opening; the vertebrae are gently biconcave,
precisely like those of the early or mesosuchian crocodiles; there
is always an opening of considerable size, called the preorbital
foramen, in front of the eyes, as in some crocodiles, many dinosaurs,
and most pterodactyls; there is also an opening through the back
part of the mandibles as in crocodiles; and the double-headed ribs
are attached exclusively to the transverse process of the arch,
precisely as in the crocodiles, dinosaurs, and pterodactyls. From
all these it is evident that the phytosaurs are related most nearly
to the crocodiles and dinosaurs, and are probably an early branch
of the stem from which they, the pterodactyls and the birds, arose,
a branch that persisted only a short time, geologically speaking,
and went entirely out of existence at the close of Triassic times,
leaving no descendants behind. Nevertheless, in this comparatively
brief life-span they developed not a few distinctive forms and became
widely distributed over the earth. Their remains are known from the
Upper Trias of Germany, England, and Scotland, India, South Africa,
and from Massachusetts, North and South Carolina, and many places
in the Rocky Mountains. No true phytosaurs are yet known from South
America, but in all probability they will be discovered there when
the Triassic deposits of that continent have been better explored
for fossils. In the Rocky Mountains, especially, their remains are
widely scattered, they have been found in many localities in Wyoming,
Colorado, Oklahoma, Utah, and New Mexico. Though for the most part
their known remains from these localities are yet fragmentary, not
less than four distinct genera have been described from these regions:
“_Belodon_,” _Angistorhinus_, _Paleorhinus_, and _Episcoposaurus_.
From the Carolinas and Massachusetts a single genus, though described
under numerous names, has been made known, originally called by Emmons
_Rutiodon_ (_Rhytidodon_). And from Europe and India at least as many
more different genera are known. All these genera are, however, so
closely allied that they are placed in the single family Belodontidae.
[Illustration: FIG. 92.—Restoration of _Mystriosuchus_, an Upper
Triassic phytosaur.]
[Illustration: FIG. 93.—Skull of _Mystriosuchus_, a phytosaur: _pm_,
premaxilla; _m_, maxilla; _na_, nasal; _f_, frontal; _p_, prefrontal;
_l_, lacrimal; _pf_, postfrontal; _po_, postorbital; _pa_, parietal;
_sg_, squamosal; _qj_, quadratojugal; _pl_, palatine; _t_, transverse;
_in_, internal nares; _en_, external nares; _pt_, pterygoid; _bs_,
basisphenoid; _eo_, exoccipital. (After McGregor.)]
[Illustration: FIG. 94.—Dorsal vertebrae of phytosaur: _az_, anterior
zygapophysis; _pz_, posterior zygapophysis; _d_, _c_, articulations of
rib.]
[Illustration: FIG. 95.—Scapula and coracoid of _Rutiodon
carolinensis_, an American phytosaur. (After McGregor.)]
[Illustration: FIG. 96.—_Belodon_; restoration of head, from above.]
[Illustration: FIG. 97.—_Mystriosuchus_: restoration of head, from
above.]
In _Belodon_ (Fig. 96), the earliest known and most typical genus,
perhaps, the moderately elongated face has a high crest reaching nearly
to its front end, and this type is known both from Europe and from New
Mexico. Others have the face long and slender, even longer and more
slender than in the ancient teleosaur crocodiles or the modern gavials.
In some forms the teeth are cylindrical and slender throughout, and
there may be as many as fifty on each jaw, or two hundred in all; while
in others only the anterior teeth are cylindrical and the posterior
teeth are flattened and serrate along their cutting edges. In the body
not very great differences have been observed. Some are more slender
than others, and there are minor differences in the shapes and sizes
and numbers of the bony scutes along the back and on the throat.
But they are all alike in their essential characters, a very long
beak with numerous teeth; the foremost ones on the expanded, more
or less spoon-shaped front extremity, are more or less, sometimes
greatly, elongated. The jaws may be likened to a long and slender
pair of tongs with nipping teeth at the front end. The strong, long,
and flattened tail is sufficient evidence that the phytosaurs were
excellent swimmers, but, aside from that and the posterior location of
the external nostrils, directly over the internal, few other aquatic
adaptations are observed in the skeleton. There are no sclerotic bony
plates about the eyes, or at least none have so far been discovered,
although among the numerous known specimens they would confidently be
expected were they really present in the skeleton; and the presence of
bony armor negatives markedly aquatic habits.
Doubtless on the whole the habits of the phytosaurs were not very
unlike those of the modern gavials, which they so strongly resemble
in form, size, and general characters. But they differ very greatly
from the gavials in the extreme posterior position of the nostrils,
and in the greatly elongated teeth of the front end of the beak,
teeth which must have had some especial and peculiar use. Nor is the
position of the nares to be accounted for satisfactorily by reference
to aquatic habits. It has been suggested that the creatures used the
very long and slender beak in prodding and probing in the sand and
mud for soft-bodied invertebrates, worms and the like, for which the
teeth would be especially fitted; and that the posterior position of
the nostrils may be in part, perhaps wholly, accounted for by this
habit, which permitted the reptiles to breathe without extricating the
beak from the mud or shallow waters. That the animals were wholly and
intensely carnivorous in habit is attested by their teeth; although
they are called “plant saurians,” they never had anything to do with
plants in the way of food. Unfortunately so far no specimens have
ever been found showing the remains of stomach contents, nor have any
been found showing impressions of the form of the body or of any of
its parts. Until such specimens are found, as they doubtless will be
eventually, one can be less sure of the precise details in their life
reconstructions. However, the skeleton is now known nearly completely,
and this suffices to give a very approximately correct idea of what the
animals were like when alive.
CHAPTER XV
CROCODILIA
The order of reptiles to which the name Crocodilia is technically
applied comprises less than twenty-five living species, popularly
known as crocodiles, alligators, caimans, and gavials. They are often
of great size, ugly and repulsive in appearance, cruel and vicious
in habit, wholly carnivorous, and denizens, almost exclusively, of
fresh-water lakes or rivers in tropical and subtropical regions; a few
only venture into the sea near the shores. They are all excellent and
powerful swimmers, but are by no means exclusively aquatic in habit,
many of them spending a large part of the time on the shores; and they
invariably seek the land for the deposition and hatching of their eggs.
In size they are the largest of living reptiles, some of the existing
species reaching a length of twenty-five feet, while some extinct
species were probably fully twice that length.
The geological history of the crocodiles is a very ancient one,
reaching back at least as far as the early Jurassic and probably to
the Triassic. The culmination of the order, at least so far as size,
variety, and numbers are concerned, was doubtless before the close of
the Mesozoic. The early crocodiles, however, were of a more generalized
structure in some respects, though specialized in others, because of
which naturalists in the past have usually divided the order into three
or four chief subdivisions, or suborders, two of which, the Mesosuchia
and the Thalattosuchia, became extinct before or during Cretaceous
time. The third suborder, the Parasuchia of many textbooks, has now
been unanimously separated by paleontologists from the Crocodilia as an
independent order. The fourth suborder of the textbooks, the Eusuchia,
a word meaning _true_ crocodiles, appeared in geological history, so
far as we yet know, shortly before the extinction of the Mesosuchia,
and is best known from the forms now living, all of which belong to
it. Although the modern crocodiles can hardly be called, as a group,
purely aquatic reptiles, we shall find it of interest, because of their
intimate relation to the older and more strictly marine forms, to speak
of them somewhat in detail.
MODERN CROCODILES, EUSUCHIA
The crocodiles of the present—and we use the word in the technical
sense of Crocodilia—because of their general resemblance to the
lizards, or true “saurians,” were classed with them by the older
naturalists, whence comes the popular name alligator, a corruption
of the Spanish _el lagarto_, or “the lizard,” given to some of
the South American forms by early explorers. But this resemblance
is a superficial one only, as was early recognized by comparative
anatomists. The crocodiles, indeed, are only remotely related to the
lizards.
[Illustration: FIG. 98.—Senegal crocodile. (By permission of the New
York Zoölogical Society.)]
The head or cranium is flattened and broad, the facial part or snout
sometimes greatly elongated and slender, and the external nostrils
are always situated at the front end. The bones of the upper surface
of the cranium and face have many pit-like depressions. The neck is
short and stout, and but little movable. The body is somewhat depressed
and flattened, not cylindrical and slender, as in the more typical
water reptiles. The tail is much elongated and compressed, forming a
powerful means of propulsion in swimming, its breadth being increased
by a vertical row of horny plates above. The limbs are of the ordinary
elongated type—ambulatory rather than swimming legs—and are not of much
use for propelling the body in the water; the front legs indeed are
usually held close to the body while the animal is swimming. The toes,
however, are partially connected by webs, to a slight extent only in
the alligators and crocodiles, but much more so in the long-snouted
gavials. The feet have five toes in front and four behind; and the loss
of the fifth toe can only be ascribed to terrestrial habits. The body
is covered more or less with horny scutes or scales, beneath which are
several rows of thickened, pitted, bony plates on the dorsal side, and
sometimes also on the under side, forming a more or less extensive
bony armor. The eyes have movable lids, as in most lizards, and the
ear-opening is small.
But the external appearance of these reptiles is not sufficient to
distinguish them widely from other groups, and we must resort to
the internal structure, especially that of the skeleton, for the
more essential differential characters. The most crucial of these,
the one which more than any other determines their relationships,
consists in the position and shape of the bone with which the lower
jaw articulates, the quadrate bone, so characteristic of reptiles. As
in the plesiosaurian and ichthyosaurian skulls, it is firmly united
with the adjoining bones, not articulating freely with them, as in the
lizards and snakes. But this fixed relation of the bones is very unlike
that of the plesiosaurs, ichthyosaurs, and turtles, in that it is held
in place by _two_ bony bars or arches, the upper extending forward to
unite with the bones at the back part of the orbit, the lower, with
the hind extremity of the upper jaw. The lower jaws are rigidly united
in front, sometimes for a long distance; they have, almost always,
a hole or opening through the hinder part, known in but few other
reptiles. The bones of the palate are all firmly united, forming a
nearly complete roof, very unlike the condition in the mosasaurs and
lizards. The palate also is very peculiar in the development of a
plate of bone below the nasal chambers, forming a complete bony canal
on each side through which the respiratory air passes far back to the
internal opening of the nostrils above the pharynx, and not, as in
other reptiles—save the immediate ancestors of the mammals—entering the
mouth at the front end. This peculiar arrangement of the air-passages,
so like that of ourselves, has much to do with the water habits of the
crocodiles, as we shall see.
The teeth are conical in shape, and are always restricted to the edges
of the jaws, above and below. They are inserted deeply and firmly in
sockets, and are replaced frequently by new ones growing beneath them,
pushing the older ones out as their usefulness becomes impaired by
injury or by use. In some species there are as many as thirty teeth in
each side of the jaws, above and below, although the broad-headed kinds
have a smaller number.
[Illustration: FIG. 99.]
[Illustration: FIG. 100.
FIG. 99.—Skull of _Alligator mississippiensis_, from below.
FIG. 100.—The same, from above: _bo_, basioccipital; _bs_,
basisphenoid; _f_, frontal; _j_, jugal; _l_, lacrimal; _m_, maxilla;
_n_, nasal; _p_, parietal; _pa_, palatine; _pm_, premaxilla; _pf_,
prefrontal; _pr_, postfrontal; _pt_, pterygoid; _q_, quadrate; _qj_,
quadratojugal; _tr_, transverse.]
The neck is short, as has been stated, but it always includes in living
forms nine vertebrae, a number probably slightly in excess of that
of their terrestrial forbears. By the peculiar mode of attachment of
the short “hatchet-shaped” ribs, much lateral movement of the neck
is prohibited, nor is any very great vertical movement possible. The
vertebrae of the whole column, save the atlas, the second sacral, and
the first caudal—which is a very remarkable anomaly—are concave in
front and convex behind, agreeing in this respect with those of all
other living reptiles, save the turtles, the tuatera, and some lizards.
The ribs of the neck have their two heads attached, one to the body of
the vertebra, the other to the arch, but those of the dorsal region,
though double-headed, have both become attached to the transverse
projection of the arch, a seemingly trivial character, but one which
immediately distinguishes all crocodiles from all other water reptiles,
and from all terrestrial reptiles, indeed, save the Parasuchia,
Pterosauria, and Dinosauria. The pelvis is firmly attached to the
spinal column by two sacral vertebrae.
[Illustration: FIG. 101.—Vertebrae of gavial from the side (cervical),
and from in front (dorsal): _az_, anterior zygapophysis; _pz_,
posterior zygapophysis; _d_, diapophysis; _r_, cervical rib; _c_,
articulation for head; _t_, for tubercle of dorsal rib.]
The collar-bones, or clavicles, are wanting in crocodiles; there is a
slender interclavicle; and the shoulder-blade and coracoid are well
developed (Fig. 102). The bones of the pelvis are loosely united with
each other as they are in most reptiles (Fig. 104). The pubes, the
anterior bones below, unlike those of all other reptiles, do not help
to form the acetabulum or socket for the articulation of the thigh
bone, nor do they articulate with each other. This single character
sharply distinguishes a crocodile from all other reptiles, living or
extinct, and is almost the only character that separates the order from
the dinosaurs, aside from the peculiar structure of the nasal passages
in the skull. On the under side of the body, connected with the front
end of the pubes, there are seven or eight pairs of abdominal ribs,
corresponding to the plastron of the turtles and similar to those of
the ichthyosaurs and plesiosaurs. The mosasaurs have no such ribs.
[Illustration: FIG. 102.]
[Illustration: FIG. 103.
FIG. 102.—Scapula (_sc_) and coracoid (_cor_) of gavial.
FIG. 103.—Front foot of crocodile: _u_, ulna; _r_, radius; _re_,
radiale; _ue_, ulnare; _p_, pisiform.]
Furthermore, the crocodiles differ from all other living reptiles in
having a four-chambered heart, like that of birds and mammals, that
is, a heart with two auricles and two ventricles. This more perfect
structure of the circulatory organs does not, however, insure at all
times a complete separation of the pure or arterial blood from the
impure or venous blood, since the blood may be more or less intermixed
outside of the heart by a connection between the venous and the
arterial systems. Whether these imperfectly developed organs, so
suggestive of a higher and more perfect mode of respiration, are the
vestiges of what were once among some reptiles functional structures,
or whether they are rudiments of a higher organization, developing
independently in these creatures, cannot be positively determined,
but it seems very probable that, far back in geological times, some
reptiles, especially the pterodactyls and dinosaurs, had their
respiratory and circulatory systems more like those of the birds and
mammals of today. Unfortunately, however, if such was the case, we may
never be able to prove it, although proof would not be impossible;
stranger things than fossil hearts have been found by paleontologists!
The stomach, moreover, in the crocodiles is fashioned somewhat after
that of the birds, with an imperfect division into crop and gizzard.
Some crocodiles of today have the habit of swallowing hard pebbles,
as do many birds. There is an old myth that the crocodile of the Nile
swallows a pebble on each of its birthdays, thus giving reliable
information as to its age by the number found in its gizzard at
its death! And this habit has been suggested for some of the most
ancient crocodiles, the teleosaurs, by the recurring presence of
siliceous pebbles found with the remains of their skeletons. And we
have seen this pebble-swallowing habit was also characteristic of the
plesiosaurs, with whose remains “stomach-stones,” or gastroliths, as
they have been called, are often found.
[Illustration: FIG. 104.—Pelvis of crocodile: _il_, ilium; _is_,
ischium; _pu_, pubis.]
All of these various characters of the skeleton and fleshy parts are
pretty conclusive evidence that the crocodiles, ugly creatures that
they are, today enjoy the highest rank among cold-blooded animals. They
are perhaps in some respects of not so high a type of reptiles as were
some of the extinct reptiles, but that they have survived so long, so
many millions of years, is pretty good evidence of endurance, to say
the least.
Living crocodiles belong to three distinct groups or families: the
true crocodiles and alligators; the long-snouted crocodiles or Borneo
gavials; and the true gavials of India. Members of the first of these
families are really only subaquatic, or amphibious in habit; they move
about on land with entire freedom, and often seek their food there.
Certain marked aquatic characters they do possess, in the skull and
tail, as we shall see. They are indigenous to southern China, India,
Africa, Madagascar, the southern part of the United States, Central
America, and the northern part of South America. The members of this
family are distinguished by the more or less broad and flat head, the
possession of comparatively few teeth of large size, and by having the
toes less completely webbed. The crocodiles proper differ from the
caimans and alligators especially in the arrangement of the teeth.
During later geological times, that is, during early Tertiary times,
the geographical range of the Crocodilidae was much more extended
than it is at present, the remains of many often very large species,
being found in the lake deposits of the northwestern parts of the
United States, Great Britain, Germany, France, etc. Yet earlier, in
the late Cretaceous rocks of the United States, in Texas, and Wyoming
especially, there have been found rather scanty remains of a gigantic
crocodile which must have been nearly fifty feet in length when living.
The second family, the Tomistomidae, or long-snouted crocodiles,
comprises but two living species, both restricted at the present time
to Borneo. These crocodiles have a moderately slender snout, because
of which they are sometimes called gavials, though it is not nearly so
slender as that of the true Gangetic gavial. This family is probably
older than either of the other living ones, and is the only one known
with certainty to have lived during much of the Cretaceous times,
several species of considerable size having been found in New Jersey
and Europe. Their feet are better webbed than are those of the true
crocodiles and alligators, the front feet partly, the hind feet wholly
so. In general structure they seem to be the most primitive of the
living Crocodilia, and may have been the ancestors of all modern forms.
[Illustration: FIG. 105.—Gavial. (By permission of the New York
Zoological Society)]
The third family, the Gavialidae, also comprises but two living
species, both restricted in habitat to the rivers of India. Of these
the famed gavial of the Ganges is the better known and the more highly
specialized. The skull of this species has an exceedingly long and
slender snout; the teeth are numerous, small, and slender; and the feet
are more fully webbed than are those of other members of the order. In
habits the gavials are more distinctly aquatic than are the crocodiles
and alligators. They feed almost exclusively upon small fishes, for
the seizure and retention of which their small and sharply pointed
teeth are admirably adapted. The hind feet are relatively long, a
character that will be better understood when comparison is made with
those of the Thalattosuchia. Although attaining a large size, fully
twenty-five feet in length, they are comparatively harmless animals,
never attacking human beings or other large animals, as do some of the
crocodiles proper. The gavials have lived a long time in the Indian
regions, the Gangetic gavial itself having been found in deposits
of Pleiocene age, perhaps the oldest known of all living species of
air-breathing vertebrates. Some of the extinct gavials of the same
region attained a length of nearly or quite fifty feet, possibly the
longest, if not the largest, of all swimming reptiles of ancient or
modern times. Extinct gavials have been reported from South America,
but are not yet fully known.
While the fish-eating gavials swallow their prey whole, the crocodiles,
caimans, and alligators prey upon all living animals that come within
their reach, whether large or small, and they will often leave the
water to seize their intended victims, such as pigs, sheep, birds, or
even human beings. Their teeth, as has been already stated, are much
larger, longer, and more irregular in size than those of the gavials.
Their victims are often drawn under the water and drowned, the peculiar
posterior position of the internal nostrils permitting the animals to
breathe with the mouth and to firmly hold their prey under water, while
the extremity of the snout and the external nostrils are above the
surface.
As the firm, unyielding bony palate, the fixed position and
articulation of the lower jaws, and their rigid attachment to each
other in front do not permit creatures of large size to enter the
gullet whole, the crocodiles and alligators must tear their food to
pieces, which they do by quick, strong jerks from side to side, aided
by the powerful tail; or they may twist off a limb or some other part
of their victims by a rapid rotation of the whole body, two assisting
in this operation, rotating in opposite directions.
Living crocodiles lay from twenty to sixty eggs, according to the
species; these eggs are sometimes the size of a goose egg, and are
covered with a hard shell. They are laid either in a deep excavation
in the sand and covered over by the parent; or under leaves and straw.
The female remains on guard until the eggs are hatched, of which she
is apprized, it is said, by a peculiar noise uttered by the partly
imprisoned young. She thereupon reopens the nest, and guides her
liberated infants to the water, where she leaves them to their fate.
Whether this remarkable habit is one that has been acquired in recent
times or not is uncertain, but because it has been observed in a
number of unrelated forms, it is probable that the instinct is of long
inheritance, and may account for certain peculiarities of structure in
some of the ancient members of the order. Doubtless the habit arose
because of the unprotected places in which the eggs are necessarily
laid on the shores and beaches, and because the eggs are comparatively
so few in number. The sea-turtles likewise lay their eggs in hollows
scooped out of the sand of the beaches, but the parents give no further
care to their eggs, nor to their newly hatched offspring, a neglect
which is compensated for by the much larger number of eggs they lay,
because of which the chances are much greater that a few will survive
the more numerous vicissitudes to which the eggs and young turtles are
exposed.
ANCIENT CROCODILES, MESOSUCHIA
The name Mesosuchia, meaning “middle crocodiles,” by which the ancient
members of the Crocodilia have generally been known, was given by
Huxley in the belief that they were intermediate between the “true”
or modern crocodiles and an ancient group which he united with the
order under the name “Parasuchia.” A fuller and better knowledge of
the members of this last group has proved very conclusively that they
are really less allied to the crocodiles than are some other orders of
reptiles, the dinosaurs for instance, and should be properly classed
by themselves as a distinct order. And, more recently, it has also
become quite apparent that the old crocodiles should not be separated
so widely from the modern ones as Huxley proposed; that the differences
distinguishing them from the recent members of the order are really
not of more than family importance. We thus have left but two chief
divisions of the Crocodilia, the Eusuchia and Thalattosuchia; and the
latter group even, by some authors, perhaps rightly, are included under
the true crocodiles as a family only.
[Illustration: FIG. 106.—_Teleosaurus_; skull, from above.]
These older crocodiles, the Mesosuchia of Huxley, comprise a
considerable number of extinct forms which lived as far back as the
early part of the Jurassic, and continued nearly to, if not actually
into, Cenozoic time, that is, to the Eocene. They differ from all
living forms, chiefly in having, not concavo-convex but biconcave
backbones, that is, the more primitive vertebrae with which all
reptiles began. Nor was the internal opening of the nasal passages so
far back in the mouth as in the later forms. In other respects they
did not differ very greatly from some of those now living. All the
earliest kinds that we know of—the teleosaurs—had a long, slender
snout, resembling very much that of the modern gavials. And they were,
for the most part, incased in a more complete bony armor, on both the
dorsal and the ventral sides; and the front legs were smaller than
those of the gavials even. The resemblance of the living teleosaurs to
the modern gavials must have been very great, although the heavier bony
armor indicates a less exclusively aquatic life. They probably lived
more in the shallow waters of the seas near the shores.
Near the close of the Jurassic appeared for the first time, so far as
we now know, broad-headed mesosuchian crocodiles, forms having less
numerous and stronger teeth, and resembling closely modern alligators.
It has been believed that these broad-headed kinds were of later origin
than the more slender-nosed teleosaurs, but a moment’s consideration
will make it evident how improbable such an evolution must be. The
crocodiles must have descended from strictly terrestrial reptiles, and
no terrestrial reptiles have a slender nose. That they should have
acquired a slender face in adaptation to water habits and then returned
to the more primitive land type with a broad face and less strictly
aquatic habits is contrary to all our experience in paleontology. From
this it is altogether probable that broad-faced crocodiles of later
times must have been the descendants of broad-faced kinds that were
in existence during all the Jurassic times, but of which we as yet
have no knowledge. These broad-faced Jurassic crocodiles were, for the
most part, small creatures, much smaller than the teleosaurs even, and
smaller than any species of crocodiles now living. Their remains are
known only from fresh-water or shore deposits, and are, for the most
part, associated with those of land and fresh-water animals. About the
time of their first known appearance in geological history, the small
mammals and birds had also become more or less abundant, and it was
suggested by Owen that these land animals had something to do with the
development of the ancient amphibious crocodiles. Perhaps this was the
case with respect to their greater abundance and development, and with
certain peculiarities of their structure, but that the gavial-like
teleosaurs should have come back to the land and reverted to a more
primitive form seems quite improbable.
During Cretaceous times, especially in America, numerous forms of
these old mesosuchian crocodiles were doubtless in existence, both
slender-nosed and broad-nosed, and some of them must have been of
considerable size, though none known was as large as some of the
late crocodiles. This type, with biconcave vertebrae, continued to
live on, in both North and South America, to the latter part of
Cretaceous times, and it is even possible that some continued on
into the Tertiary. But long before the close of the Cretaceous,
the modern kind appeared, those with concavo-convex vertebrae, and
more posterior internal nostrils. The earliest are known from New
Jersey (_Thoracosaurus_, _Holops_), so like the Borneo gavials of
today that they are properly classified in the same family, the
Tomistomidae or Gavialidae. If all the later, procoelian type, that
is, those with concavo-convex vertebrae, originated from a single form
when the amphicoelian or mesosuchian type became extinct, Huxley’s
classification into the Mesosuchia and Eusuchia would perhaps be
proper, but we have much reason to suppose that the change in the kinds
of vertebra and in the position of the nostrils was only incidental,
and may have occurred in more than one line of descent, that is, it
may have occurred in the broad-headed kinds of the Jurassic to the
broad-headed crocodiles of today, as also in the gavial-like forms of
the Cretaceous to the gavials of the present. And this is the reason
why naturalists no longer recognize the classification of Huxley,
which, partly perhaps because of the prestige of his name, has so long
been accepted in our chief works on natural history.
MARINE CROCODILES, THALATTOSUCHIA
While the ancient crocodiles of which we have spoken resembled the
modern ones so closely in form of body and probably in habits, there
were certain others of the old Jurassic seas which departed so widely
both in structure and in habits, from their associates that they are by
some authors given a place wholly by themselves as a distinct group.
This has been called by Professor Fraas the Thalattosuchia, a word
meaning “sea-crocodiles.” They were a very early side-branch from the
great genealogical tree of the Crocodilia, a branch which departed so
widely from their associates in adapting themselves to a peculiar and
aberrant mode of existence that they cannot be considered as typical
crocodiles, although so closely related to them in other respects
that therecannot be the least doubt regarding their ancestry. None of
the crocodiles which we have considered, whether ancient or modern,
can truthfully be called purely aquatic. They never ceased to use
their limbs for locomotion on land, more or less of the time, or for
the support of the body; and many of them have subsisted, in part at
least, on land animals. How easy it may have been for some of them
to become almost wholly emancipated from land habits we may easily
conjecture. The gavials, as we have seen, are more at home in the
water than upon land; their food is chiefly found in the water; but,
so long as their habits restrict them chiefly to fresh-water, or to
the vicinity of the shores, their limbs continue to be used as much
for crawling as for swimming. Were the gavials to be driven out to
sea by the stress of fresh-water conditions or attracted thereto by a
greater abundance of more easily obtainable or better food, so far from
land that they no longer would have much use for their legs for the
support or propulsion of their bodies, in the course of time they would
doubtless change to a more purely aquatic type. And in that change
there would be material modifications of their structure: their limbs
would become better adapted to movements in the water; the skin would
become smoother, and the bony and horny scales would be lost, since
they would be not only useless in the water, but actually detrimental
to the well-being of the animals; and the tail would develop into a
more powerful organ of propulsion, as a means of increasing their speed
in obtaining food or in escaping their enemies, such as the sharks.
In fact, we can only imagine that in the transformation precisely
those modifications would occur which we actually find in these old
sea-crocodiles, the Thalattosuchia. And they are of especial interest
to us here because nowhere do we find a better example of Nature in the
act of transforming a terrestrial or subterrestrial animal into a truly
aquatic one.
[Illustration: FIG. 107.—_Geosaurus_, an Upper Jurassic thalattosuchian
crocodile, drawn from restoration and figures by Fraas.]
The group comprises only a few forms, so far as known. All were of
modest size among extinct reptiles, from ten to twenty feet in length,
and all are from the Upper Jurassic deposits of Europe. They did
not exist very long, probably because they found it impossible to
discard old habits and old structures entirely and become absolutely
emancipated from the land; their breeding habits possibly were too
deeply impressed into their structure readily to change, as did those
of other sea-reptiles. Some of their remarkable aquatic adaptations
have long been known, but only within a dozen years has our knowledge
of them become at all complete. Three or four genera have been
described, but only a few forms are well known, of which _Geosaurus_
may be taken as most typical. To this we shall confine our descriptions.
[Illustration: FIG. 108.—_Geosaurus_; skull from side and from above.
(After Fraas.)]
The skull of _Geosaurus_ is rather small in comparison with the length
of the body, smaller proportionally than in any living crocodile, but
not much smaller than that of the teleosaurs. The snout is long and
slender, much like that of the teleosaurs and gavials, but the bones
of the whole upper surface are quite smooth, not roughened and pitted
like those of modern forms. The skull of _Dakosaurus_, another genus
of thalattosuchians, is much less elongate than that of _Geosaurus_,
but has the other characteristics of _Geosaurus_. The eyes are provided
with a stout ring of sclerotic bones, with a pupillary opening of less
than one inch. We have seen that all other strictly aquatic reptiles
have similar eye bones, but no other crocodiles have them. The internal
openings of the nostrils are large and long, but they are not situated
far back, as in the modern crocodiles, not even so far back as in the
early teleosaurs. They had no need of the peculiar breathing apparatus
of the amphibious crocodiles, since all their prey must have been
water-breathing creatures. Their eyes were directed laterally, not
more or less upward, as in their nearest relatives. Nearly all other
crocodiles have an opening through the hind end of the lower jaw, but
the thalattosuchians did not. The teeth were about as numerous as in
the modern gavials, but they projected freely only a short distance
above the gums in life, and they were very slender and sharply pointed,
excellently well adapted for catching smooth and slippery fishes.
Their vertebrae, like those of all other reptiles of their time,
were biconcave. Those of the neck resembled those of the teleosaurs,
save that there were only seven, fewer than is the case with any
other members of the order. In becoming adapted to their peculiar
mode of life these crocodiles lost two vertebrae from the neck. All
modern crocodiles have two ribs attached to the first vertebra; the
thalattosaurs had but one, another evidence of primitive characters.
While the number of vertebrae in the neck was reduced, in the back it
was increased to eighteen; all other crocodiles have but fifteen or
sixteen. The trunk was long, another adaptation to water life. There
were two firmly united vertebrae in the sacrum, as in the modern forms.
The reason for the persistence of this terrestrial character we shall
see later.
[Illustration: FIG. 109.—Tail, scapula (_sc_), and coracoid (_c_) of
_Geosaurus_. (After Fraas.)]
The tail was very long and strong, nearly as long as all the remainder
of the body, and relatively much longer than in other crocodiles. It is
of interest to observe that the head, neck, body, and tail had almost
the same relative proportions as those of the great sea-lizards, the
mosasaurs. The terminal bones of the tail are very peculiar, and very
different from the corresponding bones of other crocodiles. While the
spines of the tail bones along the anterior part are only moderately
stout and long, and are directed obliquely backward, near the terminal
part they become suddenly much broader and are directed upward, and, a
little farther along, obliquely forward. The chevron bones on the under
side also here become broader and longer. The end of the tail curves
markedly downward to end in a slender point. It will be remembered that
a similar downward curvature of the end of the tail observed in nearly
all specimens of ichthyosaurs induced in Owen the belief that the
animals had a fleshy terminal fin, a belief which later discoveries of
the carbonized remains of the flesh confirmed. The peculiar structures
observed in various specimens of these sea-crocodiles, even though no
impressions or remains of the fleshy parts have been discovered, is
quite conclusive evidence that these animals also had a broad, fleshy,
terminal fin. No other explanation of the structure is possible.
[Illustration: FIG. 110.—_Geosaurus._ Elongate hind leg, and
paddle-like front leg. (After Fraas.)]
The ribs are not at all stout and are not much curved. They are
directed posteriorly in the known specimens preserved in the matrix
in such a way as to indicate a slender thorax and abdomen, not the
broad body of the modern crocodiles. The abdominal ribs, that is,
those protecting the region on the under side of the body between the
breast bone and the pelvis, are strongly developed in _Geosaurus_. The
sternum, always present in other crocodiles, is wanting in _Geosaurus_.
The shoulder-blades and coracoids, often changed in shape in water
reptiles, are not unlike those of the amphibious crocodiles, but are
smaller and flatter.
The fore limbs, to use Professor Fraas’s words, “are among the most
interesting of all the peculiarities of _Geosaurus_,” and show very
clearly that these animals were excellent swimmers. The humerus is
remarkably short and broad; the two bones of the forearm, the radius
and ulna, are broad, rounded, or angular plates, not long bones, as
in other crocodiles; the wrist bones also are broad and plate-like.
The three bones of the thumb, that is of the digit which received
most strongly the impact of the water in swimming, are also broad and
flat. All of these bones are marvelously aquatic in type, as will be
evident from a comparison of them with the corresponding bones of the
ichthyosaurs and mosasaurs. The bones of the other fingers, however,
were not much changed from the ordinary crocodilian form, as a further
comparison of them with the fingers of a land crocodile will show.
Furthermore the whole limb or paddle was very small in comparison with
the hind leg, and it was attached very near to the head. The relative
proportions of the front and hind limbs in the geosaurs, gavials, and
alligators are instructive as showing the progressive decrease in size
of the front legs from the subaquatic, through the semiaquatic, to
the almost purely aquatic type. The hind legs, strangely enough, were
not very different in size and structure from those of the gavials or
teleosaurs. The thigh bone is long and slender, though the bones of the
leg and ankle are somewhat shortened and flattened, as are also those
of the first toe. There were no claws on the hind feet, a distinctly
aquatic adaptation, and the toes were certainly webbed. The pelvis,
while not especially stout, is of good size, and was firmly attached to
the sacrum.
Perhaps all these characters may best be summed up in the words of
Professor Fraas, as translated:
We recognize in _Geosaurus_ an unusually slenderly
built crocodile, in appearance very different from all true
crocodiles. The smooth, rounded skull, with its greatly
elongated and slender snout, and the deep-lying, small eyes,
reminds one most of the ichthyosaurs. The skull merges into
the slender, elongated trunk without a visible neck, and
the body was provided neither above nor below with horny or
bony armor, but was, probably, as in the whales, covered
with a smooth, soft skin. The anterior extremities, attached
far forward, are developed as paddles, and served rather
as organs of equilibration than as a means of propulsion,
which was the function of the elongated hind legs and the
extraordinarily strong and powerful tail, which supported
at its end a large fin. The entire impression given of
the animal is that of an excellent swimmer, with all the
peculiar aquatic adaptations. In the skeleton, however, all
the characters of the original crocodiles are preserved.
Most remarkable are the laterally placed eyes, protected
by the stout sclerotic bones, and the overhanging bones of
the orbits. So, too, the large temporal openings of the
skull, doubtless due to the absence of the bony plates in
the integument, give to the animal a strangely abnormal
appearance for a crocodile.
We have observed that all the truly aquatic air-breathing animals,
save the plesiosaurs, have either lost the hind legs or else have them
greatly reduced in size, and the disproportionately large size of
these members in _Geosaurus_ seems inexplicable. But an explanation is
not, I think, hard to find. In the adaptation to water life the first
to become modified for the control of the body are the front legs.
The hind legs never have any really important use when the tail is a
powerful propeller. The hind legs of the geosaurs are still essentially
legs and not paddles, and they were doubtless used either occasionally
for propulsion on land, or perhaps for pushing the body about on
the bottom of shallow waters. And the presence of a well-developed
ventral armor of bony ribs possibly also indicates more or less of the
terrestrial crawling habit. As soon as the hind legs cease to be used
for crawling they take on only a feeble use for the equilibration of
the body, and speedily become small, until finally they disappear.
That the hind legs of these creatures were of some use in the water
is certain, because of the modifications in their structure, and
especially because of the loss of the claws; but that they were of
important use as propellers is hardly probable. We may therefore infer
that the thalattosuchians, while distinctively sea-reptiles, had not
entirely lost their land habits. Moreover, it is highly probable that
their egg-laying habits, which would hardly change with a greater
adaptation to water life, compelled the animals recurrently to visit
the shores. To have finally lost their hind legs they must have become
viviparous in habit, since it seems to be impossible for any true
air-breathers to be hatched in water. Perhaps this insurmountable habit
was the final cause of their extinction in competition with the truly
viviparous aquatic flesh-eaters. The thalattosuchians had but a brief
existence in geological history, during the latter part of the Jurassic
period only, so far as certainly known, nor did they become widely
dispersed over the earth; they are known from Europe, possibly from
Brazil.
CHAPTER XVI
CHELONIA
No order of reptiles of the past or present is more sharply and
unequivocally distinguished from all others than the Chelonia or
Testudinata. No order has had a more uniformly continuous and
uneventful history. None now in existence has had a longer known
history, and of none is the origin more obscure. The first known
members of the order, in Triassic times, were turtles in all respects,
as well or nearly as well adapted for their peculiar mode of life as
are those now living, and were they now living they would attract no
especial attention from the ordinary observer and but little from the
naturalist. From time to time some have gone after better things,
and have come to grief, but the main line has remained with fewer
improvements, fewer evolutional changes, than any other group of higher
vertebrates. The turtles seem very early to have adapted themselves
so well to their peculiar mode of life, to have intrenched themselves
so thoroughly in their own province, that no other creatures have
been able to overcome them, or to drive them from it. The remains of
no other air-breathing vertebrates are so omnipresent in the rocks
as those of the turtles; they may be expected wherever fossils of
air-breathing animals are found, though unfortunately often only in
scattered and broken fragments. The loose union of their skeletal bones
and their general habits of life in shallow waters left their bodies as
food for scavengers, or for dismemberment by the tides and currents.
Relationships with other reptiles they really have none. Some have
thought that the plesiosaurs were their first cousins, others the
Placodontia, an indeterminate group of extinct reptiles usually placed
with the Anomodontia. But their relationship with neither of these is
closer than with the crocodiles, dinosaurs, or pterodactyls. They are
the only reptiles that we know, besides the cotylosaurs, which have no
holes in the temporal roof of the skull, and as the cotylosaurs were
the most primitive and the oldest of reptiles, this fact incontestably
proves that the turtles had a very ancient origin, though we know
them no farther back than the later Triassic. They are the only order
of reptiles of which not a single member is known to have teeth, or
even vestiges of them. Until recently only a single specimen has been
known from the Trias, and of that only the casts of the shell; but
the shell was as fully developed and as complete as that of a modern
alligator snapper, which it resembled much in form and in size. And
doubtless the habits of this ancient _Proganochelys_ were similar
to those of the alligator snapper. The early cotylosaurian reptiles
were all littoral-or marsh-loving animals, and more or less aquatic,
and doubtless the early turtles continued in the same environments
and with the same habits after acquiring a shell for protection and
losing their teeth, which for some inexplicable reason they seemed no
longer to need. Until near the close of the Jurassic period probably
all turtles were amphibious animals of the marshes, spending much,
perhaps the larger part, of the time in the water, good swimmers, and
yet good crawlers. With the beginning of the Cretaceous, however, some
of them became ambitious for new and untried modes of life. Various
ones went down into the sea and became marine animals, reaching the
zenith of their prosperity and the maximum of size before the close
of the period, but continuing in diminished size and numbers to the
present time, if we may consider the leather-back turtle as really
their descendant. Others in the Cretaceous took to the rivers and
ponds, and became almost as thoroughly aquatic in their thin shape
and soft covering; and their lineal descendants still continue in the
rivers of the Northern Hemisphere. Still others, in the Age of Mammals,
took to the upland, and competed with the mammals in the open places
and prairies, reaching their maximum in Miocene-Pliocene times, when
for some unknown reason the giants among them were driven from the
mainlands to continue a precarious existence to the present time in
some of the larger islands.
[Illustration: FIG. 111]
[Illustration: FIG. 112
FIGS. 111 and 112.—_Graptemys._ (From Hay.)
FIG. 111.—Carapace: _cp1_, _cp2_, etc., costal plates; _cs1_, _cs2_,
etc., costal scutes, horny; _n1_, _n2_, etc., neural bones; _nup_,
nuchal bone; _nus_, nuchal scute; _py_, pygal bone; _spy_, suprapygal;
_spy 2_, second suprapygal, or postneural; _vs1_, _vs2_, etc.,
vertebral scutes; 1, 2, 3-12 on right side, marginal scutes; 1, 2, 3-12
on left side, peripheral bones.
FIG. 112.—Plastron: _ab_, abdominal scutes; _an_, anal scutes;
_ent_, entoplastron (interclavicle); _epi_, epiplastron (clavicle);
_fem_, femoral scute; _g_, gular scute; _hum_, humeral scute; _hyo_,
hypoplastron bone; _hypo_, hypoplastron; _in_, inguinal scute; _py_,
pygal bone; _xiph_, xiphiplastron.]
Were there no turtles living we should look upon the fossil forms
as among the strangest of all vertebrate animals—animals which had
developed the strange habit of concealing themselves inside of
their ribs, for that is literally what the turtles do. The box or
shell of an ordinary turtle is composed of the backbones and ribs,
to which are soldered a shell of bony skin plates above, with the
clavicles, interclavicle, and ventral ribs below. Except in the strange
leather-back turtle described farther on, these plates form definite
series. Ten of them cover the spines of the dorsal vertebrae, in the
mid-line, one over each, of which the turtles have the smallest number
of any known reptiles. There are eight on each side over the ribs,
united by suture with each other and with the middle series; and, in
addition, there are twenty-six bones surrounding them and attached to
them. All these bones compose what is called the carapace, which forms
a complete roof in the more terrestrial types, more or less imperfect,
with vacuities between the bones in the marine forms. On the under
side, in addition to the clavicles and the interclavicle, there are
three pairs of enlarged ventral ribs that go to form the plastron,
solid and complete in land turtles, with openings in the water forms.
And in the land forms the plastron is more or less firmly united with
the carapace.
[Illustration: FIG. 113.—_Toxochelys_; coracoid and scapula.]
In the skeleton contained within the box thus formed is the very
peculiar pectoral girdle, composed of scapula and coracoid, the scapula
so peculiar that the controversy as to its homologies is not yet quite
settled. Most authors, until recently, have believed that its peculiar
shape (Fig. 113) is due to the co-ossification of the procoracoid
with the scapula instead of as usual its loss or union with the true
coracoid, so called. We are now pretty sure that this is not true,
since in reality there is no such bone as the procoracoid, the bone
so called being the real or true coracoid; and because, in the second
place, the long anterior projection called the procoracoid is really
only an outgrowth of the scapula itself and not a fused, separate bone.
Hence the bone is properly called the scapula-proscapula, and not the
scapula procoracoid, as it usually has been. The coracoids are elongate
and flattened and without the usual supracoracoid foramen, so generally
present in reptiles. The only other reptiles having a similar structure
of the scapula are the plesiosaurs, and it has been because of this
apparent resemblance that some good paleontologists have thought the
turtles and plesiosaurs were allied. The sacrum is composed of two
vertebrae only, and the pelvis of the usual three bones, the ilium, the
ischium, and the pubis, all covered over by the shell.
[Illustration: FIG. 114.—Pelvis of _Chelone_, from below: _pu_, pubis:
_is_, ischium; _il_, ilium (in acetabulum).]
In every known turtle the neck is composed invariably of eight
vertebrae, but they are peculiar in many respects. In the earliest
known turtles the neck vertebrae were, as would be supposed, biconcave,
but they soon became very variable in all; in each neck some are
biconcave, some biconvex, some opisthocoelous, and some procoelous. And
Dr. Hay tells us that the neck has increased in length in the later
forms.
The skull also is very peculiar in that it has some very primitive
characters and others very aberrant. The temporal roof, as has been
said, has no holes through it, though it is often reduced by the
emargination of the borders, whether from below or behind, until in
some the whole temporal region is exposed, and not at all covered over.
There is no parietal foramen, so constantly present in all the early
reptiles and in the lizards and the tuatera of modern times. There
are no teeth or vestiges of teeth, but the jaws have usually a horny
cutting edge, which seems to be quite as serviceable; in the river
turtle the lips are fleshy. There is no transverse or transpalatine
bone. There is a single vomer only, not paired as in other reptiles,
whence comes the doubtful theory that the vomers of other reptiles
are not the real vomers originally so named in mammals, and hence
often called prevomers. The vomer of the turtles under this theory is
believed to be the real homologue of the mammalian bone. The palate is
always slightly, sometimes nearly wholly, underfloored, as in mammals,
carrying the internal nostrils far back in the mouth. In the occipital
region of the skull there is a separate bone on each side called the
paroccipital or opisthotic, which has been indistinguishably fused with
the exoccipital in all other reptiles except the ichthyosaurs since
Triassic times.
[Illustration: FIG. 115]
[Illustration: FIG. 116
FIGS. 115 and 116.—_Trachemys._ (From Hay.)
FIG. 115.—Skull from above: _fr_, frontal; _ju_, jugal; _pa_, parietal;
_paoc_, paroccipital; _pfr_, prefrontal; _pof_, postfrontal; _pro_,
proötic; _qu_, quadrate; _sq_, squamosal; _soc_, supraoccipital.
FIG. 116.—Skull from below: _alv_, alveolar surface of maxilla; _boc_,
basioccipital; _bap_, basisphenoid; _exoc_, exoccipital; _mx_, maxilla;
_pal_, palatine; _paoc_, paroccipital; _pmx_, premaxilla; _pro_,
proötic; _pt_, pterygoid; _qu_, quadrate; _qj_, quadratojugal; _sq_,
squamosal; _vom_, vomer.]
In the feet the numbers of phalanges—that is, the bones of the free
digits—are like those of mammals, that is, two in the first and three
in each of the other four digits. The land tortoises have lost some of
these, while the river turtles have either gained one or two in the
fourth finger and fourth toe, or else have enjoyed an uninterrupted
descent from the primitive reptiles which normally possessed that
number. All other reptiles, save those phylogenetically allied to the
primitive mammals, that is, the Theriodontia and their allies, have
normally the phalangeal formula 2, 3, 4, 5, 4. It was partly because
of this similarity of the numbers of toe bones that the turtles have
been classed in the great group of reptiles that includes the ancestors
of the mammals; that is, under this theory, the turtles would enjoy a
nearer relationship to the mammals and to man himself than any other
living reptiles! But this classification has been shown to be quite
artificial.
[Illustration]
[Illustration: FIG. 117.—Limbs of _Colpochelys_, a recent sea-turtle:
_H_, humerus; _R_, radius; _U_, ulna; _r_, radiale; _i_, intermedium;
_u_, ulnare; _p_, pisiform; _c_, centrale; _T_, tibia; _F_, fibula;
_a_, astragalus; _m_, fifth metatarsal. (From Wieland.)]
From what has been said, it will be surmised that the Chelonia
represent in themselves one of the primary subdivisions of the class
Reptilia, and that, unlike most others, the order has enjoyed a most
remarkable longevity. And doubtless they are one of the primary
branches of the reptilian stock, which has remained distinct since
Permian times at least, if not since Carboniferous, isolated and
remarkably homogeneous, giving off no branches which departed far from
the main stock, and on the whole leading a singularly placid existence
for ten or more million years.
In most textbooks the order Chelonia is divided into three suborders,
the Pleurodira, the Cryptodira, and the Trionychoidea. In recent
years, however, the earlier members of the older group of Pleurodira
have been separated into a fourth suborder, the Amphichelydia, a group
characterized by some not very important differences in the plastron
and skull, and including those forms in which the cervical vertebrae
are amphicoelous. This group continued to Eocene times before it
became extinct, and consisted of archaic forms which persisted after
all the other suborders had come into existence. The Cryptodira,
especially characterized by the manner in which they withdraw the head
and neck within the shell by an =S=-like vertical flexure, are known
from the Lower Jurassic and are still the dominant group of today,
with more than one hundred and forty living species. The Pleurodira
in the narrower sense are first known from their remains in the Upper
Cretaceous of North America and are still represented by about forty
species, living in the Southern Hemisphere. They are distinguished from
the other groups by the manner in which they withdraw the neck and head
into the shell, by a horizontal, sidewise flexure. The third suborder,
the Trionychoidea, also began in Cretaceous times, so far as we know,
and are represented by about seventy living species, chiefly in the
Northern Hemisphere. They are especially characterized by the absence
of bony marginal plates and the soft epidermis.
With the exception of the land tortoises, all turtles from the
beginning of their career as an order to the present time have been
more or less at home in the water. In some, like the marine forms, the
adaptation to aquatic life has produced marked changes in structure: in
the loss of the horny dermal shields and in the loss of bone tissue;
in the flattening of the shell, and in the development of the front
legs into swimming flippers, with a loss of the claws. In the absence
of a guiding tail, which is always small in the marine turtles,
propulsion must of course be wholly by the aid of the limbs. As oar
propellers the marine turtles show some of the peculiar characters
of the plesiosaurs. With a like short and broad body, a more or less
elongated and flexible neck, there could be no sinuosity of the body in
swimming. As an oar-like organ the humerus became flattened, and its
muscular attachments, as in the plesiosaurs, descended far down the
shaft, giving greater mechanical advantage. Unlike all other aquatic
vertebrates, the turtles never developed real hyperphalangy. Only in
the river turtles is there a possibility of an increase in the bones of
the fourth digit.
To discuss in general the structure and habits of the living chelonians
would extend this chapter to an undue length, and would add nothing
to the many excellent works on natural history accessible to the
student. We have therefore contented ourselves with a brief outline of
the geological history of the order, with especial reference to their
aquatic habits.
SIDE-NECKED TURTLES. PLEURODIRA
The suborder of Chelonia, generally known as the snake-necked or
side-necked turtles or tortoises, comprises about forty living
species, confined to South America, Africa save the northernmost part,
Madagascar, New Guinea, and Australia. In Australia they are the only
members of the order known—another instance of the peculiar isolation
of the fauna of that region. In the past they lived in North America
during Upper Cretaceous times, the earliest known forms of the group
in its restricted sense, of which seven species are described by Hay.
In Eocene times they are also known from Europe and Asia, from both of
which regions they have long since disappeared.
The Pleurodira, as the term indicates, are easily distinguished from
all other turtles by the way in which they withdraw the head within the
shell. Instead of withdrawing it by an =S=-shaped flexure of the neck
between the shoulder-blades, as do other turtles, these bend the neck
laterally in a horizontal plane, bringing the head within the margins
of the shell in front of one or the other foreleg; and the margins of
the shell are produced here in an eave-like fashion for the greater
protection of the head. In the structure of the shell, which is always
fully developed into a box, these turtles do not differ very much from
the Cryptodira, though there may be some extra bones in the plastron,
as also in the skull. The nasal bones are always, the lacrimals
sometimes, well developed; the latter never, the former rarely, found
in other groups. The lower jaws articulate a little differently,
and the external ear is always fully surrounded by bone. Very
characteristic is the bony union of the pelvis with the plastron below,
which never occurs in other turtles, unless it be the Amphichelydia.
The side-necked turtles are all of fresh-water habit, similar to that
of the fresh-water tortoises spoken of farther on. The neck is often
very long and snake-like, which accounts for one of the names given
to these turtles; because it is withdrawn into the shell sidewise, it
has more distinctively ball-and-socket joints between the vertebrae,
with distinct transverse processes for the attachment of the necessary
side-moving muscles. The feet in all are more or less webbed and armed
with strong claws.
The largest and perhaps the best known of all living side-necked
turtles is the giant Amazon turtle of South America, which sometimes
has a shell nearly three feet in length. Its feet are broadly webbed,
and the shell is rather flat in the adult; it is an excellent swimmer
in the waters of the Orinoco and Amazon. Six or seven species of
the genus to which it belongs are known, all of them South American
except one that lives in Madagascar and one fossil found in the Eocene
of India. This remarkable distribution is but one more of the many
instances known in zoölogy and paleontology that seem to prove an early
land connection between India and South America. Had the migration
between the two continents occurred by way of Asia and Bering straits,
as did that of hosts of mammals, one would certainly expect to find
some evidence of it in the North American Tertiary rocks, which, so
far, is lacking.
CRYPTODIRA
The chief families of the Cryptodira turtles are the Chelydridae,
or snappers; the Emydidae, or marsh tortoises; the Testudinidae, or
land tortoises; the Chelonidae, or sea-turtles; the Protostegidae, or
ancient sea-turtles; and the Dermochelydidae, or leather-backs. Other
doubtful or smaller groups, both living and extinct, may be omitted, or
incidentally mentioned.
SNAPPING TURTLES
The family of snapping turtles, the Chelydridae, are of interest
because of their peculiar geographical distribution at the present
time. Only four species are known, three of them from North America,
the fourth from New Guinea. The family is one of the most primitive of
living turtles, though no members of it are known with certainty from
earlier rocks than the Oligocene. In all probability, also, they have
retained, more than have any other group of turtles, unless it be some
of the fresh-water tortoises, the primitive habits of the earlier or
earliest turtles, though of course there have been modifications, both
in structure and in habits. The three species of the United States
include two of the snapping turtles proper and the alligator turtles of
the southern states, which sometimes reach a length of three feet. All
the species are largely aquatic in habit, powerful and active swimmers,
with webbed feet and strong claws, and both on the land and in the
water they are bold and fierce. They have a relatively large head and
very strong jaws. Agassiz saw one bite off a piece of a plank an inch
in thickness, and they can usually be raised from the ground by any
object which they seize. The carapace and plastron are much reduced,
and are rather loosely united. The shell is not large enough for the
complete withdrawal of the head and legs within it, and the tail is
unusually large and strong. The common snapping turtle, _Chelydra
serpentina_, is found from Canada to Ecuador, and its remains have been
found with those of the mammoth and mastodon in Pleistocene deposits;
and related species of the same genus have been reported from the
Miocene of England.
FRESH-WATER OR MARSH TORTOISES
The family of turtles or tortoises (Emydidae) represented at the
present time by the common terrapin, painted tortoise, and box tortoise
of the United States, and commonly called fresh-water turtles or
tortoises, comprises the largest group of living chelonians—nearly a
third of all existing members of the order. They are widely distributed
over all parts of the earth except Australia, and are of very varied
habits. Some are almost exclusively aquatic; others, like the painted
tortoise, are partially so, while others, especially the common box
tortoise, are almost as exclusively terrestrial as the true land
tortoises, dying even, if forced to live long in water.
The shell in the more aquatic forms is depressed or flattened, but
in the terrestrial kinds may be as highly arched as in the true land
tortoises. The feet are adapted primarily for walking, but nearly
always have the toes webbed, and the digits are longer than are those
of the land tortoises. Only the two or three middle toes have claws.
Some species have developed hinges in the plastron, whereby they may be
completely closed up after the head and legs are withdrawn within the
shell. Most of the species are carnivorous in habit, but a few, like
the box tortoise, are strictly vegetarian.
Geologically the fresh-water tortoises have a not very ancient history,
going back no farther than do the land tortoises, that is, to the
beginning of the Cenozoic or Age of Mammals. Fully fifty species
are known from the Tertiary rocks of North America, or more than
three-fourths as many as are now living upon the earth.
The family at most can be said to be only amphibious in habit, and has
no striking aquatic adaptations, since the shell is well developed and
is covered with horny shields. The flattened shell of the more aquatic
forms is characteristic, as is also the greater degree of webbing
between the toes.
LAND TORTOISES
Perhaps the last of the more noteworthy specializations of
the Chelonia, and indeed among the last of the more important
specializations of the Reptilia, are the upland tortoises, of which
the common “gopher” of the southern states is almost the only remnant
in North America. They formed a part of the great hegira of forest and
marsh animals to the open prairies, away from the lowlands and water
which the turtles had inhabited almost exclusively for millions of
years.
[Illustration: FIG. 118.—_Testudo sumeirei_, a giant upland tortoise.
(From Hay, after Rothschild.)]
They began their career, Dr. Hay thinks, at about the beginning of
the Cenozoic, that is, with the great development of the mammals, and
reached the maximum of their development in the Miocene; and they
have been on the decline ever since. In the Northern Hemisphere, at
least, the slowly cooling climate throughout the Eocene, and a decided
decrease in moisture, brought about the prairies and prairie plants
before its close. Just as the horses, rhinoceroses, camels, and other
herbivorous mammals took to these open places for the better and more
abundant food found therein, so also the lowland tortoises found better
food and fewer enemies there, for they are all strictly herbivorous in
habit. The mammals became more conspicuous to their enemies when they
went into the open, and it was only by the development of speed, more
sober coloration, and perhaps greater cunning that they found safety
from them. The tortoises were handicapped by low intelligence, and they
could not develop speed, for they were not constructed to that end. But
they did find protection in their bony shell, which became thicker,
higher, and more convex, and with smaller openings. To quote Dr. Hay:
“We may suppose that it would be much more difficult for a carnivorous
animal to effect an entrance into such a shell than into one depressed,
and whose borders may be spanned by the jaws of their enemies.” Perhaps
also the highly arched form of the shell gave greater capacity for the
lungs, and the tortoises in general, it is said, do have better lung
capacity than the more aquatic or lowland types of turtles. Possibly,
also, the heavier shell lessened the evaporation of the body fluids,
and made the tortoises less dependent upon the vicinity of water.
Certain it is that the common box tortoise, of like form and habits,
occurs not rarely on the arid plains, far from water.
The neck and legs became fully retractile within the shell; the digits
were shortened up, without a vestige of webbing membrane between them;
the phalanges were reduced in number to two in each toe, and nearly all
the toes have well formed claws. The feet are placed squarely upon the
ground, and the body is elevated in walking. They can swim, when by
accident they are thrown into the water, only as any terrestrial mammal
can.
About forty species of land tortoises are known throughout the world
at the present time, though North America, the probable original home
of the tribe, has but three, all small. The larger species are all now
denizens of islands, especially the Galapagos Islands, where the giant
tortoises have long been famous. And many of our living forms have
changed but little since Eocene times. In the Oligocene and Miocene
they inhabited western North America in enormous numbers. In the Bad
Lands of South Dakota one can often see the remains of a dozen or more
of these giant tortoises at one time, specimens varying from one to
three feet in length of shell. In river deposits, those of the late
Miocene or early Pliocene, the writer has seen areas of an acre or more
literally strewn with their remains, as though droves of them had been
overwhelmed and perished together. About fifty species of these land
tortoises are known from the American Tertiary, thirty-two of them
belonging to the modern genus _Testudo_, which comprises the giant
tortoises of the Galapagos. The largest known species of the group is
one of _Testudo_ from the Pliocene of India, which had a shell six feet
in length. Why the larger species became extinct in Pliocene times on
the mainland to survive only in the islands is not known; possibly
their carnivorous enemies became too cunning and too numerous.
SEA-TURTLES. CHELONIDAE
The sea-turtles, or Chelonidae comprise five or six living species,
inhabitants for the most part of tropical and subtropical oceans, of
which the green or edible turtle (_Chelone_), the hawksbill turtle
(_Caretta_), and the loggerhead (_Eretmochelys_ [Fig. 119]) are the
best known. They are all thoroughly aquatic in habit, and of large
size, from three to five feet in length. The carapace is heart-shaped,
and reduced, that is, with large openings between the ribs; the
plastron also is reduced and loosely united to the carapace. The neck
is short and the head is not retractile within the shell. The temporal
region of the skull is roofed over. The four legs form large and
powerful flippers, and the hind legs are relatively small. The body is
flattened and the tail is small. The aquatic characters of the limbs
are seen especially in the broad and strong humerus, with the radial
crest for the attachment of powerful muscles situated far down on
the shaft; in the relative shortness of the radius and ulna, and the
large size of the latter bone; in the flattened carpal bones; and in
the great elongation of the digits and the absence of all but one or
two of the claws. Unlike the leather-back turtle and the Cretaceous
sea-turtles, the carapace and plastron are completely covered with
horny shields, from which indeed the tortoise shell of commerce is
derived. Except the green turtle, all members of the family are
carnivorous.
[Illustration: FIG. 119.—_Eretmochelys_, loggerhead turtle. (By
permission of the New York Zoölogical Society.)]
Extinct members of the family are known from scanty remains in Cenozoic
and late Cretaceous rocks. From the earlier Cretaceous deposits of the
plains more primitive allied forms occur, often classed in distinct
families of which _Toxochelys_ (Fig. 120) and _Desmatochelys_ are the
more noteworthy. The latter genus, especially, might well have been an
ancestor of all the modern forms. About three feet in length, it had
all the essential characteristics of the sea-turtles, in its thin form,
roofed-over skull, reduced carapace, loose plastron, and flipper-like
limbs. The single known specimen, preserved in the museum of the
University of Kansas, came from the lower rocks of the Upper Cretaceous
of Nebraska. Yet earlier, at the close of the Jurassic, there were
shore turtles of considerable size that had begun to develop a fondness
for the open seas; to acquire a depressed form and lightened shell, the
limbs still retaining, however, more of the terrestrial or crawling
form. They are grouped as a separate family, the Thalassemydae,
and include the first of the Chelonia to depart from the marsh and
fresh-water habits which for long ages, perhaps, had limited the
activities and evolution of the turtles.
[Illustration: FIG. 120.—Carapace of _Toxochelys bauri_, an Upper
Cretaceous sea-turtle: _ep_, epineural. (After Wieland.)]
ANCIENT SEA-TURTLES. PROTOSTEGIDAE
[Illustration: FIG. 121.—_Toxochelys latiremis_; front leg: _hum_,
humerus; _rad_, radius; _ul_, ulna; _int_, intermedium; _uln_, ulnare;
_p_, pisiform; _cen_, centrale. (From Wieland.)]
[Illustration: FIG. 122.—_Desmatochelys lowii_; skull from above and
below.]
Forty-four years ago the late Professor E. D. Cope, one of the
greatest naturalists America has ever produced, in almost the earliest
exploration of the great Cretaceous fossil deposits of western
Kansas, discovered and collected a remarkable specimen of one of
the most extraordinary turtles that is known even yet. By an error
somewhat natural for those times, when the theory of evolution was
just beginning to attain acceptance by naturalists, he thought that
the specimen, notwithstanding its monstrous size, represented a very
primitive kind of turtle, and gave to it the name _Protostega gigas_,
meaning gigantic first roof! The late Professor George Baur, to whom
paleontology owes so much, showed that, far from being a primitive
turtle, _Protostega_ was really one of the most specialized types of
the order. Professor Cope’s account of the discovery of the specimen is
of so much interest that it may be quoted here:
“In the very young tortoise or turtle the ribs are separate, as in
other animals. As they grow older they begin to expand at the upper
side of the upper end, and with increased age the expansion extends
throughout the length. The ribs first come in contact where the
process commences, and in the land tortoise they are united at the
end. In the sea-turtles the union ceases a little above the ends. The
fragments of the _Protostega_ were seen by one of the men projecting
from a ledge of a low bluff. After several square feet of rock had
been removed, we cleared up the floor and found ourselves well repaid.
Many long, slender pieces of two inches in width lay upon the ledge.
They were evidently ribs, with the usual heads, but behind each head
was a plate-like the flattened bowl of a huge spoon, placed crosswise.
Beneath these stretched two broad plates, two feet in width, and no
thicker than binder’s board. The edges were fingered and the surface
was hard and smooth. All this was quite new, among fully grown animals.
Some bones of a large paddle were recognized, and a leg bone. The
shoulder-blade of a huge tortoise came next, and further examination
showed that we had stumbled on the burial place of the largest species
of sea-turtle yet known. But the ribs were those of an ordinary turtle
just hatched, and the great plates represented the bony deposit in the
skin, which, commencing independently in modern turtles, unite with
each other at an early day. But it was incredible that the largest
of known turtles should be but just hatched, and for this and other
reasons it has been concluded that this ‘ancient mariner’ is one of
those forms, not uncommon in old days, whose incompleteness in some
respects points to the truth of the belief that animals have assumed
their modern perfection by a process of growth from more simple
beginnings.”
Later studies by Doctors G. Baur, E. C. Case, O. P. Hay, and especially
G. R. Wieland, of the abundant and excellent material, preserved in the
museums of Yale and Kansas universities and the Carnegie Institution,
and especially the discovery by Wieland in 1895 of an allied and yet
larger form which he called _Archelon_, have determined practically
every detail of the structure of this remarkable group of sea-turtles.
A surprisingly complete specimen of _Archelon_ is mounted in the museum
of Yale University.
[Illustration: FIG. 123.—_Archelon ischyros_; skeleton from above: _n_,
nuchal, _r_, _r_, _r_, ribs; _m_, _m_, peripheral bones; _h_, humerus;
_r_, radius; _u_, ulna; _t_, tibia; _fi_, fibula. (From Wieland.)]
About a half-dozen species and two genera of the family have so far
been described, all coming from the Upper Cretaceous deposit of Kansas
and South Dakota, the genus _Archelon_ from later rocks than those
which have yielded _Protostega_.
The general form and structure of _Archelon_ will best be understood
from the accompanying figures after Wieland (Figs. 123, 124, 125) and
the restoration of the living animal as interpreted by the writer
(Fig. 126). If the leather-back turtle, described farther on, is really
the descendant of these or allied turtles, as many authors believe, it
of course represents the very highest aquatic specialization of all
Chelonians. If, on the other hand, as some believe, the leather-back
is the end of a long and independent line of descent, then _Archelon_
represents the highest aquatic specialization of all other turtles.
In size, at least, _Archelon_ attained the maximum of the order,
reaching a length of more than twelve feet, and a weight of more than
three tons. Except that the shell was not heart-shaped or elongated as
in all modern sea-turtles, but nearly circular in outline, it had all
the aquatic adaptations of the sea-turtle in a yet higher degree.
[Illustration: FIG. 124.—_Archelon_ from below, without plastron: _h_,
humerus; _r_, radius; _u_, ulna; _sc_, scapula; _c_, coracoid; _p_,
pubis; _i_, ischium. (From Wieland.)]
[Illustration: FIG. 125.—_Archelon_; skeleton from below: _hp_,
hyoplastron; _hpp_, hypoplastron. (From Wieland.)]
The shell was depressed; the dermal plates covering the ribs had almost
entirely disappeared, remnants only of their upper ends remaining; the
skull (Fig. 127) had the temporal region wholly roofed over; the neck
was short and not retractile. The front legs were strong flippers,
the humerus was long and stout, with the crest for the attachment
of muscles far down on the shaft; the digits were greatly elongated
and clawless, etc. The plastron only was less reduced than in the
case of the modern sea-turtles. No traces of horny shields have been
discovered. As to the nature of the covering and the general appearance
of the turtle when alive, Dr. Wieland has kindly given the writer his
views, as follows:
“After direct study or fairly close examination of all the fossil
material of importance thus far collected representing the
Protostegidae, it seems certain that in all the members of the group
an external leathery layer was well developed. In no instance is there
the slightest trace of horny shield sulci, or grooves; though it seems
probable that there was some gradation toward a thin and perhaps even
slightly horny hide. In _Archelon ischyros_ the reduced condition
of the carapace and the presence of the continuous row of large,
median, supraneural elements render it quite certain that there was a
development of leathery hide comparable to that of _Dermochelys_. The
same may be said of _Protostega gigas_. But _Archelon Marshii_ had a
less reduced carapace, and the leathery skin was probably less well
developed; and _Protostega Copei_, in which no trace of supraneurals
remains, must have made some approach to the horn-shield condition.
A more distinct suggestion of transition from the leathery to the
horny shield covering may be seen in the very different contemporary
Cretaceous form, _Toxochelys Bauri_, where ossified epi-or supraneurals
occupy quite exactly the nodal relation of the five vertebral horn
shields of later turtles, like _Lytoloma_, though there are not the
slightest traces of sulci.
[Illustration: FIG. 126.—_Archelon ischyros_, a gigantic sea-turtle
from the Upper Cretaceous of South Dakota.]
[Illustration: FIG. 127.—Skull of _Archelon ischyros_: _pa_, parietal;
_f_, frontal; _pm_, premaxilla; _pf_, prefrontal; _ptf_, postfrontal;
_m_, maxilla; _j_, jugal; _qj_, quadratojugal; _sur_, surangular; _d_,
dentary; _an_, angular. (After Wieland.)]
“From a purely anatomical standpoint I have suggested that _Archelon_
had seven dorsal keels corresponding to those of _Dermochelys_. There
is much excellent reason for regarding dermogene ossification as
essentially double-layered throughout the Reptilia.
“In any restoration of _Archelon ischyros_ only the mid-line should
be accentuated as a series of rather sharp supraneural crests. These
are shown to have been present by the characteristic groove-like
median pits with radiating striae, which are such a prominent feature
of epineurals. It is reasonable to believe that the pits mark the
attachment of horny crests developed in the leathery hide. Such
were doubtless projected, more or less keel-like, to a height of
one or two inches, and thus gave to the mid-line of the carapace,
when seen laterally, a distinctly sinuous outline not unlike that of
_Toxochelys_.”
As regards the habits of these ancient sea-turtles, we may offer
tolerably certain conjectures. In the opinion of the writer, the
less reduced plastron indicates a bottom-feeding habit, a view that
is strengthened by the more rounded form of the shell, like that of
the river turtle. All in all it would seem that _Protostega_ and
_Archelon_ lived habitually on the soft bottoms of the shallower
seas, feeding upon the hordes of large shell-fish, for which their
powerful parrot-like beak was admirably adapted. That the species of
_Protostega_ did not commonly frequent the deeper oceans is indicated
by the general absence of their remains in the deeper water deposits.
The writer, in a long collecting experience, always found their remains
associated with those of the smaller _Toxochelys_, toothed birds,
pterodactyls, and the smaller mosasaurs.
Perhaps no one can speak more authoritatively as to the habits of these
gigantic sea-turtles of the Cretaceous than Dr. Wieland:
“With regard to the general habits and appearance of _Archelon_ much
might doubtless be said if the present-day sea-turtles were more
familiar objects. Dr. Hay thought that _Archelon ischyros_ was a clumsy
or even a sluggish, mainly littoral animal, moving slowly about the
bottom of quiet inlets in quest of shell-fish; I, on the contrary,
much struck by the powerful flippers, and especially by the flattening
of the humerus, with its low radial crest and obviously strong
musculature, have held that unusual swimming power and adaptation to a
strictly marine life were indicated. Perhaps, as usual where experts
differ, it is probable that both views are in part correct, and that
_Archelon_ was only a moderately good swimmer. It may be noted that,
notwithstanding the almost circular body, the femoral notch, that for
the hind leg, lies far back, so that it is not necessary, on the score
of bulk, to assume slowness of motion, or the inability to pursue a
sea-going life. Furthermore, it is now known that the development of
the digits fell little short of that seen in _Colpochelys_ (Fig. 117)
or _Eretmochelys_, truly marine turtles.
“Therefore, while there can be no doubt that _Archelon_ was strictly
carnivorous in habit, and well able to navigate the open seas, it
is not likely that it fed on other than relatively slow-moving prey.
Lydekker looked upon the broad mandibles and broad palate of _Lytoloma_
as specializations for a mussel diet; and very similarly in _Archelon_,
while the decurved beak would easily transform him into a most
formidable enemy, the heavy premaxillaries and vomer, and the flat but
deep lower jaw, suggest an adept crusher of crustaceans. The presence
of vast quantities of _Nautilus dekayi_, which I found accompanying one
of the specimens, was doubtless accidental, but it plainly suggests
that this cephalopod was one of the teeming sources of food in the
_Archelon_ environment.
“The huge bulk of the mature _Archelon_ might account for the shearing
off and loss of the flippers of younger forms caught between the shells
of the ‘elder boatmen of the Cretaceous seas,’ as Cope has called them,
during any sudden rush while herding on the shores. But probably the
young turtles did not much frequent the shores at either egg-laying or
other times. Whence it is much more likely that it was a mosasaur or
some of the gigantic fishes like _Portheus_ which bit off the right
hind flipper in the type-specimen of _Archelon ischyros_, well above
the heel, as I have described it. That this happened rather early in
life is shown by the arrested growth of the right femur and remaining
portions of the tibia and fibula, which are all uniformly 10 per cent
smaller than the corresponding bones of the left flipper.”
While there were many small fishes in the Niobrara seas which the
Protostegas inhabited, the most striking thing in the fauna is the
great abundance of molluscal shells, especially _Ostrea congesta_.
And with them were great hordes of larger pelycypod mollusks, some of
them of enormous size. Some of the largest reach a diameter of nearly
four feet, with shells so thin that one can hardly understand how they
could have supported such large, oyster-like creatures. One can imagine
that such shell-fish might have afforded an almost inexhaustible
source of food for the large turtles; and several times the writer has
found remains of _Protostega_ associated with such shells. From all
of which evidence it seems very probable indeed that Dr. Wieland is
right in imputing to these gigantic turtles a shell-feeding habit, a
habit which required neither speed nor great prowess; and perhaps the
formidable beak was used more in social quarrels than for food-getting.
That these marine turtles departed from the usual reptilian habit of
laying their eggs upon land is improbable. The tortoise shell turtles
of the Bahamas lay three or four hundred eggs in a hollow scooped out
in the sand and then leave the young to their own devices; certainly
many a one is gobbled up by birds of prey or other enemies on their way
to the water. Perhaps the young _Archelon_ lost its hind leg in some
such mishap.
LEATHER-BACK MARINE TURTLES
The most remarkable member of the Chelonia now living is _Dermochelys
coriacea_ (Fig. 128), the great leathery or leather-back turtle of
the warmer parts of the Atlantic, Indian, and Pacific oceans, the
sole member of the family Dermochelydidae. It is the largest of
all living turtles and the most thoroughly aquatic of all, whether
living or extinct. It sometimes reaches a length of six feet, or
half that of the largest known extinct forms, and weighs a thousand
or more pounds. Agassiz saw a specimen that he said weighed a ton.
Unlike other turtles, it has a carapace quite peculiar to itself,
composed of a layer of thin, irregularly polygonal bones forming a
mosaic, completely hidden in the thick skin, and entirely free from
the skeletal bones beneath them. The larger of these skin bones form
seven rows above, which appear in the living animal as sharp keels
running the whole length of the shell. On the under side there are five
rows of smaller-sized bones, under which there are vestiges of bones
representing the normal plastron of turtles. The limbs are powerful,
flattened paddles, not unlike those of _Eretmochelys_, but wholly
destitute of claws. The front paddles are much larger than the hind
ones; the humerus is long and flattened, and the digits are elongated.
The leather-back is a powerful and effective swimmer, going long
distances. Its habits are not well known; its food is chiefly fish,
crustaceans, and mollusks.
[Illustration: FIG. 128.—_Dermochelys coriacea._ (From Brehm.)]
So very different is the structure of its shell that some excellent
naturalists regard _Dermochelys_ as the equivalent in rank of all other
turtles combined, the sole representative of the suborder Athecae, as
distinguished from the Thecophora. Dr. Hay, whose authority on fossil
turtles is of the highest, believes that its line of ancestry has been
distinct from that of all other turtles from Triassic times at least.
Others believe that the leather-back is merely a highly specialized
form derived from the ordinary shelled type, a descendant of some of
the marine turtles of Cretaceous times. In support of the primitive
ancestry of the leather-back Dr. Hay offers the following:
“The writer holds the view that the earliest turtles possessed
practically two kinds of shell, one purely dermal, consisting probably
of a mosaic of small bones arranged in at least twelve longitudinal
zones. Each zone probably consisted of a row of larger bones bordered
on each side by smaller bones. Each of these bones was covered by a
horny scute. The nearest approach to such a dermal shell is in our days
seen in _Dermochelys_. Beneath the skin there seems to have existed a
carapace more or less complete, which consisted of a nuchal, a median
row of neurals, eight pairs of costals, a pygal, probably one or more
supraneurals, and about eleven peripherals on each side. To what extent
the neurals and the costal plates had become anchylosed to the neural
spines and the ribs respectively, it is now impossible to determine.
Nor can we say to what extent the various elements of the carapace had
become connected with one another. There was a subdermal plastron which
was composed of at least eleven bones.
“According to the author’s view, as time went on the external,
mosaic-like shell disappeared in most turtles, while a more efficient
armor was developed out of the subdermal elements. In the ancestors of
_Dermochelys_, however, the dermal armor was retained, while the more
deeply seated one disappeared, with the exception of the nuchal bone.”
Such a hypothesis as the foregoing satisfactorily explains the
extraordinary mosaic shell of the leather-back, and is perhaps an
acceptable explanation of the rather strange fact that the horny
shields of turtles do not correspond with the bones below them, as
might be expected. Unfortunately this hypothesis lacks sufficient
proof. About the only evidence that is offered in its support is the
existence of a row of bones along the middle line in the Cretaceous
_Toxochelys_, and notably in _Archelon_, both aquatic forms. It is
urged that these bones, the epineurals of Wieland, are really the
remains of an external layer that persisted in these turtles. However,
they might have been new ossifications, such as we know did occur in
not a few of the land tortoises later over the tail and limbs. Aside
from _Proganochelys_ in the vast interval of time from the Triassic to
the Eocene no other evidence of such an external dermal layer has been
discovered. The chief argument against such divergent ancestry of the
turtles in two chief lines of descent is the fact that in its other
structure _Dermochelys_ shows great resemblance to other sea-turtles of
the Cretaceous times—so much resemblance that it seems impossible that
the ancestors of the leather-back should have paralleled them in almost
everything except the shell.
On the other hand, those who disagree with this view believe that the
modern leather-back is the descendant of such Cretaceous marine turtles
as _Protostega_ or _Archelon_, some of which had lost nearly all of
the costal plates and had the neurals and marginals reduced. It is
urged that some of these early marine forms, after they had practically
lost the ordinary bones of the carapace, for some reason or other found
a bony shell again necessary for their welfare. Possibly they had
become littoral in habit; possibly they again became subject to new and
dangerous enemies in their unprotected condition, notwithstanding their
great size; perhaps the zeuglodons were among their enemies. Now, as we
have seen, an animal never takes a back track and recovers a thing it
has once lost. It was impossible for the ancestors of the leather-back
again to acquire an orthodox shell, and they forthwith proceeded to
acquire quite another kind that would serve the same purpose.
Possibly the truth lies between the variant views, in the theory
recently expressed by Versluys: “The shell of tortoises and turtles is
formed by a combination of two layers of dermal ossifications, a thecal
layer and a more superficial epithecal layer, the latter generally
represented by the marginals only. The leather-back is a member of the
Cryptodira, and is allied to the other marine turtles. The problem of
the origin of the aberrant shell of the leather-back seems to find its
solution in the hypothesis that it is a secondary proliferation of the
marginals and such other epithecal elements as were present in its
thecophorous ancestors.”
In other words, Versluys believes that Hay’s and Wieland’s views
of the primitive double layer of exoskeletal bones is essentially
correct, but that _Dermochelys_ was derived from later forms in which
some of them, only, as in _Archelon_, had remained. Baur’s contention
that “_Dermochelys_ is not the least, but the most specialized marine
turtle” seems to have been fully justified.
RIVER TURTLES. TRIONYCHOIDEA
No reptile is more familiar or more exasperating to the river fisherman
than the turtle, variously known as the river, soft-shelled, or mud
turtle. It lives, often in great numbers, in most of the rivers, ponds,
and bayous of the interior east of the Rocky Mountains, and especially
in those of sluggish current and muddy bottoms. It is voraciously
carnivorous in habit, feeding upon the smaller fish, mussels, and such
other living food as it can capture. With its long, sinuous neck and
snake-like head, and soft, mottled skin, it is repulsive enough to most
persons, but is especially annoying to the fisherman, since it devours
with impunity his bait so long as he feeds it, and can seldom be caught
on the hook because of its hard and bony mouth, in which only by good
luck will the hook catch. And the luckless string of fish that the
fisherman leaves in the water may be almost completely devoured in a
few hours by these fiercely predaceous feeders. However, if so annoying
while seeking for better game, it in part makes up for the annoyance it
causes by furnishing in its own body a not unpalatable food for those
who like to eat reptiles.
[Illustration: FIG. 129.—_Trionyx_, river turtle. (By permission of the
New York Zoölogical Society.)]
The river turtles will be readily recognized from the accompanying
illustration (Fig. 129). They are very flat, covered with a soft,
smooth skin, with a long, sinuous neck and a small, snake-like and
vicious-looking head which has a protuberant snout with the external
nasal orifice at its end. Their feet are webbed and somewhat
paddle-like, but always with three stout claws—whence comes the name of
the group—on the anterior digits, which are used for burrowing in the
mud and excavating holes for their eggs. These turtles burrow more or
less in the mud, with the long neck free, lying in wait for their prey,
and coming to the surface from time to time to breathe. As the shape
of the body and the paddle-like feet would suggest, they are active
swimmers and purely aquatic in habit, never leaving the water unless
compelled to. They bury their hard-shelled eggs on the shores only a
few feet from the water, and leave them to their fate. If the pools
in which they live dry up, they burrow deeply in the mud and await
the rains and floods. In captivity they feed upon all kinds of food,
vegetable as well as animal, and are active and aggressive.
[Illustration: FIG. 130.—_Aspideretes_, a trionychoid turtle from the
Basal Eocene of New Mexico; skull from above. (From Hay.)]
Because of certain peculiarities, they are usually classed in a
separate suborder all their own, the Trionychoidea, especially
distinguished from the Cryptodira, which in general they resemble in
most respects, aside from the absence of the usual horny dermal plates,
in the lack of a marginal row of bony plates around the carapace—not a
very important distinction. Less than thirty living species are known,
all of them exclusively, or chiefly of fresh-water habit. Six species
are known from North America; the remainder inhabit Africa, south of
the Sahara Desert, southern India, and most of the East Indian islands;
none is known from Australia. No species lives in South America and
none is known to have lived there in past times. During Eocene,
Oligocene, and Miocene times these fresh-water turtles lived in the
region of Europe in great numbers, but for some inexplicable reason
they became extinct there and never returned. Nearly seventy species of
the Trionychoidea, belonging in two families, are described by Dr. Hay
from the Tertiary rocks of North America, more than twice the number
now living throughout the world. Some of these were of relatively
large size, measuring fully two feet in the length of the shell. And in
some places they must have been very abundant. The writer has seen, in
the Bad Lands of the Continental Divide, their weathered-out remains so
numerous that they might be raked into windrows miles in length along
the sloping bluffs, all in small fragments, for their bones, like those
of most turtles, are only loosely united by sutures and readily drop
apart before fossilization. Their shells may be readily distinguished
from those of all other turtles by the granulated, pitted, or
sculptured exterior surface, that was covered by the skin in life;
other turtles have the surface smooth below the horny shields, the
margins of which are marked on the bones by grooves or sulci; the few
marine turtles of the past that were probably covered with a soft skin
instead o£ horny shields had the shell smooth and much less completely
ossified.
[Illustration: FIG. 131.—_Aspideretes_, a trionychoid turtle from the
Eocene of New Mexico; front leg. (From Hay.)]
As to the origin of the soft-shelled turtles there has been not a
little difference of opinion. The earliest ones known in geological
history date back only to about the middle of the Cretaceous; perhaps
they branched off from the horny-shelled turtles somewhat earlier, but
probably not much. There are some, however, who think that this group
of turtles was very primitive, perhaps the most primitive, but the
writer agrees with Dr. Hay in rejecting this view. Unlike those of all
other turtles, the fourth digit in front and hind feet has one or two
more phalanges than have other turtles. We have seen that the oldest
known reptiles had the digital formula 2, 3, 4, 5, 3 or 4. Most other
turtles have the same numbers of bones in the digits that mammals have,
that is, two phalanges in the thumb and big toe and three in each of
the other digits. The river turtles have a larger number in the fourth
digit, either four or five. It seems to be a law that evolution is
irreversible, and if so could the river turtles have been descended
from forms with a less number of phalanges? But, the skeleton of the
Trionychoidea resembles the more specialized turtles in so many ways
that one can hardly believe they were all accidental or parallel.
We may then assume that at about the time that the ordinary marsh
turtles took to the sea to become marine, others took advantage of
the fresh-water ponds and rivers, and in doing so, like the marine
turtles, lost their horny epidermal shields, and became thinner in
shape, thereby reducing the resistance to the water. Instead, however,
of reducing the costal plates over the ribs, they retained them intact
and complete for some reason or other, but lost instead the marginal
row of bones, unlike the marine turtles which retained them even after
they had lost nearly all of the costal plates. Possibly also they
regained additional bones in the fourth digit, a sort of hyperphalangy
like that of the more strictly aquatic reptiles. Or, possibly, they may
have descended from some branch of the turtles which had not yet lost
these bones, retaining them because they were still serviceable for
swimming. We know nothing yet about the structure of the feet of the
early turtles, and it is possible that not all had acquired the reduced
phalangeal formula.
In the development of aquatic habits the river turtles do not show
the same degree of specialization in the limbs that the strictly
marine forms do. The humerus (Fig. 131) is a slender bone, with the
tuberosities for the attachment of the muscles situated near the
proximal end. The radius and ulna are relatively short, and the foot is
long. The hind legs, as would be supposed, are less highly specialized
as swimming paddles, and are relatively smaller. Nevertheless the
Trionychoidea present an interesting type of adaptation to water
habits, both in body and in limbs.
INDEX
Adaptation to aquatic life, 59.
Aetosauria, 187.
Aigialosaurs, 146.
Alligator, _197_.
Amblyrhynchus, _142_.
Andrews, C. W., 75.
Angistorhinus, 190.
Anomodontia, 102.
Aquatic reptiles, adaptation of, 59.
Araeoscelis, _133_, 138.
Archelon, 234, _236_, _238_, 239.
Archosauria, 33.
Aspideretes, _246_, _247_.
Baptanodon, _113_, _114_, _117_.
Baur, George, 119, 120, 185, 234.
Belodon, 185, 189, _191_.
Belodontia, 185.
Bêche, De la, 73.
Bogalobou, 75.
Broom, Robert, 5, 102, 103, 129.
Brown, Barnum, 93, 179.
Buckland, Dean, 75, 77.
Cacops, _35_.
Camper, Adrian, 149, 166.
Camper, Peter, 149, 166.
Captorhinus, _49_.
Caretta, 230.
Carpus, 38.
Casea, _55_.
Case, E. C., 234.
Champsosaurus, _179_, _180_, _181_, _182_.
Chelone, _220_, 229.
Chevrons, 32.
Choristodera, 178.
Classification of reptiles, 13.
Clavicles, 37.
Cleithrum, 36.
Clidastes, _147_, _154_, _155_, _157_, 166.
Collection of fossils, 10.
Colpochelys, _222_.
Conybeare, Rev., 73, 110, 149.
Cope, E. D., 126, 176, 178, 185, 231, 233.
Coracoid, 36.
Cotylosauria, 16.
Cryptodira, 226.
Cretaceous of Kansas, _8_.
Crocodiles, ancient, 204;
marine, 207;
modern, 195.
Crocodilia, 15, 194.
Crocodilus, _195_.
Cuvier, Georges, 73, 97, 107, 110, 149.
Darwin, Charles, 142.
De Fond, St. Faujas, 148.
Dermochelys, 241, _242_.
Desmatochelys, 231, _232_.
Dimetrodon, _36_, _51_.
Dinosauria, 18.
Dolichosaurs, 145.
Dolichobrachium, 54.
Dollo, Louis, 167, 179.
Edaphosaurus, _23_.
Elasmosaurus, _79_, _84_, _86_, _87_.
Enaliosauria, 75.
Eosauravus, 52.
Episcoposaurus, 190.
Eretmochelys, _230_.
Eryops, _31_, _47_.
Eusuchia, 195.
Exoskeleton, 43.
Extinct reptiles of North America, 52.
Femur, 41.
Fibula, 41.
Foot, 42.
Fraas, Eberhard, 74, 75, 99, 111, 115, 185, 213.
Gastroliths, _92_, 200.
Gavial, Borneo, 201;
Gangetic, _198_, _199_, _202_.
Gavialidae, 203.
Geological Ages, 46.
Geosaurus, _208_, _210_, _211_, _212_, 214.
Gervais, Professor, 126.
Gilmore, Charles, 113, 167.
Globidens, _167_.
Goldfuss, August, 151.
Graptemys, _218_.
Hadrosaurus, 56.
Hand, 38.
Hauff, B., 122.
Hay, Oliver P., 220, 227, 228, 234, 242.
Hofmann, Dr., 148.
Holops, 207.
Home, Everard, 110.
Homo diluvii testis, 108.
Huene, Friedrich von, 131, 185.
Humerus, 38.
Huxley, Thomas, 185, 207.
Hydrus, _169_.
Hyperphalangy, 118.
Hypocentrum, 36.
Ichthyosauria, 17, 107.
Ichthyosaurus, _108_, _112_, 119, 121, _122_.
Iguana, _140_.
Ilium, 39.
Interclavicle, 37.
Intercentrum, 30.
Ischium, 39.
Jaeger, George, 184.
Jaekel, 185.
Karoo beds, 102.
Koenig, 110.
Labidosaurus, _22_, _26_, _50_.
Laecertilia, 140.
Lariosaurus, _99_, _100_.
Leather-back turtles, 241.
Leidy, Joseph, 151.
Limnoscelis, _20_, _47_.
Lizards, 140;
flat-headed, 144;
Galapagos, 142.
Lull, Richard, 54.
Lystrosaurus, 103, _104_.
Lortet, M., 135.
McGregor, J. H., 126, 129, 131, 185, 186, 189.
Mandible, 25.
Mantell, Dr., 75.
Marsh, O. C., 54, 185.
Merriam, J. C., 112, 120, 171, 174.
Merriamia, _118_.
Mesosaurus, 126, _127_, _128_.
Mesosuchia, 185, 204.
Meyer, Hermann von, 75, 87, 132, 134, 184.
Mixosaurus, 119.
Monitor lizards, 144.
Mosasauria, 148, 166.
Mosasaurus, 148;
hofmanni, 149;
maximiliani, 151.
Münster, Georg von, 97.
Mudge, B. F., 93.
Mystriosuchus, 185, _188_, _189_, _191_.
Nares, 23.
Nectosaurus, 175.
Nothosauria, 95.
Nothosaurus, _96_, _97_, _98_.
O’Fallen, Major, 151.
Ophiacodon, _30_, _32_, _37_, _38_, _40_, _42_.
Ophidia, 168.
Ophthalmosaurus, _118_, 125.
Orders of reptiles, 16.
Osborn, H. F., 126.
Ostodolepis, _33_.
Owen, Richard, 75, 103, 110, 111, 185.
Paleorhinus, 190.
Paliguana, 5.
Parasuchia, 18, 184.
Parietal foramen, 23.
Pectoral girdle, 34.
Pelvic girdle, 39.
Pelvis, 39.
Pelycosauria, 186.
Pelycosimia, 187.
Phytosauria, 18, 187.
Phytosaurus, 186, 187.
Pineal foramen, 23.
Platecarpus, _151_, _155_, _156_, _158_, _159_, 166.
Platynota, 148.
Plesiosauria, 77.
Plesiosaurus, _74_.
Pleurodira, 224.
Pleurosaurus, _134_, _136_, 177.
Polycotylus, 80.
Proatlas, 32.
Proganochelys, 217.
Proganosauria, 17.
Proteosaurus, 110.
Protorosauria, 17, 132.
Protorosaurus, 132.
Protostega, 233, 234.
Protostegidae, 231.
Pseudosuchia, 185.
Pterosauria, 18.
Pubis, 39.
Pythonomorpha (Mosasauria), 166.
Range of Reptilia, _45_.
Rhachitomous vertebrae, 17.
Rhynchocephalia, 17, 176.
Rhytidodon, 190.
Ribs, 33.
River turtles, 244.
Rutiodon, _190_.
Sacrum, 32.
Sapheosaurus, _178_.
Sauranodon (Baptanodon), 125.
Sauropterygia, 17, 73.
Scheuchzer, 107.
Seeley, H. G., 75, 126, 132.
Seymouria, _21_, _48_.
Shoulder-girdle, 34.
Simoedosaurus, 179.
Skeleton of reptiles, 19.
Skull of reptiles, 21.
Snakes, 168.
Spener, 132.
Sphenodon, _24_, 132, _176_.
Squamata, 17, 138.
Stegocephalia, 48.
Stereosternum, 126.
Sternum, 37.
Stomach-stones, 200.
Tarsus, 42.
Teeth, 21, 25.
Teleosaurus, _205_.
Temporal openings, 23.
Testudo, _228_.
Thalattosauria, 17, 171.
Thalattosaurus, _172_, _173_, _174_.
Thalattosuchia, 207.
Therapsida, 16.
Theromorpha, 16.
Thaumatosaurus, _74_.
Thoracosaurus, 207.
Tibia, 41.
Tomistomidae, 201.
Tortoises, 216;
fresh-water, 226;
land, 227;
marsh, 226.
Toxochelys, _219_, _231_, _232_.
Trachemys, _221_.
Trimerorhachis, _25_.
Trinacromerum, _77_, _81_, _83_, _85_, _88_, _89_.
Trionychoidea, 244.
Trionyx, _245_.
Tuatera, 176. _See_ Sphenodon.
Turtles, 216;
river, 244;
sea, 229;
ancient sea, 231;
side-necked, 224.
Tylosaurus, _153_, _155_, _157_, _160_, _165_, 166.
Varanus, _144_.
Varanops, _53_.
Ventral ribs, 34.
Vertebrae, 28;
notochordal, 29;
rhachitomous, 31.
Versluys, 244.
Watson, D. M. S., 103.
Wieland, G. R., 234, 236, 239, 243.
Woodward, A. S., 126.
Zittel, Carl von, 185.
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