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Title: Fossils: A Story of the Rocks and Their Record of Prehistoric Life - Denver Museum of Natural History, Popular Series No. 3
Author: Markman, Harvey C.
Language: English
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                                FOSSILS
                          A Story of the Rocks
                                  and
                    Their Record of Prehistoric Life


                          By Harvey C. Markman
                  Curator of Geology and Paleontology

                       Cover Design and Murals by
                           Mary Chilton Gray


                            DENVER MUSEUM OF
                            NATURAL HISTORY

                          Popular Series No. 3
                        Alfred M. Bailey, Editor

                        Third Edition, Reprinted
                            October 1, 1954

    [Illustration: At Work on a Dinosaur Skeleton]



                                CONTENTS


                                                                     Page
  Introduction                                                          5
  The Prehistoric Record                                                5
      Varieties of Fossils                                              8
      Fossilization                                                     9
      Floras and Faunas                                                13
      Formations                                                       16
      Geological Time                                                  18
      Explanation of the Time Chart                                    23
      The Geological Section                                           25
  Before the Age of Reptiles                                           31
      The Pre-Cambrian Complex                                         31
      Cambrian Life                                                    33
      The Ordovician Record                                            35
      Silurian Events                                                  36
      Devonian Progress                                                37
      Carboniferous Forests                                            40
      Permian Hardships                                                43
  The Age of Reptiles                                                  47
      Dinosaurs                                                        48
      Plant Life and Climate                                           56
      Coal and Fossil Footprints                                       57
      Mesozoic Invertebrates                                           58
      Extinct Birds                                                    60
      Ancestors of the Mammals                                         61
  The Age of Mammals                                                   64
      Prehistoric Horses                                               70
      Mastodons and Mammoths                                           80
      The Rancho La Brea Fossil Pits                                   88
  The Age of Man                                                       93
  Supplementary Reading                                                95



                         LIST OF ILLUSTRATIONS


                                                                     Page
  At Work on a Dinosaur Skeleton                                        2
  Fossil Bones in Rock Formation                                        7
  Insect Fossils                                                       10
  Restoration of Rhinoceros                                            12
  Dinosaur Tracks                                                      17
  Time Chart                                                           22
  Geological Section Showing Positions of Formations                   26
  Marine Beds of the Benton Formation                                  28
  Plesiosaur Bones in Place                                            28
  Invertebrate Fossils                                                 34
  Modernized Fishes                                                    38
  Prehistoric Plants                                                   41
  Marine Reptiles                                                      46
  Diplodocus                                                           49
  Trachodon                                                            51
  Stegosaur                                                            52
  Sea Turtle                                                           54
  Murals, Hall of Mammals                                              63
  Uintatheres and Contemporary Life                                    65
  Moropus                                                              67
  Titanotheres                                                         69
  Oligocene Mammals                                                    71
  Pleistocene Horse                                                    73
  Structure of Molar Teeth                                             75
  Grazing Type of Molar Teeth                                          77
  American Mastodon                                                    79
  Long-Jawed Mastodont                                                 81
  Molar Tooth of Mammoth                                               83
  Nebraska Mammoth                                                     85
  Rancho La Brea Fossils                                               87
  Folsom Bison                                                         90
  Man and Mammoth                                                      92



                                FOSSILS



                              INTRODUCTION


In the recent growth of knowledge there has been rapid progress in two
directions. The commercial exploitation of natural resources, being
fundamental to modern civilization, attracts a liberal share of the
talents and energies of workers trained for the industrial professions.
A second trend has specialized in the further development of the
sciences which are characteristic of our time. Such activities, in the
natural history field, deal largely with the refinements of exact
definition, nomenclature and classification, all of which means little
or nothing to the layman who is otherwise engaged.

For the latter, however, there is a quality of interest which may be
described as a wholesome curiosity about what has happened, how it
happened, how we know it happened, and what it may signify to one who is
neither industrialist nor scientist. This booklet is intended for the
many who feel that there is more to be obtained from a natural history
museum than an occasional glimpse of a bewildering “marvel.” In addition
to being a guide to fossil exhibits it supplies parts of a great story
which specimens alone can not relate.



                         THE PREHISTORIC RECORD


All that is known of the extinct plants and animals which inhabited the
earth before man began the practice of recording his observations has
been obtained from a study of the rocks. The few possible exceptions to
this rule, in which animal and plant remains have been preserved by
freezing or drying, are so unusual as to be hardly worth mentioning.

Explanation of this is that plant and animal tissues quickly decay under
ordinary conditions when life ceases. Unless protected from destructive
agencies which are especially active at the surface of the ground, even
the heavier bones of animals and the large trunks of fallen trees will
soon crumble into shapeless masses. The usual method employed by nature
to prepare a fossil specimen is so closely related to the natural
process of rock making that a little knowledge of that subject will be
necessary in order to know what fossils are and how they are preserved
for so long a time.

It should be understood first that a fossil is some record, commonly
preserved in rock, of a kind of plant or animal which no longer exists
as a living type. This, at least, is the ordinary sense of the word and
more elaborate definitions are of small service to anyone. It may be
necessary to add, however, that all things which have lived at any time
are regarded as either plants or animals.

Nature’s way of producing rocks and fossils remains a mystery to many of
us because we are so wrapped up with the importance of finding names for
things and materials that we frequently neglect the consideration of
sources and histories. Everyone knows a rock when he sees it in a large
mass, but when he looks at sand, mud, dust, or soil, he seldom thinks of
it as related in any way to rocks. Although the difference is almost
entirely a matter of size, our use of words makes it seem unreasonable
to speak of the finer particles as rock.

There can be no reality or meaning in the natural record for an
individual who has failed to observe a few simple facts which involve
changes going on in all parts of the world at the present time. With
regard to rocks, it is supposed that what happens in our day also
occurred under like circumstances ages ago. Anyone wishing to do so may
see for himself that rock masses break down wherever they are exposed to
the elements, that the larger pieces are reduced to smaller fragments,
and that the final product is sand or dust.

He may also note that this finely ground material is being moved and
sorted, by rain, wind, and streams, transported to lower levels and
accumulated in great quantities wherever it finds a resting place. Along
with it go sticks and leaves, bugs, shells, bones and carcasses of
animals, some of which in time may become fossils. In large lakes and
seas there is a steady distribution of such materials over broad areas,
yesterday’s accumulation of sediments being buried by the contributions
of today, the most recent of the settlings always resting upon older
ones until something happens to disturb that arrangement.

Not so readily observed are other parts of the process, such as the
consolidation of sands and muds into the firm sandstones and shales
which we again recognize as rocks. Much of this requires more than the
few score years of a human lifetime for its accomplishment, but many of
us have seen muds become so solidified, by merely drying, that they
could hardly be distinguished from prehistoric shales. It is to be noted
also that some ancient fossils come from sandstones which are scarcely
more rock-like than the loose sands of an ocean beach. Thus we learn
that firm consolidation of rock-making materials is not always a sign of
great antiquity, and that hardness of rock is not always essential to
the preservation of imbedded plants and animals.

    [Illustration: Rhinocerous Bones as Found in the Rock

    This exhibit was taken from the famous fossil quarry at Agate,
    Nebraska. The fossilized remains are still partially imbedded in the
    sandstone which preserved them for millions of years.]

The rocks themselves must explain the many things which have happened
during the course of millions of years, and this they do remarkably well
when carefully studied, for many of the factors involved in their
histories leave characteristic marks. Changing climates, the draining of
seas, the uplifting of mountain ranges, all have ways of registering
their occurrence which are as convincing and reliable as anything ever
written by man. Piece by piece the story has been patched together
through the efforts of thousands of investigators. Parts of the
narrative remain buried at inaccessible depths, and whole chapters, no
doubt, have been destroyed by the same forces that composed this
tremendous record of prehistoric times.


VARIETIES OF FOSSILS

It would be a serious mistake to regard nature as divided into a number
of distinct and independent schools of fossil making, each refusing to
use the methods and devices of another. There are, however, certain
features which stand out so prominently that a little classification
becomes helpful. While this procedure brings out differences it should
be understood that processes actually work together, several of them
usually being involved in the production of any individual specimen.

(1) Impressions of animals and plants, or parts of these, are frequently
left in soft sand or mud which later becomes converted into more durable
rock. This type of fossils is represented by animal foot-prints and the
imprints of leaves, flowers, insects, and like objects which may be
mingled with the finely ground materials of the common sedimentary
rocks.

(2) Parts of plants and animals may be gradually replaced by mineral
matter with little or no change from original form and texture. Fossils
of this class are said to be petrified or turned to stone. They are also
known as replacements. The fleshy parts of animals do not petrify.

(3) Many animals among the invertebrates use mineral substances for
protective or supporting structures. Small plants of various kinds
follow a similar practice. These structures, being produced in stony
materials, are readily converted into fossils. The shells of mollusks
are the best known illustrations in this field, and all that is required
for a shell to become a fossil is the extinction of the species of
animal that produced it. Fossils of this type are extremely abundant.

(4) Preservative substances other than those which produce common rocks
may be mentioned among fossil-making possibilities. Bones are known to
have been preserved in asphalt, and insects in resins, but such cases
are few in comparison with the products of other methods.

(5) In rare instances there has been preservation of extinct creatures
by the process of drying or by refrigeration. Occasional mummies are
found with shriveled flesh and skin still in place, but better
preservation of all tissues occurs when the temperature is quickly
reduced below freezing point and held there without interruption. This
can happen only in the colder parts of the earth and is always subject
to climatic change. The effect of drying also may be undone at any time
by a slight increase in the amount of moisture.

(6) Coal beds often produce fossils of an unusual sort. In the formation
of coal, plant material gradually loses some of its more perishable
substances but retains carbon which has better lasting qualities and
slowly accumulates to produce the seams and beds that are mined. In the
early stages of the process the original vegetation undergoes little
change in appearance but eventually practically all of its character is
lost. Many fossil leaves are found as thin layers of carbon, bedded in
the clays which are commonly associated with coal deposits.

(7) Concretions, which are hardened lumps of mineral substances
occurring commonly in sandstones and shales, are often mistaken for
fossils because of their peculiar shapes. However, there are localities
in which the mineral solutions have been concentrated and deposited
around shells, leaves, seeds, or similar objects, thus producing an
abundance of fossils which may be obtained by opening the concretions.
Fossils of this type are well known from Mazon Creek and other districts
in Illinois, Kansas, Colorado, and elsewhere.


FOSSILIZATION

Footprints need little explanation other than a consideration of the
factors which make it possible for them to be preserved. The sand or mud
must be neither too soft nor too hard to take the form of the foot and
retain its shape when the foot is withdrawn. Then in some manner the
impression must be protected while the rock-making process goes on. When
such protection is obtained it is usually in the form of more mud and
sand, deposited over the surface which received the impression. At a
later time the covering may be separated from the lower part of the
deposit, which serves as a mold, and if the separation be accomplished
successfully a natural cast of the foot will be obtained as well as the
mold in which it was produced. Since conditions for perfect work are not
always present in a laboratory of this kind, it is not surprising that
fossil footprints are very rare considering the number and variety of
tracks left by wandering animals.

Impressions of leaves are explained in much the same way except that the
leaf remains under its protective covering until it decays. Similar
impressions may be obtained from the bodies of delicate invertebrate
animals but they are seldom preserved because of the softness of the
tissues. The smaller fishes provide much better material for the
production of fossils according to this method. While the fish is being
flattened by the weight of surrounding sediments, scales, fins, and soft
bones retain their positions and provide the necessary resistance to
leave an impression of the body form when the flesh is gone.

    [Illustration: Insect Fossils (enlarged)

    Fine specimens of this type are obtained from an old lake bed at
    Florissant, Colorado.]

The larger and more spectacular fossils, such as skeletons, skulls, and
detached bones are nearly always of the replacement type. Replacement of
plant and animal substances by mineral matter is a slow process and in
younger fossils the change is rarely completed, some of the original
material being present in a partially altered condition or not modified
at all. Since air does not often carry the necessary materials and
provide other essential conditions, replacement may be regarded as
something which happens underground or in water. It is perhaps best
explained in connection with limestones, because calcite or “lime” is
frequently the replacing substance although other minerals, especially
quartz, may serve the purpose.

Besides converting bony or woody objects into rock substance, mineral
replacements may assist in the production and preservation of fossils in
another manner. It often results in the filing of cavities with some
rock-making substance which retards destruction through crushing or
other injury. In many cases, so-called fossil shells are not shells at
all; instead, they are merely a stony filling which was once surrounded
by shell substance. In other instances the original shell remains as it
was during the life of its former occupant, preservation of the shell
being due largely to the substitution of a mineral filler for the soft
animal tissues once present.

Limestone comes into existence through a more elaborate process than
that which produces sandstone and shales. It is one of the three types
of common rocks, known collectively as the sedimentaries, in which
fossils are found. It differs from sandstones and shales, however, in
that much of its substance has been dissolved in water instead of being
transported in the form of finely ground rock particles. Lime occurs in
many varieties of rock which are exposed to the wear and tear of the
elements throughout the world. Slowly but more or less continuously it
is taken from this source by ground and surface waters coming in contact
with it. Particularly active is carbonated water, moving underground
through pores and crevices.

This underground circulation of mineral matter in a dissolved condition
explains the occurrence of fossils in land areas which have not
necessarily been submerged during any great length of time, for it is
well known that plant and animal remains are not invariably washed into
lakes or seas, and that all sedimentary deposits have not been built up
in large bodies of water. Here we are dealing with what is known as the
continental type of sedimentation and such fossils as dinosaurs,
mastodons, three-toed horses, and other former inhabitants of land
areas.

In order to become properly fossilized, certain conditions are
absolutely necessary, and only a small percentage of the once-living
multitude secures the required treatment. There must be present, soon
after death, some protection from the activities of the carnivorous
birds and beasts that would separate and scatter the parts of a carcass,
also from the smaller gnawing animals that would continue the
destruction, and finally from wind, sun, rain, frost, and bacterial and
chemical activities which in the course of only a few years would remove
everything but possibly a few scraps of tooth enamel, which is the
hardest of animal tissues.

A slight covering of earth substance in any form serves to check the
disintegration, and this may be acquired in several ways. Animals that
perish in bogs or quicksands are soon covered over; in many localities
wind-blown dust and sand do the work; and flooded river valleys provide
an abundance of mud for the necessary burial of others. Even
underground, the decay of soft tissues is too rapid to permit of
replacement by mineral substance in a manner that would reproduce form
and texture. Skin and flesh are almost invariably lost, although in a
few instances the thick scaly hides of dinosaurs are known to have
produced natural molds and casts by the method explained in connection
with footprints and other impressions.

    [Illustration: The skeleton of this prehistoric American rhinoceros
    is mounted in a “half shell” which was modeled over the bones to
    show the form of the living animal. The artist’s reconstruction
    appears in the painting above the fossil exhibit.]

With regard to the more durable tissues found in the teeth, bones, and
shells of animals, or the woody parts of plants, the case is different.
These parts become firmly imbedded in the ground, but moisture still has
access, and it begins to work immediately; for all water moving
underground finds soluble substances which it picks up and carries with
it wherever it goes, and much of the load consists of mineral matter
which may be unloaded again when the necessary conditions are found.

Mineral-laden waters will drop one kind of substance to take up another
which dissolves more readily, and this happens sooner or later when a
buried bone or log is encountered. Complications of various sorts enter
into the process, but the final outcome frequently is a complete change
from one chemical composition to another which is more enduring, the
transformation being brought about so gradually and thoroughly that in
many fossils the inner structure of the original tissue is as accurately
reproduced as the fine detail of surface features.

Converted into stone, however, the result is still far from permanent.
While yet underground the fossil is subjected to distortion and breakage
due to earth movements which bend and dislocate the rock deposits. What
causes these upheavals and depressions of the earth’s surface remains
the subject of much discussion, but that they have occurred on a large
scale and continue to occur is clearly evident. At higher altitudes the
surface rocks and fossils are exposed to a larger variety of destructive
activities than at lower levels where protective coverings are more
likely to be provided and retained. Once stripped of that protection
there is little chance for a fossil to survive. Beyond a doubt there are
many thousands of tons of prehistoric remains damaged or destroyed each
year, by weather and stream erosion.


FLORAS AND FAUNAS

As the various types of sediments continue to accumulate on land and in
water they produce deposits of sandstones, claystones, and limestones
which in time may acquire great thickness and cover wide areas of sea
floor, or continental surface. Usually there is more or less mixing of
sediments resulting in sandy limestones, limy clays, and other
combinations. Quite commonly, however, the types remain fairly pure but
become arranged in layers which alternate from one kind of material to
another. At all times the character of the deposit will depend upon the
nature of the rocks which supply the materials, and any fossils that may
be produced will consist of such plants and animals as live and die
during the time the rock is in the making.

Some of the rock layers will be rich in plant and animal remains, others
quite barren, the difference being due partly to conditions influencing
the life of the region. In addition, the character and amount of
rock-making materials at the time may be favorable or unfavorable to the
preservation of fossils. Seas, lakes, and valleys may at any time be
drained, or enlarged and deepened, by changes in the elevation of
underlying rocks. The amount and variety of mineral substances dissolved
in the waters of a region not only affect the character of rock deposits
but also the plants and animals living in the water. Some of these
chemical solutions provide cementing materials which bind together the
grains of sands and mud; others have a detrimental effect upon cementing
material previously deposited, and so construction and destruction go on
continuously, more or less hand in hand, to produce complicated and
often puzzling results.

A little more salt, or a little less of it, may change completely the
variety of life inhabiting a body of water. A slight change in the depth
of the water often accomplishes the same thing, for plants and animals
are so delicately adjusted to their environments that conditions fatal
to one race of creatures may provide the exact life requirement of
another. This is a matter of practical knowledge which is being used
today in the cultivation of plants and animals for market purposes. It
is being demonstrated continuously, also, upon living subjects in
experimental laboratories throughout the world; and, in a bigger way,
the facts are observable wherever life is considered in relation to
habitat. That anything so obvious should be regarded as guesswork or
theorizing, or opposed to truth, when applied to former inhabitants of
the earth, is somewhat surprising. And, it may be added, the cultural
worth of fossil study comes to a focus on this very point, for men and
women are now meddling, consciously or unconsciously, wisely or
unwisely, with an all-important environment about which they have
learned very little—one called, among other things, “civilization.”

For any portion of the world a complete-list of the different kinds of
plant inhabitants comprises the _flora_ of that region, and a like
summary for the animal life is known as the _fauna_ of the district. It
is generally understood that different species of both plants and
animals inhabit different regions of the earth, but outside of
professional circles it is only beginning to be recognized that changes
in floras and faunas occur from time to time, that slight differences
may be noted in the course of observations extending over a period of
only a few years, and that everything in a fauna or flora eventually may
be displaced by new forms.

It is, however, a convenient practice to use these terms in connection
with time periods, rock beds, and types of environment, as well as
geographical areas. Thus we have such phrases as a “Cretaceous fauna”
(attaching the name of a geologic period), a “Benton fauna” (with
reference to the fossils of a rock formation), a “marine flora” (using
the name of an environment), an “Arctic flora” (which applies to a
definite portion of the earth surface and its plant inhabitants).

Faunas include animals which many persons do not recognize as such.
Sponges, corals, insects, worms, crabs, oysters, and a host of other
boneless creatures are grouped together as _invertebrate_ animals, while
another group includes the fishes, amphibians (toads, frogs, and
salamanders of today), reptiles (crocodiles, lizards, snakes, and
turtles being well known varieties), birds, and mammals. This second
lot, provided with backbones and skeletons, comprise the great division
of _vertebrate_ animals.

Floras also include types which are commonly seen but not popularly
identified as plants. The algae are perhaps best known as seaweeds,
water-silk, and pond scums; fungi as toadstools and moulds. Both groups
are large and of important rank in the vegetable kingdom; only the
algae, however, are recognized as important fossil producers. Better
known types of plants are the mosses, ferns, evergreens, grasses, and
the more conspicuous flower-bearing forms, from weed size to tree size.

Many rocks owe their character to the work of large colonies of plants
or animals, for the living organisms are frequently the active agency
which takes dissolved mineral substance from the solvent liquid and gets
it back into solid form. The liquid is, of course, the water in which
the creatures live, while the mineral substance often becomes a
commodity required by a plant or animal in its mode of living. Mollusks
have a way of using lime in the production of shells, and many a bed of
limestone consists almost entirely of this by-product of molluscan life.
Tiny coral polyps build complicated and beautiful structures from the
same mineral substance. Either intact or in broken condition, these
structures contribute in a large way to the making of limestones. Algae,
among the lowliest of plants, have done extensive work along similar
lines, and numerous invertebrate animals could be named as important
factors in the production of rocks. Many of the shells and other
fabrications retain their peculiar patterns long after the extermination
of their makers, and a highly informative part of the fossil record is
provided in this manner. It is also by far the larger portion of the
record, for the earlier ages of prehistoric time failed to produce a
vertebrate animal of any kind, while the invertebrate record dates back
to pre-Cambrian time.


FORMATIONS

If in some part of North America there had been steady accumulation of
sedimentary materials under constantly favorable conditions since the
beginning of Cambrian time, the result would have been a deposit of
sandstones, claystones, and limestones measuring nearly fifty miles from
bottom to top. These figures are based on actual production in North
America where extensive measurements have been made in many localities.
When other parts of the world are as thoroughly investigated and older
deposits included in the calculations, the total thickness of such beds
will probably be more than one hundred miles.

No single pile of rocks offering a complete cross section of the
geological record has ever been produced, but portions of the section
are exposed to view on all the continents. In order to carry on
desirable investigations and make comparisons, it has been necessary to
divide this great composite section into small units which may be named
in some way and placed definitely with relation to lower and higher, or
older and younger, layers. To serve this purpose there has been
developed the idea of rock _formations_, and here we have a word which
is not defined readily, even for the use of those who are familiar with
it. Nevertheless it is used so commonly that some understanding of its
meaning becomes desirable.

A _formation_ may be regarded as an extensive rock mass, variable, in
thickness and other proportions, as well as in composition, but
representing a period of time during which there was no great change in
the character of plant and animal life, and no serious interruption in
the depositing of the rock-making materials. Occasionally the lower and
upper limits of a formation are well defined and readily located.
Frequently, however, the transition is gradual, one formation merging
into another with no apparent mark of separation. In such event the
original description serves to establish more or less definitely the
boundaries of a formation.

Descriptions are published whenever a worker believes he has discovered
a significant part of the great section which has not previously been
named. The usual practice is to apply a name taken from the locality in
which the beds were investigated, and in this manner the names of
formations become associated with towns, rivers, counties, mountains,
states and other geographical features. The locality which supplies the
name is then regarded as the “type locality” for the formation, but
wherever these same beds may be traced or otherwise identified the one
formation name applies.

    [Illustration: Dinosaur Tracks

    An ancient trail in sandstone of the Dakota formation. East slope of
    the hogback, west of Denver.]

The “Dakota formation,” to use a convenient illustration, is mentioned
in scores of reports bearing on the geology of Colorado, Iowa, Kansas,
Nebraska, New Mexico, Texas, Utah, and Wyoming, as well as the Dakotas.
On the geological map of Colorado it appears on both sides of the
Rockies, scattered in strips and patches from north to south boundary
lines. The beds are easily located in the foothills district west of
Denver because of their tendency to produce the prominent ridges known
as hogbacks.

Many formations are exposed over much less territory, some have even
greater extent. Thickness may vary from a few inches to thousands of
feet, and no two exposures will be exactly alike though some similarity
necessarily prevails throughout. “Exposures” are simply portions of the
beds which are not concealed by loose rock, soil and vegetation, or
overlying formations. Canyon walls, steep cliffs and mountain slopes,
gullies, and badlands provide a large variety of natural exposures. In
such places rocks and fossils may be studied to best advantage.

Since a formation may contain a variety of beds, including sandstones,
shales, limestones, and all sorts of mixtures, there is sometimes need
of subdividing it; but formations are the smallest units commonly shown
on geological maps. They are actual rocks which fit into a historical
scheme of things and may be regarded aptly as the pages of a book which
nature has done in stone.


GEOLOGICAL TIME

“How old are they?” “How can you learn their names from the rocks?”
These are typical examples of questions most frequently asked concerning
fossils. The second question follows the usual reply to the first, for
prehistoric plants and animals are as old as the rocks in which they are
found. The answer, as to age, must come from the rocks and what we have
learned about them through many years of hard work, thoughtful
observation, and careful study. Names, however, come from a different
source. Nature, apparently, managed for a long time to carry on without
the use of words. Since man began talking he has had no trouble
inventing names for things which interest him.

Early students of rocks and fossils likewise accomplished a great deal
without being able to date events in terms of years although many of
their efforts and interests centered on the problem of discovering a
continuous sequence of events in the fragments of evidence that had been
uncovered. This relatively simple problem has not been fully worked out,
and some of the breaks in the record are recognized as “time gaps” which
may never be converted into history.

The question of time, expressed in years, has been a puzzle which
attracted some attention even in the earliest days of investigation. Its
solution was attempted by several methods long before there was
sufficient information to make them work satisfactorily, which accounts
in part for the extreme variation in results of the calculations. Even
now it is to be expected that changes will have to be made as long as
pertinent studies are continued. Two of the most promising methods of
investigation in late years have been producing figures which are
surprisingly large. More accuracy than ever before is probably present
in modern estimates but, except for comparatively recent time, there is
yet no way of knowing within a range of millions of years when a
creature lived.

Astronomy and physics were used in early calculations but, although
taken seriously by some geologists, it was soon recognized by others
that certain events revealed by earth history could not be explained
with so short a time allowance as these methods indicated. One of the
first estimates provided a total of only twenty-five millions of years
and included a great stretch of time during which the earth, according
to prevailing theory, was more sun-like than rock-like, a time when
planets were being born and the earth could not have been in its present
physical condition, which is the chief concern of the geologist. Since
those earlier conditions could not have supported life as we know it,
our knowledge of cosmic history renders small service in the study of
fossils.

Among the methods suggested by astronomy and the laws of physics is one
which is based on the probable rate at which the earth cooled from its
molten condition to present temperature. It is believed now that the
heat of the earth is not necessarily due to an original molten state and
that a steady rate of cooling cannot be ascertained. Any figures based
on such procedure, therefore, are discredited today.

The amount of salt in the oceans, and the time required for its
concentration there by natural processes, offers another way of
attacking the problem. It is a well known fact that salt is being added
to the seas at a fairly constant rate; sea water, then, must become
saltier from year to year. The salt comes from rocks exposed on land
surfaces and is transported by the rivers which drain these areas. By
analyzing the river waters it is possible to estimate the amount of salt
annually dumped into the oceans and, also by chemical analysis, it is a
comparatively simple matter to figure the total amount now present in
the oceans. Some recent calculations indicate that thirty-five million
tons of salt are being added each year, and this figure divided into the
total amount for all the years places the age of the oceans at three
hundred sixty millions of years.

However, there are certain other factors which complicate the problem.
For instance, it is known that land areas exposed to surface drainage
have not always been of their present size, and the annual production of
salt by the different types of rocks exposed at various times in the
history of the earth has not always been as it is now. The rocks also
must be older than the oceans, but how much older cannot be determined
by means of figures obtained in this way.

Until the beginning of this century there was little anticipation of a
better measuring stick than one in use at the time which placed its
reliance on the total thickness of the sedimentary deposits and the
length of time required to produce this great accumulation of material
which is known as the geological column. Since the total thickness, or
height of the column, was not accurately known, and with recognized time
gaps to bridge, there was little hope of working out a complete
chronology by this device, but it has supplied highly desirable and
reliable information concerning parts of the record.

The system has been somewhat improved since its earliest use, and one of
its latest applications gives us an age, for known sedimentary rocks, of
nearly half a billion years, this being based on a total thickness of
one hundred miles and an average rate of 880 years for the building up
of one foot of sediments. Its greatest weakness is due to the absence of
a reliable factor to take care of long stretches of time in which the
sedimentary rocks are known to have been subjected to destructive
processes. A yardstick of this character cannot be applied to rocks that
have been destroyed, and there are excellent reasons for believing that
these interruptions may account for several times the lapse of years
indicated by the amount of rock remaining in the column which has been
pieced together.

Following the discovery of radium, however, the present century provided
a new field of knowledge which has contributed greatly to the
measurement of geologic time. The penetrating rays produced by radium
and other radioactive substances are due to extremely slow but violent
disintegration of the material. Uranium and thorium are radioactive
elements which occur in the rocks of many parts of the world. There is
little or no loss of material as the so-called disintegration proceeds;
instead there is a complicated series of transformations in which other
elements are produced, radium itself being one of these. Helium and lead
eventually take the place of the less stable elements and the known rate
at which these products accumulate provides the highly desired key to
the age of the rocks.

Part of the gas, helium, may escape, but except in rare instances where
chemical alteration might occur, there probably is no loss of lead.
Fortunately, when this metal is produced by radioactivity it differs
slightly in atomic weight from ordinary lead; otherwise the presence of
the latter would introduce a misleading factor. Since the speed at which
the change goes on cannot be increased or decreased, it is assumed that
throughout past ages it has never been faster or slower. The amount of
such change that has been completed in any body of radioactive minerals
may be measured by techniques employed in physics and chemistry. If it
is found that the amount of helium or lead present requires a hundred
million years for its production at the working speed of the parent
elements, the mineral deposit must be at least that old.

Certain conditions of course complicate the problem seriously: knowing
the age of a piece of rock which happens to contain some radioactive
element is of small service in historical studies unless the rock can be
definitely associated with a flora or fauna, or some outstanding event
disclosed by geological investigations. But there have been a few
instances in which most of the necessary conditions were present, and
more and better opportunities to apply this method will no doubt appear.
Other elements, or their radioactive isotopes, are already being
employed with good results. Some of these, such as carbon 14, are more
sensitive indicators for the accurate dating of events in comparatively
recent time.

When it can be used, this type of measurement is far less subject to
uncertainties than any other. It promises to eliminate all need for
guessing, and comes close to a degree of accuracy which is satisfactory
to the scientist, a person who thoroughly dislikes uncertainties of any
kind. If suitable material can be found in just the right places it
should accomplish what the preceding method cannot do—the accurate
measurement of the great time breaks which interrupt the geological
record in many places. Something along this line already has been
accomplished, for radioactive material has been found in some of the
oldest of the rocks. Regardless of the destruction going on in other
localities, these rocks have continued to register the passing of time,
and a tremendous antiquity for the earth and some of its first
inhabitants has been indicated.

Tests made on radioactive minerals from Gilpin County, Colorado, have
established the age of late Cretaceous or early Cenozoic rocks at sixty
million years, providing a convenient and reasonably accurate date for
the beginning of the Age of Mammals. In Russia, one of the oldest
mineral deposits yet studied in this way and regarded as early
Pre-Cambrian, produced the astonishing figure of 1,850,000,000 years;
what we commonly refer to as geological history may therefore be
regarded as covering a range of approximately two billions of years. The
earth, in some form or other, has in all probability passed through an
earlier history of another billion years or more.

Wherever we may roam, a portion of the prehistoric record is to be found
in the rocks underfoot and not far from the surface. Formations as
already mentioned may be regarded as the pages—often torn and badly
scattered—of nature’s own book, in which the geological periods are
chapters. But instead of numbering these pages and chapters we have
_named_ them, in order to get the parts reassembled in orderly fashion
and restored to a condition which makes the book legible. However, the
names cannot render the service intended except in connection with a
time chart and an outline of earth history.

    [Illustration: GEOLOGICAL TIME
    Figures to the left denote millions of years that have elapsed up to
    recent time]

  CENOZOIC
     Age of Man
             RECENT              Man and his Culture
          1  PLEISTOCENE         Last of Mammoths & Mastodons
     Age of Mammals
          7  PLIOCENE            Horses modernized
         20  MIOCENE             Grasses and Grazing Animals
                                 Three-toed Horses, Rhinos, Camels
         35  OLIGOCENE           Specialization of Primitive Ancestors
         60  EOCENE              Decline of archaic types
                                 Mammals flourishing
  MESOZOIC
     Age of Reptiles
        125  CRETACEOUS          Last of Great Reptiles
                                 Specialization of Dinosaurs
        160  JURASSIC            Bony Fishes thriving
                                 Flowering plants advance
                                 Cycads
                                 Birds and Flying Reptiles
        200  TRIASSIC            Few small mammals of lower orders
                                 Dinosaurs become prominent
  PALEOZOIC
     Age of Amphibians
        225  PERMIAN             Reptiles advancing
                                 Amphibians dominant insects
        300  CARBONIFEROUS       Dense forests of spore-bearing plants
     Age of Fishes
        350  DEVONIAN            Shark-like Fishes
                                 Land floras established
        375  SILURIAN            First land animals (scorpions)
                                 Armored Fishes prominent
     Age of Invertebrates
        425  ORDOVICIAN          Corals and Bryozoa
                                 Progress among Mollusks
        500  CAMBRIAN            Brachiopods gaining
                                 Trilobites dominant
                                 Advance of shelled animals
  PROTEROZOIC
     EARLIEST LIFE
       1000  UPPER PRE-CAMBRIAN  Small marine invertebrates
                                 Lowest Forms of Plant and Animal Life
                                 Few Fossils
  ARCHEOZOIC
       2000  LOWER PRE-CAMBRIAN  Some chemical evidence of life
                                 No fossils

Such aids have been devised and revised from time to time. No figures
have been offered as final or absolutely “right” since the beginning of
scientific investigations. Time divisions have been proposed that are
not yet in common use while others have been abandoned or modified.
Sources of information are so numerous that appropriate credit cannot be
given fairly for anything that is up-to-date. The combined chart and
outline here provided is based on time calculations of recent date but
with figures slightly rounded off for the sole purpose of making them
easier to remember. In view of the still existent probability of error
it is felt that the slight alteration of figures may justify itself. It
need not be regarded as misleading if the present purpose be
considered—the stimulation of a natural history interest which is not
vitally concerned with the little difference between a thousand million
years and nine hundred ninety-nine million years.


EXPLANATION OF THE TIME CHART

The whole of geological time has been divided and subdivided according
to varying practices. The development of life is perhaps the one
outstanding feature of the time divisions, but for the most part the
changes in floras and faunas have been gradual rather than abrupt, and
this makes it very difficult to draw sharp lines or to visualize
beginnings and endings of the various stages of development.
Occasionally there is good excuse for drawing a line, where the record
is broken and resumed again after a long lapse of time. The principal
cause of such breaks is the elevation of great land masses, which brings
on an interval of erosion and surface destruction for the areas
uplifted.

These movements of parts of the earth’s crust have been exceptionally
pronounced at certain times, often culminating in the production of
mountain systems, and because of the extreme changes they introduce are
known as revolutions. The major divisions of prehistoric time have been
established, at least in part, by such _revolutions_; crustal, climatic,
or other _disturbances_, on a smaller scale and recurring with greater
frequency, may be regarded as establishing boundaries for the minor
divisions. Hence we have five great Eras of geological history, and
these are divided again into Periods. The time chart shows an
arrangement commonly used in America. In the first column the names of
the Eras are stated in technical form. Closely coinciding with these
terms are the popular names of the Ages which appear in the second
column. These names, describing the dominant life of each age, are very
convenient. The more scientific terms used for the eras, while serving
essentially the same purpose, are a little more systematic and
generalized in that they refer to ancient life (Paleozoic), middle life
(Mesozoic), and recent life (Cenozoic), without being specific as to any
class of animals or plants for any one division of time.

The period names, in the central column, have been derived from
miscellaneous sources, some of them from geographical districts, some
from descriptive references to prominent features of the rocks, others
indicating a degree of approach to recent time. In paleontology (fossil
study) it has long been a practice to cut the periods into lower,
middle, and upper divisions, and in a few cases it has been found
desirable to make two periods out of an old one. What was once known as
the Lower Carboniferous is now commonly recognized as the Mississippian
period while the upper portion has become the Pennsylvanian. The Lower
Cretaceous is now the Comanchean of some authors.

Both old and new practices are responsible for a little confusion at the
present time. A former division into Primary, Secondary, Tertiary, and
Quaternary eras has been partly abandoned, but the term “Quaternary”
still applies to the Age of Man, while “Tertiary time” remains in good
usage for the balance of the Cenozoic era. Among the newer introductions
may be mentioned the use of a Paleocene period which precedes the
Eocene. Geologists are not entirely in agreement as to the necessity for
this addition and many would not give it equivalent rank with other
periods. In the interest of simplicity these modern refinements have
been omitted from the chart.

The figures appearing in the third column, between the Ages and Periods,
indicate the millions of years that have elapsed up to present time.
They denote the age of the rocks at the beginning of each period. The
age of a plant or animal which lived in Eocene time would be, according
to this scale, somewhere between 35 million and 60 million years. In
practice it is usually possible to determine whether a fossil was
embedded in the rocks during an early or late portion of the period, and
thus its age may be established within a shorter range, but it is
impossible to be exact, even in terms of millions of years, with regard
to anything as far back in prehistory as the Eocene period.

The period in which we are living today is known as Recent. It began at
the close of the Ice Age or Pleistocene period about ten thousand years
ago and represents so little of earth history since the beginning of
life that a chart many times the length of this page would be required
to show the rest of the periods in proportion. The Cambrian period is an
early chapter in which the story of prehistoric life suddenly becomes
clear and richly varied. It is, however, much farther from the beginning
of the record than it is from the present, and the Pre-Cambrian eras
would require a great deal more space in order to show their relative
lengths. The Archeozoic and Proterozoic eras have to some extent been
divided into periods, but the great antiquity of the rocks has obscured
much of their history, and divisions established for one locality have
been of little service elsewhere. Consequently, the period names are in
less general use and the common practice is to refer to all this great
stretch of time as Pre-Cambrian.

In the last column, at the right of the chart, some of the historical
features are indicated. This column should be read from bottom to top in
order to get the proper development of the story, and at best this
sketchy outline of events can be no more than suggestive of the progress
and decline through which the earth’s inhabitants have passed.

Rocks of every period except probably the Silurian are known to have
been deposited somewhere in the Colorado area, although in most cases
the record for each period is far from complete. Formations are too
numerous and too varied locally to be shown on a chart of this type.


THE GEOLOGICAL SECTION

In the study of fossils there are two important field aids usually
available. For any locality there should be a geological map and a
section showing the sequence and character of the strata. On a
small-scale map many of the local details have to be omitted, but the
position of the larger exposures is indicated and, with this information
at hand, the fossil-bearing strata may be located with the help of a
geological section. The latter is frequently obtained from technical
reports published by State and National Geological Surveys. Frequently,
however, it is possible to obtain only a general plan for a given
locality, and a great deal of literature may have to be scanned in order
to get that. Excellent geological maps of Colorado have been published
by the Colorado Geological Survey and the United States Geological
Survey.

It often happens that a formation is not where we expect to find it,
this being due to several possible factors. The sediments may not have
been deposited there, or they may have been removed by erosion. Where
the structure has been disturbed by folding and faulting, a multitude of
complications is introduced. The expected sequence is sometimes inverted
and repeated through a series of folds. Formations also may be moved
miles out of place by faulting. Both thickness and character of
sediments may vary considerably within a formation. In some regions the
geology is very simple, in others extremely difficult to understand.

    [Illustration: FORMATIONS of the DENVER-FOOTHILLS REGION
    A GENERALIZED SECTION SHOWING SOME OF THE SURFACE FEATURES
    RED BEDS    HOGBACKS    TABLE MOUNTAIN    DENVER DISTRICT

REGION OF MOUNTAIN-MAKING UPLIFT

Formations bordering the mountains have been bent into upright
positions.]

                PERIODS

  RECENT
  PLEISTOCENE
  PLIOCENE
  MIOCENE
  OLIGOCENE
  EOCENE
  CRETACEOUS                                              THICKNESS
  SOFT SANDSTONES GRITS & CLAYS        DENVER & ARAPAHOE   2000 ft.
  SANDSTONES, SHALES & LIGNITE         LARAMIE             1000 ft.
  YELLOWISH SANDS & SHALES             FOX HILLS           1000 ft.
  SOFT DARK GRAY OR RUSTY SHALE        PIERRE              5000 ft.
  LIMESTONES & SHALES                  NIOBRARA             500 ft.
  DARK SHALES & LIME                   BENTON               400 ft.
  GRAY OR BUFF SANDSTONES & CLAYS      DAKOTA               300 ft.
  SHALES, SANDSTONE & LIME             MORRISON             200 ft.
  JURASSIC
  TRIASSIC
  PERMIAN
  DEEP-RED SANDY SHALES, LIME, GYPSUM  LYKINS               700 ft.
  CARBONIFEROUS
  MASSIVE PINK OR WHITE SANDSTONE      LYONS                200 ft.
  RED OR BROWN SANDSTONE &             FOUNTAIN            1500 ft.
  CONGLOMERATE
  DEVONIAN
  SILURIAN
  ORDOVICIAN
  CAMBRIAN
  PRE-CAMBRIAN
  METAMORPHIC & INTRUDED ROCKS         IDAHO SPRINGS
  SCHIST, GNEISS, QUARTZITE            (PART)
  BASEMENT ROCKS of IGNEOUS ORIGIN

A generalized section for the western part of the Denver Basin is
introduced here for the use of local students. The formations normally
present in this region are shown in their usual position. They are
briefly described on the chart, and their thickness is indicated by
figures which may be regarded as near the maximum for the district. The
section will apply to most of the foothills area between Morrison and
Boulder though surface features and thickness of beds will vary
considerably from place to place.

Certain of the formations are known to be fossil bearing, others barren
or nearly so. When fossils are present they are usually restricted to
certain localities, and these may be widely scattered. The following
remarks apply to the possibilities for finding fossils in the formations
named.


                         _Denver and Arapahoe._

Leaf impressions of palms, ferns, and numerous species of well-known
trees and shrubs are common in many localities. Petrified wood is fairly
abundant, and a few scattered bones of reptiles and mammals have been
found. The two formations are treated as a unit because the Arapahoe is
neither conspicuous nor sharply defined. Denver beds are well exposed on
the slopes of Table Mountain at Golden; fossils, however, have been
obtained from several localities nearer the city of Denver, notably from
the hills just west of Overland Park.


                               _Laramie._

Plant material is locally abundant, principally the leaves of familiar
deciduous trees, palms, and ferns. Many of the clay pits being worked
near Golden are in this formation. Oysters and a few other mollusks may
be found in some places.


                              _Fox Hills._

Better exposures of this formation are located to the north of Denver.
Marine mollusks are most frequently found.


                               _Pierre._

In addition to the characteristic dark shales, this formation includes
some limy material and sandstone beds, both of which are fossiliferous
in places. Two types of marine mollusks are characteristic:
_Inoceramus_, generic name for several species of clam-like bivalves
readily identified by concentric elevations which produce a rippled
effect on the shell surfaces; and _Baculites_, cephalopods with
straight, chambered shells which often break at the suture lines, where
the fossil is weakened by the chamber walls. Small oyster shells are
fairly common also. The formation is to be found some distance to the
east of the prominent hogback where it weathers into smooth surfaces in
the form of broad valleys and flats, with rounded contours on the few
elevations that may be present. It forms a soft, flaky soil when dry, is
a sticky “gumbo” when wet. The clay is generally of a rather dark
grayish color when freshly exposed but it takes on a rusty appearance
after weathering. At various levels there are numerous iron-cemented
concretions, many of which contain fossil shells.

    [Illustration: Marine Beds of the Benton Formation, Northeastern
    Colorado

    A stream channel has cut deeply into the formation, uncovering and
    partly destroying a plesiosaur skeleton which was found at the level
    where the men are standing.]

    [Illustration: Plesiosaur Bones in Place

    Benton formation. Surface rubble has been cleared away, and several
    vertebrae are partially uncovered in the area at the right of the
    hammer.]


                              _Niobrara._

The formation contains fossils rather similar to those of the Pierre.
Shark’s teeth have been found in some of the lower beds. Limestone is a
prominent feature, often forming a well defined ridge near the foot of
the eastern slope of the main hogback. The limestones commonly have a
chalky character.


                               _Benton._

The formation is not especially productive in this region. Marine shells
are numerous in some localities, and bones of marine reptiles have been
found at various places. As usually seen, it is almost entirely composed
of impure clay shales, very dark, brownish-gray to almost black, and
commonly interbedded with thin patches of white bentonite, yellow ochre,
gypsum, and limestone.


                               _Dakota._

This formation produces the high hogback which is usually present some
distance east of the Red Rocks. There are generally two or three layers
of massive, light-colored sandstone separated by clays which are used
extensively in the making of bricks and pottery. Leaf impressions and
some fish scales are found in the clays and occasionally in the
sandstone. The hogback is a good marker from which to locate other
formations, because of its prominence in the foothills landscape.


                              _Morrison._

Good dinosaur material has been taken from the Canon City and Morrison
districts. The formation is to be found on the lower west slope of the
Dakota hogback. It consists of continental deposits of the stream and
lake types. There is considerable sandstone in this formation and a
little limestone is to be found here and there, but the most
characteristic feature is in the shales. When freshly exposed, the
shales are delicately tinted with gray, green, and maroon, a
bronze-green being rather prominent. This formation is highly variable
in character, with much of the clay often buried under the valley floor.
In addition to the bones of reptiles, there are plant fossils, usually
of poor quality, and fresh-water gastropods more or less abundant in
some localities.


                               _Lykins._

Outcrops are not prominent, owing to the small amount of
weather-resisting materials. The sandy clays are commonly of a deep red
color mottled with spots of light gray. A white limestone is sometimes
present near the middle of these deposits, and gypsum beds are included
locally. The formation is often indicated only by red soil in the
depressions between ridges. Few fossils have been reported.


                                _Lyons._

This formation is usually prominent as the eastern wall of the uplifted
Red Rocks series. In some localities it forms a ridge of pink or white
sandstone distinctly separated from the older sediments to the west.
Very few fossils are found.


                              _Fountain._

Exposures usually are brown to red in color, though sometimes a dirty
white. The prominent rocks are rather coarse sandstone, commonly with a
gritty texture due to the angular character of the sand or gravel from
which they were made. These are the westernmost of the Red Beds and the
oldest of the uplifted sedimentary rocks bordering the foothills in most
of our area. Fossils have been found in the formation, but it is
practically barren for the territory here considered.

                            * * * * * * * *

This geological section also illustrates a method of dating crustal
movements and the birth of mountain ranges, for the folding of the
strata along the flanks of the Rocky Mountains has a great deal of
significance in this connection. The sedimentary layers were originally
deposited over much of the present mountain area in a horizontal
position, and only those formations in existence at the time could be
distorted by the upheavals which produced the new elevations. Of the
series generally involved in the movement the Laramie beds are the
youngest. Since these beds had not been formed until near the close of
the Cretaceous period it is to be assumed that the mountains must be of
more recent date, younger than the topmost of the deformed beds and at
least as old as the lowermost of the undisturbed formations overlying
them.

Some disturbance is evident also in the Arapahoe and Denver beds which
overlie the Laramie, but this is believed to have occurred sometime
after the occasion of the first great uplift. Volcanic materials in
these beds lead to the belief that the sediments were deposited during a
period of volcanic activity brought on by the crustal folding which
terminated the Mesozoic era. Hence the conclusion arises that the age of
the Denver and Arapahoe beds must coincide closely with some of the
earlier stages in the history of the mountain system. This interval is
often referred to as Post-Laramie time.



                       BEFORE THE AGE OF REPTILES


THE PRE-CAMBRIAN COMPLEX

The rocks of Pre-Cambrian time have been buried deeply under the
accumulation of younger sediments, and the resulting pressure in many
places has been tremendous. In addition to the effects of pressure there
also is recorded in these ancient formations the repeated movements of
the materials since they were first deposited. Vertical and side
adjustments of parts, with relation to other parts, have distorted the
original arrangement of the rock particles to such an extent that
ordinary fossils would eventually become unrecognizable. These crushing,
grinding, and kneading forces working through millions of years alone
would account for the absence of fossils from the older deposits.
Frequently the rocks have become so changed in form that their original
character can only be conjectured, and because of this change they are
known as metamorphic rocks.

A few beds of Archeozoic age remain in nearly their original condition,
but they are either without fossils or they have produced very
questionable and unsatisfactory specimens. The existence of life during
these early stages of earth history is indicated largely by chemical
rather than fossil evidence. Much of the ancient limestone has been
converted into marble, but it is not unreasonable to believe that plants
and animals were instrumental in the production of this type of rock as
they are today. Certain varieties of iron ore deposits are now being
built up by the aid of plants, and similar ores in the ancient rocks may
have had a like origin. The presence of great quantities of carbon, in
the form of graphite, may be regarded also as a sign of life, for this
substance is accumulated on a large scale by living plants, and may be
retained in a solid form after the partial decay of the plant tissues.

So far as the direct evidence goes, there is no sign of any creature of
large size or of such complicated structure as the common plants and
animals of today. The chemistry of the mineral deposits is not entirely
convincing as to the presence of life, but it is regarded as highly
probable that microscopic, single-celled plants and animals, comparable
to modern algae and protozoa, were in existence during Archean time.
Throughout later eras there is unmistakable evidence of gradual
development from simpler to more elaborate life-forms and the Archeozoic
is commonly regarded as a time of preparation during which simple
organisms of some kind were becoming adapted to early conditions which
could not support life on a higher plane. The importance of the work
done by such lowly creatures in the preparation of suitable environments
for more advanced modes of living is overlooked almost entirely.

During the next era, the Proterozoic, the record of life becomes
somewhat clearer. Fossils are hardly to be regarded as abundant but
there were several well-defined types of animals which left shells and
other parts composed of mineral matter. Among these may be mentioned the
Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced
delicate, often lace-like shells of many patterns adorned with the
radiating filaments or spines which have suggested the name for this
group. Foraminifera produced minute shells, sometimes many chambered,
and often bearing a confusing resemblance to the work of snails. Common
chalk is composed almost entirely of such shells and fragments of them.

Sponges and Bryozoa are animals of slightly higher organization. They
are many-celled instead of one-celled and the cells have special work to
perform, which is a most important step in the direction of the
specialization which characterizes the structural and life pattern of
later arrivals. The Bryozoa lived in moss-like colonies which have been
important rock-makers; the fossil forms bear some resemblance to corals.
Sponges are too well known to require description although the familiar
article of commerce is merely the framework of once-living animals. They
represent the earliest organization of true animal bodies even though in
appearance they may have a resemblance to plants.

Actual plants of this era were of the algae class, aquatic in habit as
were their animal neighbors, the first to leave a record in the form of
fossils. This record, obscure and distorted, has long been a source of
perplexity to investigators. Without well-defined floras and faunas to
guide them, and with rocks frequently in chaotic relationships, early
geologists were content to regard it all as a “Pre-Cambrian complex.”
Recent studies have contributed a great deal of information not
available some years ago. It is quite possible that more advanced types
of life were in abundance before the close of the second era, but
material on which to base sound opinion is still scarce.

Rocks of Pre-Cambrian age are plentiful in the foothills region west of
Denver. The schists, gneisses, and quartzites exposed for some miles
immediately beyond the red-beds are part of this great complex. The
Idaho Springs formation is known to be one of the oldest in this
district, although its exact age has not been determined. Other
formations are recognized among the metamorphic rocks of the region but
none has contributed to our knowledge of early life.


CAMBRIAN LIFE

There can be no mistake as to the prolific development of life in
Cambrian seas, for fossils of this age are to be found in many parts of
the world, where ancient sea bottoms now form part of the land surface.
Invertebrate animals appear to have made much progress, but plants were
either scarce or too small and delicate to be productive of fossils. It
is probable, however, that seaweeds and other algae were flourishing
along with the invertebrates, because animal life is directly or
indirectly dependent on the existence of plants. The latter sustain
themselves by taking carbon and nitrogen from air, water, and soil, but
animals must obtain their requirements by eating plants or eating each
other. They cannot obtain what they need from the inorganic world
without this help from the vegetable kingdom.

One group of animals stands out prominently above all its
contemporaries. Known as the trilobites they were by far the most
distinguished and most characteristic of Cambrian invertebrates.
Trilobites inhabited the warmer seas of this period and several later
ones, but were extinct by the end of the Paleozoic era. Hundreds of
species have been described, most of them under four inches in length.
Well-known distant relatives now living are the shrimps, and other
crustaceans. The name Trilobite has reference to the three lobes which
are apparent in the form of the upper surface, the central lobe forming
a broad ridge extending along the back. Beneath the outer lobes on each
side there was, during life, a row of short, jointed legs used for
swimming and walking, but these delicate appendages are seldom preserved
in the fossils.

Second in importance among the animals of the period were the
brachiopods or lamp-shells, not true mollusks although they were
provided with similar shells composed of calcium phosphate or calcium
carbonate. Shells are of two parts (bivalved) as in the case of clams,
but the valves are above and beneath the body instead of on the right
and left sides, which is the arrangement among mollusks. Although
abundant as individuals, there were only a few species during the
earlier part of the period; the number of species increased, however,
and the race became very persistent. About seven thousand species have
been described, and the race is not yet extinct although the number of
living species is relatively small.

Cambrian life evidently included representatives of all the great
divisions of invertebrates; sponges, jelly-fishes, worms, and primitive
corals have been reported. At the end of the period there was an
elaborate molluscan fauna. The closing of the period in North America
was apparently a gentle elevation of continental areas and a consequent
withdrawal of the sea.

    [Illustration: Invertebrate Fossils

    Only a few prominent types have been selected from thousands of
    invertebrates known to zoologists. The forms illustrated are of
    frequent occurrence as fossils.]

  CRINOIDS
  CEPHALOPODS
    Coiled types
    Ammonite
    Scaphite
    Straight-shell type
    Baculite
  TRILOBITE
  BRACHIOPODS
  BIVALVES
    Inoceramus
    Oyster
  GASTROPODS
    Snail-like Univalves
  PROTOZOA
    UNICELLULAR FORMS
      Radiolaria (Microscopic)
      Fusulina limestone
      Foraminifera (Enlarged)
    MULTICELLULAR FORMS
      Cup coral
      Reef coral
      Sponge
      Bryozoa


THE ORDOVICIAN RECORD

Extensive land areas must have subsided again early in the Ordovician
period for marine sediments were laid down over a large part of the
North American interior, and three epochs or subdivisions of the period
have been based on as many invasions of the sea. In these ancient
deposits the record of life continues to show new forms. Nothing of a
very spectacular sort is recorded other than a great increase in the
number of species among types that were established in earlier periods.

Trilobites were at their best, brachiopods continued to flourish, and
the mollusks made new progress, especially the cephalopods, a group
which includes our cuttle fishes and squids. Some of these predatory
creatures attained large size and were no doubt masters of the sea.
Typical forms were provided with tapering chambered shells that
occasionally reached a length of twelve or more feet. Most of the shells
were straight and trumpet-like or but slightly curved. Some were closely
coiled and in this respect more like the well-known nautilus now in
existence.

The bryozoans became very common in the later part of the period and
corals made slight advances. Somewhat of a novelty at this time were the
crinoids, commonly known as “stone lilies” although not plants at all.
They have been described as starfishes with back turned downward and a
thick stem attached beneath. Where they lived in great abundance the
limestone deposits may consist almost entirely of their stems. Crinoids
continued to produce a variety of forms throughout several of the
succeeding periods.

The brachiopods were commonest of all animals representing this period,
however, and their wide distribution, together with their known
preference for warm waters, is taken to be an indication of mild
temperatures prevailing over a large portion of the earth. Land plants
are indicated by spore-bearing forms related to the ferns and mosses.
Impressions of such plants have been found in Europe but, since most of
the known rocks of this age were formed in seas, the marine algae are
more abundant as fossils.

In the Colorado area, rocks of Ordovician time are exposed only in
mountainous areas where they have been lifted high above their original
levels. They are not especially rich in fossils although they have
produced some fish remains which are of interest in that they suggest an
age of vertebrates which is just ahead.


SILURIAN EVENTS

Since land floras and faunas had not yet become conspicuous the fossil
record for this period is limited to those areas which were invaded by
the sea. Apparently there was no such invasion of the present Colorado
region, for rocks of this age are not in evidence. If they exist at all
they are restricted to localized districts which are deeply buried under
sediments of later periods. There may have been no Silurian deposition
in this area, or such rocks may have been produced only to be destroyed
by elevation and consequent subjection to weathering and erosion during
a long interval of time, in which they were completely removed. In the
region of the Colorado Rockies there is no evidence of returning seas
until late Devonian time.

In other parts of the world, however, there was extensive deposition of
rock-making sediments in seas which were inhabited by algae and
invertebrates of the types previously described. Among the common
animals of the time there were still numerous species of brachiopods,
trilobites, corals, crinoids, and bryozoans. In addition to the
primitive cone-shaped, cup corals there were several advanced types but
the habit of building large reefs was not yet established.

“Sea scorpions,” really large crustaceans, flourished during Silurian
time, and late in the period there appeared a race of true scorpions
which lived on dry land or between high and low tides along the
seashore. These were smaller and much like modern descendants but
probably they did not wander far from the ocean shores where an
abundance of food was available. These little scorpions, the largest
measuring only two and a half inches in length, are the oldest
air-breathing land animals of the fossil record.

It was not until the period was well advanced that fishes became
numerous, and much of our knowledge of the beginning of an “Age of
Fishes” has been obtained from European fossils. Although fishes are
classed with the vertebrate or backboned animals there are large groups
which do not have bony skeletons but are provided instead with a simple
framework of cartilage. Among the earlier and more primitive types were
the ostracoderms or bony-skinned fishes with no internal bones and only
a small amount of bony substance in the armor-like plates and scales
which covered the forward portion of the body.

The ostracoderms comprise a small group of fishes about which very
little is known. They appear to have been inhabitants of fresh-water
streams as well as lagoons bordering the seas, and may have been related
to the small sharks of the time. They lived during the Ordovician,
Silurian, and Devonian periods, and left no descendants now recognized
among living creatures. A much larger type of armored fishes is known as
the arthrodires, a name which refers to a pointed neck and an
arrangement of the armor plates to permit a movement of the head. These
were the most ferocious fishes of the Silurian and Devonian seas, some
of them reaching a length of twenty feet though most were much smaller.
Their jaws were provided with formidable shearing and crushing plates
instead of teeth.


DEVONIAN PROGRESS

The Devonian is one of the most outstanding of all periods from the
viewpoint of life development. Dominance of the fishes is its greatest
achievement, the invertebrates remaining about as they were and the
higher vertebrates barely in evidence, but life on a large scale was no
longer confined to the seas. Fresh-water fishes became prominent and
land plants well established. The first forests appeared, with fern-like
plants predominating although woody trees of several types and
considerable size were included. It is quite possible that extensive
land areas had been well supplied with vegetation during earlier times,
but the delicate tissues of plants are far less likely to be preserved
than the limy parts of animals. The fossil record, therefore, cannot be
expected to reveal more than a suggestion of the progress made at this
level of living. The story of plant life becomes much clearer in the
next period when conditions were more favorable for the production and
preservation of plant fossils.

Land animals of the time are almost unknown. A few snails and scorpions
have been found, and some footprints made by early amphibians. Insects
probably were in existence although the evidence is not quite clear on
this point. The increasing number of fresh-water fishes, however, may be
regarded as a sure indication that inland conditions were becoming more
favorable for plant and animal inhabitants of all kinds.

The extent of development among the fishes cannot be accurately
indicated by naming a few types, for it is mainly in the number of
species and genera within the larger groups that progress is seen. In
general it may be stated that the fishes of the period had not yet
acquired the bony skeleton and typical form of familiar modern species.
Skeletons were of cartilage, partly hardened in some instances by lime.
Armor plates were customary with certain races but were not present
among all fishes. Neither were these armored forms exceptionally large,
as compared with living sharks. Although occasional giants appeared, the
majority were small. Many were sluggish creatures with poorly-developed
jaws, living as scavengers on sea and stream bottoms. Tail fins were
usually unbalanced as in the sharks, or pointed and rounded rather than
evenly forked.

    [Illustration: Modernized Types of Fishes from Eocene Shales of
    Southwestern Wyoming]

The great tribe of true bony fishes, such as the cod and perch, which
includes more than ninety percent of the fishes living today, was not
yet in existence. About one-third of the many kinds of fishes then
living were related to the sharks, a group which is relatively
insignificant in recent years. Nearly one-fourth of the total belonged
to a tribe of enamel-scaled fishes, now represented only by a few
sturgeon and gar-pike.

Lung fishes have never been a large group but it is noteworthy that they
have had existence since Middle Devonian time. Living members of the
race, inhabitants of Africa and South America, make a practice of
burrowing into the mud of stream channels during dry seasons and are
provided with lungs which enable them to breathe air in the manner of
higher vertebrates. They survive the complete drying-up of the streams
and live for months without water. Other forms, with less development of
lungs, frequent stagnant pools and come to the surface occasionally for
a breath of air. All are provided with gills also, which enables them to
obtain their oxygen as other fishes do. They are believed to be a
connecting link between the fishes and the early amphibians. More
accurately, perhaps, they should be regarded as holding an intermediate
position without being directly ancestral to any higher type of
vertebrate animal.

Still dominant among the invertebrates were the brachiopods, on the
whole averaging a little larger in size, and otherwise indicating
congenial times for that type of organism. They reached the peak of
their development during this period. Trilobites were declining although
a few new and strangely ornamented varieties made a brief appearance.
Crinoids apparently found living conditions less favorable during
Devonian time, but in a later era they again became prominent. Corals
were favored only at times and in certain localities. Along with the
crinoids they appear to have suffered from the presence of an unusual
amount of mud in the waters of their customary habitats. Both had a
preference for clear water as indicated by the absence of fossils from
limestones containing more than a very small percentage of muddy
sediments. Crustaceans, similar to the sea-scorpions and better known as
eurypterids, became prominent among fresh-water animals. Some were
unusually large for creatures of this class, lengths of several feet
being recorded from fragments. Gastropod mollusks came into prominence
in localities where living conditions were favorable. Bivalves continued
to thrive but the cephalopods had a rather meager development
considering the heights they were to achieve in subsequent periods.

In western North America the large expanse of territory known as the
Great Plains was evidently well above sea level during this entire
period, for no beds of this age are found in eastern Colorado. West of
the Front Range, however, there was some deposition of marine sediments
during late Devonian time. Formations of this age are exposed near
Salida and Glenwood Springs, on the White River Plateau, and in the San
Juan region.

The Carboniferous period gets its name from the vast deposits of coal
which were developed during that time in many parts of the northern
hemisphere. Depressed land surfaces bordering the continents, and
extending well into the interior of present boundaries, supported dense
growths of vegetation and provided the swampy conditions most favorable
to coal production. Varieties of plants which are now of small size and
lowly position in the botanical world acquired the proportions of large
trees.


CARBONIFEROUS FORESTS

Best-known fossils of the period are carbonized portions of the larger
trees, and impressions left in the muds and sands of ancient bogs.
Forest trees of several kinds reached the height of a hundred feet, with
a trunk diameter of two to six feet. This size often is exceeded in
modern forests, but by trees of an entirely different type. Considering
the amount of development among the plants of earlier periods,
Carboniferous forests provide an outstanding spectacle of advancing
life.

Quite common among the larger trees were two varieties of club-mosses,
also known as scale trees. They were cone-bearing evergreens with only
slight resemblance to modern conifers. Instead of seeds they produced
spores, a method of reproduction which is practiced among ferns. The
trunks were marked from bottom to top with uniform patterns of cushions
and scars indicating the points at which leaves were attached during the
earlier stages of growth. In the Lepidodendrons the rows of scale-like
cushions wind spirally upward while among the Sigillaria there is a
vertical arrangement of leaf-scars which resemble the imprints of a
seal, these impressions being in straight and parallel rows on a surface
which may be either ribbed or smooth. The leaves of scale trees were
stiff and slender, and arranged in grass-like tufts at the top.

Calamites, related to our horsetail rushes, were somewhat smaller than
the scale trees. Their trunks consisted of a thin, woody cylinder with a
pithy interior, and were marked at intervals by nodes which gave them
the “jointed” appearance of a bamboo stem. Leaves were arranged in
circles around the nodes of main stem or branches. Spore-bearing cones
appeared at the tips of the stems.

    [Illustration: Prehistoric Plants

    Some of the larger and better known plants of past ages are shown as
    reconstructed by artists. Finer details of the reconstructions often
    have to be neglected because of uncertainties due to the scattered
    and fragmentary character of the fossil record.]

  LEAF IMPRESSIONS
    Carboniferous Ferns
    Strap-leaf Conifer (Cordaites)
  MODERNIZED TYPES
    Sequoia Cone and foliage
    Miocene Fossils (Florissant Shales)
      Maple
      Willow
    Eocene palm (Denver Beds)
  HORSE TAIL RUSHES
    Restoration (Calamites)
    Fossils Leaves and Stem
  CYCADS
    Restoration
    Fossil Trunk
  CLUB MOSSES
    Restoration (Sigillaria)
    Fossils Trunk Impressions
  SCALE TREES
    Restoration (Lepidodendron)
    Fossil Leaf scars

Also included among the larger trees were the Cordaites or large-leaved
evergreens, tall and slender, seed-bearing but not true conifers as yet.
Leaves were strap-shaped or grass-like, the larger ones having a length
of six feet and a width of six inches. Trunks were woody, resembling
pine, but with a central pith. The flowers were small and resembled
catkins in form.

Ferns and fernlike plants were so numerous that the period has been
known as an age of ferns. Earlier knowledge of these forests was based
on fossils of a fragmentary character from which an accurate association
of parts could rarely be obtained. A general relationship with the ferns
was apparent, but careful study of additional material has given us a
rather different view of Carboniferous plant life and we note a highly
diversified array of forms with many suggestions of modern tendencies.
The flora as a whole may be regarded as highly specialized for the
conditions which prevailed at the time and were not to continue through
subsequent periods. Warm temperatures and abundant moisture were
essential especially to spore-bearing types, and the cold, arid
conditions of the next period put an end to many of the groups, or
greatly reduced their prominence.

This could be regarded equally well as an age of insects, for some of
these invertebrates acquired the greatest size they have ever had,
particularly the dragonflies with a wing-spread of more than two feet in
one of the largest fossils so far discovered. Cockroaches numbering
upwards of five hundred species have been named. Though large they are
hardly to be regarded as giants, lengths of three or four inches being
about the limit.

Some of the insect types of today quite evidently existed among the
inhabitants of Carboniferous forests, but it is apparent that there were
also some antiquated forms which may have descended from the trilobites.
Although some authorities regard this as the period in which insects
originated, there are others who maintain that definite beginnings are
not established so readily on present evidence. Spiders are believed to
have made their appearance at this time.

Four-footed vertebrates resembling salamanders were prominent animals of
the Carboniferous swamps. At first adapted to a life in water and later
to land conditions, they are known as amphibians, the name being based
on the ability to live in two different kinds of environment. Common
living representatives of this group are the toads and frogs, but these
tailless forms are not known among fossils of the Paleozoic era and are
almost unknown throughout the Mesozoic. The Age of Amphibians, as we
apply that phrase, was definitely not an age of toads and frogs.

These primitive land animals were of different types, ranging from much
smaller sizes up to the length of a crocodile. Most of them had short
legs, and feet which were suitable for locomotion upon land, but many of
the creatures probably spent most of their lives in the water. Tails
were usually high and flattened as if for swimming, sometimes long, at
other times greatly shortened in proportion to the body. Heads were
generally large, jaws long, and mouths wide.

Before the close of the period true reptiles appear, but this race of
animals is destined to make a more spectacular advance than the
amphibians and will be discussed in connection with Mesozoic life. The
amphibians, however, are regarded as being the ancestors of the reptiles
as well as the higher quadrupeds which follow them. Although living
reptiles are readily distinguished from living amphibians there is a
different situation with regard to these primitive forms, for among the
fossils it becomes increasingly difficult to separate the two groups as
new material is investigated.

Invertebrates had their ups and downs during the period. Trilobites
became scarce, and brachiopods for a time were the most abundant of the
shelled animals but later declined rapidly. In favorable localities the
crinoids established a wonderful record for new species before the
period had advanced very far. Hundreds of species of Carboniferous
invertebrates are known, and in many of the rocks of the period they are
the only fossils to be found, for the vertebrates were still unable to
venture far from the swampy districts, and much inland territory was too
well drained to support either the floras or faunas then existing.

In the Colorado area there are both marine and continental formations
but the great coal-making forests and their inhabitants were limited to
other localities. As a consequence this region is not famous for
Carboniferous fossils.


PERMIAN HARDSHIPS

For a time there was no great change in North America following the
opening of the Permian. Then began a series of mountain-making movements
and continental uplifts which drained the swamps, lakes, and inland
seas. With the passing of the vegetation which had established itself in
and around these areas much of the animal life followed. It is probable
that a considerable proportion of the marine life survived, much more
than is indicated by the fossil record, but the receding seas carried
the survivors into territory which is now inaccessible to fossil
hunters.

After Middle Permian time the climate everywhere seems to have been cold
and dry. By the end of the period there had been accomplished more
geographical change throughout the world than at any time since the
beginning of the Paleozoic era. Traces of the crustal movements which
produced new mountain ranges can be followed in Europe, Asia, and North
America. The Appalachian region was raised to a great height, possibly
in excess of three miles. A major disturbance of this character is known
among geologists as a revolution, and to this particular one the name
“Appalachian Revolution” has been given.

The elevation of continents necessarily changes their coast lines. This,
in turn, influences ocean currents which have an important bearing on
climatic conditions. In addition to this, the elevation of mountain
systems not only rearranges the distribution of hot and cold winds over
the land areas but it may produce barriers to the migrations of floras
and faunas, confining them to areas in which it is no longer possible to
live. When the effect upon plants and animals is considered, it is
easier to understand why a line is drawn across the geological time
chart at such a point and an era of prehistory is regarded as closed.

During the Permian period there was recorded in the rocks more
widespread glacial action than ever before or since. With less inland
water to provide the necessary evaporation there was a marked decrease
in rainfall, and arid or semi-arid conditions replaced the hospitable
climate that had been such an important factor in the prolific life of
the Carboniferous. The struggle for existence became intense, but
hardier types of plants and animals, with greater ability to adapt
themselves to adverse conditions, established themselves here and there,
as ancestral forms became extinct. Most of the large spore-bearing trees
died out and seed-producing varieties began to acquire prominence, among
them the coniferous evergreens. Ferns, however, proved their
adaptability by producing some new forms which became prominent in
Permian floras.

The prehistoric amphibians have been divided into three orders, one of
which includes all the larger forms. This group, known as the
labyrinthodonts, continued on through Permian time but began to show
backward tendencies, with dwindling limbs and a return to life in the
water. Among the larger land varieties are typical fossils ranging from
about fifteen inches to five feet in length. In outward appearance they
differed from Carboniferous amphibians. One of the other orders,
including a great diversity of smaller forms, became extinct during this
period, leaving no known descendants. The third order is regarded as the
oldest, and probably the ancestral group from which the modern newts and
salamanders originated.

The most successful of Permian land animals were the peculiar reptiles
that learned to live in drier regions. Like the horned toad and Gila
monster of our arid southwestern United States, the larger Permian
reptiles were four-footed animals. In size and shape they were not
greatly different from amphibians then living. An exception to this
rule, among some of them, is the development of long, bony spines above
the vertebrae of the back. A fairly common fossil of this type, found in
Texas and known as _Dimetrodon_, had a total length of six feet, about
half of this being in the tail. The tips of the spines adorning the back
reached a height of three feet or more and there was probably a covering
of skin over these bones, which would produce a sail-like structure or
“fin” of large size. Its use has not been explained but it provides an
easy name for these odd creatures—the “fin-back lizards.”

Rock deposits produced in arid regions usually have characters which are
not difficult to recognize. Gritty texture, irregular bedding, red
color, and gypsum are common features. Formations of Permian age are to
be found in Colorado but better fossil deposits have been discovered in
Kansas and Texas.

    [Illustration: Marine Reptiles: Plesiosaur (Lower Skeleton) and
    Mosasaur

    The Mesozoic Era produced many types of reptiles besides the
    dinosaurs. Two of the marine forms are shown in this illustration,
    both from Cretaceous beds of the western United States.  Plesiosaurs
    were the giants of the seas in their time, lengths of forty to fifty
    feet being not uncommon. A long flat tail provided locomotion for
    the mosasaurs whereas the plesiosaurs resorted to the peculiar limb
    structures known as flippers or paddles.]



                          THE AGE OF REPTILES


The Mesozoic, or era of middle life, was a long stretch of time during
which there was marvelous development among the reptiles. Many strange
types were produced and most of them became extinct before the end of
the era. The reptilian stock branched out in many directions. Types
emerged which differed from one another so widely that their mutual
relationships have become obscure. Hideous and fantastic creatures
suggesting sea serpents and dragons were worldwide in distribution.
Reptiles of the air and seas acquired large size and weird forms, but
greater advances were made upon land.

The flying reptiles or pterosaurs flourished in Jurassic times with some
of the larger varieties surviving until near the close of the
Cretaceous. Although these winged lizards were the first of the
vertebrates to fly they are not to be confused with birds. They were
without feathers, and the earlier forms were provided with long tails
bearing a flattened rudder-like tip. One of the best known of this type
had a length of about eighteen inches. Its jaws were long and provided
with sharp teeth. The wings were membranes attached to body and legs,
stretched and manipulated by means of greatly elongated fingers. In
later types there was a reduction in tooth equipment and length of tail.
_Pteranodon_, found in Kansas, had a wing spread of twenty-five feet, a
large toothless beak, a short body, and a mere stub of a tail. It was
one of the last of these winged monsters.

Several types of marine reptiles appeared during this era, among them
the plesiosaurs which first appeared in Triassic seas. These peculiar
animals were serpent-like with regard to the character of head, neck,
and tail, but in other respects were quite different, the short
barrel-shaped body being provided with four large paddles corresponding
to the usual limbs of quadrupeds. Fossil remains of these animals are
common in many Jurassic and Cretaceous deposits, some of the largest
exceeding forty feet in length. Mosasaurs, also marine carnivores,
inhabited shallow Cretaceous seas throughout the world and are
especially abundant as fossils in the Kansas chalk beds. These were
elongated forms with a resemblance to salamanders in some respects but
provided with long pointed jaws and sharp teeth. Swimming was
accomplished largely by the tail though probably aided to some extent by
four webbed paddles or flippers. The ichthyosaurs were more fish-like in
construction, as the name implies. The limbs were short and broad, and
there was usually present a well-developed tail-fin as well as a large
fin on the back. They were especially abundant in Jurassic time. Fossils
are fairly common in marine deposits of western North America. Mosasaurs
and ichthyosaurs were about half as long as the plesiosaurs.


DINOSAURS

Most spectacular of the prehistoric reptiles were the dinosaurs, a large
group of animals varying greatly as to size, form, and habits. They were
adapted for a life on land though many of them probably spent much of
their time partly submerged in the waters of lakes and streams. There is
little that can be said of the group as a whole other than that all of
them were reptiles. Further than that it is necessary to regard them as
belonging to several different subdivisions of the Reptilia.
Classification has been difficult and the names used for the various
subdivisions are often misleading to the layman who tries to understand
the terminology.

Ancestral reptiles were five-toed and five-fingered but among the
dinosaurs there were many departures from the standard formula. Three or
four of the digits were commonly well developed, the others when present
being shortened or reduced to mere rudiments. Early in the history of
dinosaurs there was a division of the stock into two main branches, each
of which includes a variety of types and sizes, and is again subdivided.
The two main groups are best recognized by the construction of the bony
framework which comprises the pelvic girdle or hip region of the
skeleton. In order to avoid technical difficulties, however, the
remaining discussion of these interesting reptiles will be confined to a
few names and descriptions which serve to illustrate roughly the great
amount of variation that developed from the comparatively simple
ancestral pattern. The plan according to which the dinosaurs are usually
classified is barely suggested by the types described.

The meat-eaters were active creatures provided with powerful jaws and
teeth. They were unarmored, moved about on their hind feet, and during
their time were the most highly advanced of all animals. _Tyrannosaurus_
with a length of forty-five feet or more, and _Deinodon_, nearly as
large, were among the greatest of these. Both lived in the Cretaceous
period. Their teeth were simple but strong, knife-like, curved, and
finely serrated. Skulls were large and the forelimbs were reduced almost
to a state of uselessness. Large carnivores lived also during Jurassic
time and even as far back as late Triassic. Early Triassic forms were of
smaller size.

More primitive flesh-eating dinosaurs of the Triassic and Jurassic
periods were delicately proportioned and lightly built bipeds bearing
some resemblance to birds. _Struthiomimus_, which means
ostrich-resembling, was about the size of the bird which provides the
name. It was slender in the limbs, three-toed, long necked, long tailed.
The skull was small, forelegs long for a biped. Unlike most dinosaurs it
was toothless. All these bird-like carnivores were small as compared
with other contemporary forms. Compsognathus, of Germany, and one of the
smallest of all dinosaurs, had a length of less than three feet,
including the long tail.

    [Illustration: One of the Large Jurassic Dinosaurs (_Diplodocus
    longus_)

    This magnificent specimen, exhibited by the Denver Museum of Natural
    History, has a length of seventy-five feet six inches. Two years
    were required to complete the task of removing the bones from the
    matrix rock and preparing them for mounting. Diplodocus was one of
    many large reptiles which inhabited western North America a hundred
    and fifty million years ago. The skeleton was obtained from the
    Morrison beds of eastern Utah. The same formation is exposed in many
    Colorado localities, including the foothills west of Denver, where
    it acquired its name from the town of Morrison.]

In Jurassic time there became prominent a group of large dinosaurs which
were more equally developed as to fore and hind limbs. They were
sluggish creatures, quadrupedal in their manner of locomotion,
vegetarians in regard to their diet. Some of them reached enormous
proportions and it is believed that they resorted to life in the water
in order to get part of the weight off their feet. _Diplodocus_ and
_Brontosaurus_ are the names of well-known giants in this group. They
had long necks and tails, very small skulls, were the largest of all
land animals and are known to have reached a length of eighty feet or
more. Some estimates, based on measurements of incomplete skeletons,
have exceeded one hundred feet, but these extremes are somewhat
questionable. _Diplodocus_ was the more elongated of the two, with much
of its length in the whip-like tail. Our mounted skeleton has a length
of seventy-five feet six inches, measured along the vertebrae. Its
height at the pelvis is twelve feet six inches.

The teeth of these large quadrupeds are of a slightly broadened and
blunted form which has caused some speculation as to their possible use.
It has even been suggested that the animals were fish-eaters but this
seems impossible in view of the great size and general characteristics
of the group. Although they differ extremely in some respects, they are
regarded as being more closely related to the carnivores than to the
herbivores of the second great branch of the tribe.

The unquestioned herbivores, constituting this second branch of the
dinosaurian race, also include both bipeds and quadrupeds. The better
known plant-eaters were large animals but not such monsters as
_Tyrannosaurus_ or _Brontosaurus_. Of the bipeds, _Trachodon_ is perhaps
best known. It is one of the duck-billed dinosaurs which had an average
length of about thirty feet. The duckbills were unarmored, active
animals, good swimmers as well as runners. They were prominent and
widely distributed during late Cretaceous time. Many skeletons have been
found in western North America. Natural casts and impressions of
mummified remains indicate that the hides were scaly and the feet
provided with webs between the toes. The bill was broad, flat, and
toothless, but the sides of the mouth were provided with a large number
of simple teeth closely arranged in parallel rows. The fine skeleton
exhibited in our hall is thirty feet six inches in length. Near
relatives of _Trachodon_, such as _Corythosaurus_ had hollow, bony
crests, combs, or tubular structures on top of the head. These may have
been of some service in connection with breathing while feeding under
water.

    [Illustration: A Duck-billed Dinosaur of the Cretaceous Period
    (_Trachodon mirabilis_)]

    [Illustration: Stegosaur (_Stegosaurus stenops_)]

Among the quadrupedal vegetarians an interesting family is represented
by _Stegosaurus_, a late Jurassic dinosaur having a length of about
twenty feet. These creatures had heavy limbs, all used in walking, an
arched back, and almost no brain at all. A double row of large flattened
plates standing upright and extending from the rear of the skull nearly
to the tip of the tail provided some protection for the back of the
animal, but otherwise there was no defensive armor. Several long spikes
at the end of the tail probably served as weapons. The mounted skeleton
in our collection was obtained from Garden Park, near Canon City,
Colorado, a district which has long been famous for dinosaur remains.

The ankylosaurs were more completely armored with closely set bony
plates fitting neatly over the body. They were of about the same size as
the stegosaurs but the body was broad and somewhat flattened. These
armored quadrupeds apparently lived only during the Cretaceous period,
after the disappearance of the stegosaurs. Their tooth equipment was
very poor and in a few cases entirely lacking. _Ankylosaurus_ and
_Nodosaurus_ are good examples of the type. They have been described as
animated tanks and are sometimes referred to as having the appearance of
enormous horned toads.

Among the last of the dinosaurs to come and go were the horned
quadrupeds known as the Ceratopsia. Their entire history appears to have
been confined to the Upper Cretaceous and the closing stages of the
reptilian era in America. _Triceratops_ and _Monoclonius_ are well-known
representatives of the group. Besides the horns, which appeared above
the eyes or near the center of the nose, there was a broad, flattened,
backward extension of some of the skull bones which produced a great
frill or collar reaching over the neck as far back as the shoulders.
This frill, combined with the large skull, gave the animal the
appearance of being nearly one-third head. _Triceratops_ had three
horns, _Monoclonius_ only one. The average length of the animals was
slightly under twenty feet.

Although very little is known about the ancestry of the horned dinosaurs
a valuable discovery in Mongolia may throw some light on the subject. A
small dinosaur with a well-developed frill, but no horns, once inhabited
the region of the present Gobi desert, and in recognition of the
apparent relationship it has been named _Protoceratops_. In addition to
numerous skeletons, several nests of eggs were found in association with
the bones. Until this discovery was made, dinosaur eggs had been
practically unknown. A reproduction of one of these nests is among our
exhibits.

    [Illustration: A Sea Turtle of Cretaceous Time (_Protostega gigas_)

    This marine animal belongs to a group which became extinct near the
    close of the great reptilian era, but a few related forms still
    survive. Their weight is greatly reduced by the peculiar
    construction of the shell, and the front feet are enlarged for use
    as oars, an excellent illustration of the manner in which a land
    type can become adapted to life in the sea.]

With the possible exception of a very few short-lived survivals
dinosaurs were extinct before the opening of the Age of Mammals, many of
them for millions of years. Along with them went other types of ancient
reptiles, and the cause of their extinction is a problem which may never
be solved. Conditions remained favorable for the turtles, which made
their first appearance during Triassic time, and for the crocodiles,
which date back to the Jurassic period. Snakes were only at the
beginning of their history as the era closed. The survival of these
modern forms suggests that they were favored to a greater extent than
the dinosaurs during a prolonged period of changing conditions the full
details of which are unknown to us.

In general it is to be expected that disaster would first overcome the
highly specialized creatures, such as the dinosaurs, which had become
more delicately adjusted to the particular environments in which they
lived. It appears that some of them had been too progressive up to a
certain point, but not sufficiently adaptable to get beyond that stage,
or fortunate enough to make their advances in directions that could be
followed, through fluctuations in the matter of food supply, predatory
enemies, climate, and other factors which bear upon success and failure.

The reptilian era closed with exceptional volcanic activities in many
parts of the world, but these cannot account for the disappearance of
the highly diversified and abundant reptilian life. The eruptions were
merely incidental to movements and readjustments in large masses of rock
comprising the earth’s crust or surface. Such crustal folding and
elevations always have been of serious consequences to both plants and
animals because of their effect upon drainage and climate. There were
disturbances of this kind in western North America in late Jurassic
time, with folding and uplift in the region of the Sierras and probably
extending from Mexico to southern Alaska. A great trough to the east of
this elevated district was produced in the course of these movements and
provided access to the sea from south to north. During the Cretaceous
period there were repeated invasions and retreats of the sea by way of
this great depression, consequent upon slight changes in the elevation
of the floor. Hence there are numerous marine formations in Colorado and
adjoining states, some of them rich in fossils.

Before the close of the Cretaceous period the sea had made its final
departure from this region, and the Mesozoic era was terminated by
revolutionary disturbances which brought about the uplifting of a new
mountain system. The Rocky Mountains may be regarded as part of this
system and to have had their birth at this time. The Rockies, however,
show unmistakable signs of repeated elevation, with intervals of erosion
during which there was great reduction of their total height. What we
see of them today is the result of more than fifty million years of
continuous geological activity.


PLANT LIFE AND CLIMATE

Some idea of the Mesozoic climate is obtained from the character and
distribution of the plant life. Triassic floras are not large and there
is very little fossil evidence for the earlier half of the period. It is
quite possible that arid or desert conditions prevailed for a time in
much of North America, as at the close of the Paleozoic era. Plant life
was at first not abundant, and conditions were unfavorable for the
production of fossils. In Upper Triassic rocks of Virginia, however,
there are signs of swampy conditions, with rushes and ferns
predominating. Adjoining forest areas were well timbered with large
coniferous evergreens which show no annual growth rings, as similar
trees do in regions where cold winters alternate with warm summers. This
suggests, for that time and place at least, a uniformly warm climate,
lacking seasonal variations. Warm temperature or subtropical climates
are indicated again by some of the Jurassic and Cretaceous plants, but
intervals of lower temperatures and variable climates are also apparent.
Palms, figs, and other trees, very similar to modern types now living
only in warmer regions, were widely distributed in late Cretaceous time,
and their range was extended into regions which have since become too
cold to support such growths.

The trend toward modern forms in the plant world was gradual, but
throughout the era there were occasional novelties that attract the
attention of botanists. Ferns and horsetail rushes, reminiscent of the
Paleozoic forests, soon began to lose their prominence as the
seed-bearing trees gained the ascendency. Mesozoic time could well be
called the age of cycads, because of the striking performance of this
plant group. Different varieties flourished in the three periods, with
the Jurassic standing out as the time of greatest abundance.

To the uninitiated, the usual cycad fossils resemble “petrified
pineapples,” but these are merely the scarred stems or trunks of small
to medium-sized trees with a tufted arrangement of leaves at the top,
and usually without branches. Foliage and habit of growth suggest
something more like large ferns or low-growing palms, with short, thick
trunks seldom more than fifteen feet tall and many of them under three
feet. The leaves are rarely found entire or attached to the trunks, but
occasional discoveries indicate a leaf-length of about ten feet.
Although they are classed among the first and lowest of seed-bearing
plants, and in this respect are related to the conifers, their
appearance was quite unlike that of the modern cone-bearing evergreens.

More nearly resembling the common conifers of today were the sequoias,
of early Mesozoic origin and far more abundant during Cretaceous time
than they are at present. The maidenhair trees, now represented by a
single species of _Ginkgo_ which is cultivated principally in China and
Japan, were never very prominent but are of interest as an ancient
family that persisted throughout the Mesozoic and down to our own time.
Before the close of the Cretaceous period the flowering plants had
greatly outnumbered the spore-bearing groups, such as the ferns and
horsetails which were formerly so abundant. We know little of early
flowers, however, except in connection with trees, the large gayly
colored blossoms of the type now conspicuous in woodlands, meadows, and
gardens being later arrivals and poor subjects for preservation as
fossils.

Cretaceous floras were surprisingly modern in character, far in advance
of the animal life. Poplars, plane trees, magnolias, palms, figs, oaks,
and buckthorns were abundant at the close of the Cretaceous, as
indicated by fossils of the Laramie formation, which is the surface rock
in many localities near Denver. Also abundant in various places at this
time were walnut, hazelnut, laurel, tulip, maple, beech, birch,
breadfruit, ivy, holly, and many other well-known trees and shrubs.
Sedges and grasses, which became so important to the herbivorous mammals
of the next era, made their first appearance in Cretaceous time but were
then inconspicuous.


COAL AND FOSSIL FOOTPRINTS

The abundance of plant life in the Colorado area during the Cretaceous
period is indicated by the extent of coal deposits of this age. About
one-fourth the area of the state is underlain by coal seams varying in
thickness from a few inches to fifty feet or more, most of it being
Cretaceous. In the northern Colorado district the coal-bearing formation
is the Laramie. Near Denver there is some coal in the Arapahoe formation
which overlies the Laramie and is of later age.

Coal mines often produce excellent plant fossils, and occasionally other
evidence of prehistoric life. In a mine near Canon City, Colorado, a
series of natural casts of dinosaur feet was taken from the overlying
rock after the coal had been removed. One of these, in the Denver Museum
of Natural History, is seen to consist of sandstone inside a very thin
layer of dark clay. Flattened against the lower surface is the
carbonized stem of a Cretaceous plant which grew in the swamp where the
coal deposit was formed.

Since the shape of dinosaur feet is unmistakable we can only assume that
a large reptile of this type walked over the surface of swampy ground in
which a great thickness of decaying vegetation had accumulated. A layer
of mud settled over the top and became sufficiently firm to retain the
mold of the feet as the animal moved along. Any plant material either on
the mud or included in it was pushed to the bottom of the impressions
and flattened out by the weight of the huge creature. Then sand was
washed into the footprints from some nearby source during a heavy
rainstorm.

Following these events there was probably a subsidence of the area, and
a great thickness of rock-making sediments was built over the ancient
swamp. The buried vegetation gradually became converted into coal, the
sand consolidated into a firm sandstone, and the mud produced the shales
forming the roof of the present mine, which is now at an elevation of a
mile above sea level as a consequence of the general uplifting of the
Rocky Mountain region during late Cretaceous and subsequent time.

When the coal was removed, the hard sandstone casts separated readily
from the softer shales surrounding them. A small amount of the shale
adheres to the sandstone, and some of the flattened vegetation, now in
the condition of coal, still remains attached.


MESOZOIC INVERTEBRATES

As in other eras, the invertebrates fluctuate with the periods.
Characteristic forms appear, become more or less prominent, then in many
cases decline or disappear. Variations among the mollusks are
particularly helpful in the identification of rocks which originated in
the Cretaceous seas. Clam-like bivalves of the genus _Inoceramus_, the
straight-shelled ammonids known as _Baculites_, and oysters, are locally
common in some of the formations exposed a few miles west of Denver.

The ammonids, or “ammonites,” were extremely abundant throughout the
world during Mesozoic time. Their shells were chambered like those of
the pearly nautilus, a related cephalopod mollusk inhabiting tropical
seas at the present time. While only four species of the Nautilus tribe
are living today, thousands of species of ammonids swarmed the
prehistoric seas. Many new forms came into existence in Triassic time
but few survived the period. A pronounced revival occurred in the
Jurassic, only to be followed by a decline and eventual extinction at
the close of the Cretaceous. Ammonites measuring three or four inches
across the diameter of the coiled shell were about average size, but
diameters up to three or four feet were not uncommon. Externally the
shells were ornamented with ribs, knobs, and spines; inside was a pearly
lining. The partitions were thin and composed of the same pearl-like
substance as the lining. Each partition becomes wavy as it approaches
the shell, and the line of union has a distinct pattern which is seen in
specimens which have lost the outer shell layer. This wavy suture line
becomes more complicated in the later members of the race, and the
peculiar markings developed by the repeated partitions provide a
convenient method of identification.

The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes
now living, comprise another group of carnivorous mollusks. These,
however, had lost the external shell, and the usual fossil is part of an
internal shell or “skeleton,” known as the guard. This limy structure
has the form of a pointed cigar, and is seldom over a foot long although
the total body-length of the larger animals was commonly about six to
eight feet. Several hundred species have been described, the majority
being of Jurassic age. They declined rapidly toward the close of the
era.

The invertebrate life of the Mesozoic was strongly dominated by
mollusks, with cephalopods in first place, the bivalve pelecypods and
the single-valve gastropods or snail-like forms sharing subordinate
positions. The dominating trilobites, sea-scorpions, and tetracorals of
the Paleozoic had disappeared, while the brachiopods and crinoids were
greatly modified and more like the forms which live today.

Crinoids became moderately abundant at various times, but in many ways
different from their relatives of the preceding era. Some of the largest
known species, with stems estimated as fifty feet long, have been found
in lower Jurassic rocks. A great abundance of microscopic life is
indicated by the frequent occurrence of chalk in the Cretaceous
formations. Corals of the modern reef-building type (hexacorals) were
common in the warm seas of a large part of the world.

The insects of the early Mesozoic are represented by few fossils
although it is evident some new forms were becoming established. The
warm climates prevailing throughout much of the world appear to have
been a favorable factor in the progress of insect life. In addition to
the older cockroach and dragon-fly types may be mentioned the arrival of
grasshoppers, cicadas, caddis-flies, beetles, and ants.

Several hundred species are found in Jurassic rocks, and by the end of
the Cretaceous period most of the insect families now known to us were
probably in existence. The record is seriously obscured by the erosion
of rocks which so frequently marks the end of a period, also by the
small size of the subjects, and by the exceptional conditions required
for the production of such delicate fossils. Among the last of the
familiar insects to appear were the bees and butterflies. These
evidently came in with the more advanced types of flowering plants that
produce the nectar on which many insects feed. It is probable, too, that
without the arrival of these insects and their service in the
pollination of flowers, the floras of today would be rather different
from what they are.


EXTINCT BIRDS

It is not surprising to find that birds made their first appearance in
the Mesozoic era, for of all animals they are most like the reptiles as
a class. Feathers are about the only dependable characteristic of the
entire group, nearly every other feature being matched by some reptilian
creature of great antiquity. The nesting habit, which includes care of
the young as well as the eggs, is a matter of progress which relates to
flight and to warmer body temperature. It appears to have been developed
by forest-dwelling types living among trees and nesting there in
comparative safety from enemies prowling on the ground.

The oldest known prehistoric birds were found in lithographic stone of
Upper Jurassic age. _Archaeopterix_ was discovered in 1861 at
Solenhofen, Germany. Sixteen years later a similar bird in a better
state of preservation was found in Bavaria. The latter was named
_Archaeornis_. These Jurassic fossils are regarded as true birds by some
authorities, while others believe them to be more nearly related to the
reptiles, the opinions being based on careful studies of the skulls and
other skeletal features. Both birds had teeth of reptilian character,
and it is evident that there was no beak, for the jaws were covered with
scaly skin. The bony construction of the long tail would suggest lizards
rather than birds, were it not for the presence of feathers which were
attached at each side. Head, neck, and parts of the body were covered
with scales. Wings were well provided with stout feathers but the
skeletal framework indicates that the birds were gliders rather than
true flyers. Claws on the wings served like fingers to aid in climbing
among the branches of trees, a practice which is occasionally noted
among the young of living birds. In adult birds of today, however, the
claw-like appendages of the fore limbs are greatly reduced and of little
service.

The next fossil birds of importance have been found in Cretaceous rocks
of Kansas, both of them fishers of the seas instead of forest
inhabitants. _Ichthyornis_ was a small bird, standing about eight inches
in height, a powerful flyer with reptilian jaws and teeth. _Hesperornis_
was built for diving and swimming, like the loon, but was somewhat
larger and provided with teeth. Its wings were too poorly developed to
be of use in flying.

Toothed birds became extinct with the close of Cretaceous time, and the
ancestors of modern types were in existence before the Age of Mammals,
but fossil remains are few and poorly preserved. Large ostrich-like
birds, however, are known to have lived in North America during the
Eocene period. One of these, named _Diatryma_, stands nearly seven feet
tall in the reconstructed skeleton. Its legs are heavy, wings greatly
reduced, beak massive. In its relation to modern birds it is possibly
nearer to the cranes than the ostriches.

Flightless birds of large size are known from many parts of the world
and seem to have been prominent throughout the Cenozoic era, as they are
today in the southern hemisphere. _Aepyornis_ lived in Madagascar during
the Pleistocene period and may have become extinct quite recently. Its
eggs are the largest known among fossils, several times the size of an
ostrich egg. Also in this period the moas were living in New Zealand
where their remains are still abundant. One of the largest, known as
_Dinornis_, had about the same form as _Diatryma_ but the neck was
longer, head and beak smaller, legs better fitted for running, height
about eleven feet.

A much smaller flightless bird, the dodo, became extinct in modern time.
This former inhabitant of Mauritius and other islands of the Indian
Ocean was related to the doves and pigeons, and had lost its power of
flight through disuse of the wings. It was a clumsy, defenseless bird
weighing possibly as much as fifty pounds. Actual remains are few and
incomplete, and descriptions published by the explorers who knew the
bird two centuries ago are not entirely trustworthy. In the Pleistocene
Rancho la Brea beds of California the largest of all prehistoric flying
birds has been found, a vulture bearing the name of _Teratornis_.
Re-assembled skeletons show them to be slightly bigger than existing
condors.


ANCESTORS OF THE MAMMALS

The monotremes or egg-laying types of mammals such as the duck-bill and
spiny anteaters which now inhabit Australia are almost unknown as
fossils. Marsupials, the next higher living group, which includes the
opossum and kangaroo, appeared at the end of Cretaceous time along with
the placentals or higher mammals which dominate the history of the
Cenozoic era. Nevertheless, there are a few teeth and jaws from rocks of
Triassic and Jurassic age to indicate that small mammals, from the size
of mice to slightly larger than rats, existed throughout most of the Age
of Reptiles. There is no complete skeleton of any of the earlier forms,
and little is known of their relationships either with living orders of
mammals or with probable ancestors among the reptiles. The record
becomes somewhat clearer toward the end of the era but it is obscured
again by the great disturbances which followed.

Looking back among earlier land animals for the origin of the first
mammalian stock it is necessary to go as far as Permian or even
Carboniferous time. Reptiles then living had many structural features in
common with mammals, and mammal-like forms continued to flourish until
late in the Triassic. An interesting group of such animals, named
therapsids, was one of the earliest reptilian stocks to appear, and is
well known from fossils found in the Red Beds of Texas and New Mexico,
in Europe, South Africa, and Asia. Quite a variety of types is included
in this group, with many advances in dentition, and modifications of the
skull, limbs and pelvic construction which strongly suggests a
relationship to the mammals.

    [Illustration: Murals Over Fossil Exhibits, Hall of Mammals
    Top: Eocene; Protylopus, Tanyorhinus, Patriofelis, Uintatherium,
    Turtle, Crocodile, Eohippus.
    Middle: Upper Oligocene; Mesohippus, Merycoidodon, Hoplophoneus,
    Metamynodon, Poebrotherium, Trigonias.
    Bottom: Pliocene; Teleoceras, Turtle, Synthetoceras, Amebelodon,
    Teleoceras.]



                           THE AGE OF MAMMALS


The striking feature of life development in the Cenozoic era is the
great progress and expansion over the earth of the mammalian races. The
division of the era into periods, however, was based largely on a study
of fossil mollusks. In the Paris basin of France, it was noticed by the
geologists of a century ago that the youngest of the sedimentary beds
contained the greatest number of recent or still living species.
Successively downward into the older beds the percentage of recent
species decreased until there were practically no living species
represented in the oldest rocks of the series. From the percentage of
recent forms among prehistoric ones it was proposed that the following
division be made: Eocene, meaning _dawn of the recent_; Miocene, meaning
less recent; and Pliocene, meaning _more recent_. Sometime later it was
suggested that another period be added, and to this was given the name
Pleistocene, meaning _most recent_. In 1854, the older Miocene
formations were segregated and referred to a newly provided Oligocene
period, this name meaning _little of the recent_.

Early geologists grouped the rocks in three great divisions, applying
the names Primary, Secondary, and Tertiary. To these was added
afterwards the name Quaternary, which applied to the youngest formations
of the earth. Only two of these terms remain in common use at present:
it is a frequent practice to refer to the combined Eocene, Oligocene,
Miocene and Pliocene periods as the “Tertiary” division of Cenozoic
time; to the Pleistocene and Recent periods as the “Quaternary”
division. The geology of some remote future may be clearer with regard
to the full significance of this subdivision of the Age of Mammals into
two parts. It may be that a great era was concluded at the end of
Pliocene time as others have been concluded, by the usual earth
disturbances and climatic changes and by the decline of animals once
prominent in the faunas of the world. Events of such character have
registered their occurrence but may eventually prove to have been a
series of minor events not comparable with the revolutionary changes
that terminated other great time divisions. The favored practice of
including ourselves and our times in the Cenozoic is based on a trend of
opinion which holds that no great era has been ended since the Age of
Reptiles was concluded.

    [Illustration: Skulls of the clumsy, six-horned uintathere and the
    early, hornless titanothere form part of this Eocene display. In the
    mural these animals and the little “three-toed” Eohippus, smallest
    of horses, are pictured with a contemporary turtle and crocodile.]

The oldest of Eocene rocks show a great variety of mammals and a strange
assortment of forms far in advance of the Mesozoic record. Ancestries
and successive stages of development have been only partially worked
out, though details have been better preserved for some of the groups
than for others. Some oddly shaped creatures such as the uintatheres
reached their full development in a relatively short time and passed out
of the picture before the end of the period. These animals, represented
in our collection by _Uintacolotherium_, acquired large bodies and many
horns, but a peculiar tooth equipment fitted them for a special diet
which apparently failed to be supplied in sufficient abundance at a
critical time.

On the other hand, we find in this period the ancestors of more
successful groups, some of which continue on into modern times. Only a
few of these histories can be traced in a brief sketch, but in a general
way it may be stated that the successful races had modest beginnings and
that they developed very slowly into what they are now, by a process of
adjusting themselves, or by becoming better adapted to new or previously
unused conditions in their respective environments. In their early
stages the various types had much in common; they were generalized,
rather than specialized for any particular kind of existence. What they
were fitted for is best indicated by their teeth and feet, though other
structural features frequently contribute valuable information. The rise
of mentality is indicated by skull capacities and the increasing
development of the upper lobes of the brain, as revealed by casts taken
from the interior of skulls.

The creodonts were the earliest and most primitive of the flesh-eating
mammals or carnivores. Many of them were small of body and brain, and
equipped with teeth that indicate a mixed or largely insectivorous diet,
or possibly the habit of feeding on carrion. Although there was
considerable variation among them, and some tendency toward
specialization, there was little to suggest the coming of more
progressive groups such as the cats and dogs, with teeth perfected for
the tearing and cutting of flesh, and feet especially fitted for the
life of hunters. The ancestry of the cats cannot be traced farther back
than the Oligocene but it probably connects somewhere prior to that time
with the creodonts.

_Cynodictis_, an Oligocene carnivore slightly under two feet in length
is commonly regarded as a primitive dog, but its characters are so
generalized that it probably differs but little from the ancestors of
many other carnivores. The skeleton of this animal suggests a slender
and flexible body like that of the weasel, with somewhat shortened limbs
and a long tail. It lived in forested regions and was probably more or
less of a tree dweller. The more advanced carnivores required longer
legs, better adapted for running and overtaking the prey, which is the
dog’s way of hunting, or for stalking and springing upon the quarry,
which is the method of the cat.

    [Illustration: Moropus (_Moropus cooki_)

    Though its teeth clearly indicate a diet of plant material, this
    strange animal had claws on its toes, like the carnivores. It is
    probable that the claws were used in digging for roots, as indicated
    by the artist.]

A prominent group of mammals today is that known as the ungulates, or
hoofed animals, which includes the horses, cattle, deer, swine,
rhinoceroses, tapirs, and other types both living and extinct. Their
probable ancestors were the condylarths, primitive ungulates of the
Eocene period. One of these, known as _Phenacodus_, serves well to
illustrate the general character of the early hoofed mammals. It was
about five and one-half feet long, rather large for its time, with long
tail and short limbs, low elongated skull and small brain, very similar
in many respects to the creodonts or ancestral carnivores. The teeth,
however, were partially of the grinding type so essential to the welfare
of plant feeders.

The condylarths were five-toed animals and evidently provided with small
hoofs, but the more progressive ungulates soon lost one or more of the
toes, and a division of the group into odd-toed and even-toed branches
became firmly established. Consequently, the families of ungulates
having one, three, or five toes are classed together as being closely
related to one another, and those having two or four toes are segregated
in a second lot. The odd-toed clan, known as perissodactyls, included
such animals as the horses, rhinoceroses, tapirs, and titanotheres, each
of these types being placed in a separate family. The even-toed clan has
been treated in a similar way and named the artiodactyls. In this
division are such families as swine, cattle, deer, camels, oreodonts,
and others. The odd-toed group dominated among the larger animals of
North America for a time but has been completely replaced by the
even-toed division which is still flourishing, although some of the
older families have become extinct.

Among the exhibits of the Denver Museum of Natural History may be seen
complete skeletons of extinct horses, rhinoceroses, titanotheres, and
chalicotheres representing the perissodactyls. _Moropus_ was one of the
chalicotheres, an exceptional family which never became very prominent
although it had a prolonged history and persisted in Europe and Asia
after its extinction in North America. The family is grouped with the
ungulates because of many similarities found in the molar teeth, skulls,
and other parts of the skeletons, but the toes were provided with claws
instead of hoofs. The use of these claws is somewhat of a puzzle:
possibly for defense against carnivorous enemies, for dragging down
branches in order to obtain food, or for digging roots which may have
been an important part of the diet.

Titanotheres are represented in our collection by the skeletons of the
large, horned type which was the last of the race and destined to
extinction by the middle of the Oligocene period. Smaller hornless
varieties of Eocene time are illustrated by skulls. This family of
ungulates had an unprogressive dental equipment, and a small brain in a
flattened skull. The molar teeth readily distinguish the group from
other ungulates and enable us to trace the relationship between earlier
and later varieties. These teeth were of a type which is soon destroyed
by wear, and it is evident that the animals survived only so long as
their environment provided them with an abundance of soft vegetation.

    [Illustration: Titanotheres of Oligocene Time

    The name of these animals refers to the large size though they were
    greatly exceeded in bulk by the mastodons and mammoths of later
    periods. Ancestral titanotheres, dating back to the Eocene, were
    hornless animals of much smaller size. These splendid specimens were
    obtained in Weld County, Colorado.]

The large assortment of rhinoceros material provides an idea of the
great abundance and variety of forms in this family which was once
prominent in North America but no longer among the inhabitants of that
continent. Some of the mounted skeletons have been restored on one side
to show how these animals appeared in the flesh.

Of the even-toed ungulates there are also several types illustrated by
complete skeletons. _Merycochoerus_, the subject of one of our mounted
groups, represents the oreodonts, a large family of mammals whose
history begins with the Upper Eocene and ends in the Lower Pliocene. The
oreodonts were small animals, rather pig-like in form and quite common
in the western plains region shortly after the time of the titanotheres.
Ancient swine are represented in our exhibits by two mounted skeletons
which were obtained from northeastern Colorado, where the bones were
found associated with rhinoceros and titanothere remains. Some of these
animals were of very large proportions, and the entire family is
commonly known as the “giant pigs.”

Camels and closely related forms were quite abundant in North America
from early Oligocene to comparatively recent time. Numerous types were
developed during the course of their history, some small and delicately
formed, others tall and clumsy and much like the giraffe in structure.
Parts of many of these creatures have been found but the only completely
prepared skeletons in our collection are of the little gazelle-camel,
_Stenomylus_, from Lower Miocene deposits in northwestern Nebraska.
Pleistocene bisons are represented by several complete skeletons and
numerous skulls and horncores, some of the species showing an extreme
development in the length of horns. With two of the bison skeletons are
shown prehistoric weapon points, found with the bones and indicating
that these animals were hunted by primitive men at some time near the
close of the Ice Age. The artifacts first discovered near Folsom, New
Mexico, by field workers of our Museum, have become known to
archeologists as Folsom points.


PREHISTORIC HORSES

The past history of horses is well known from an abundance of fossil
material, ranging in age from the Eocene down to the present. Modern
horses have only one toe in each foot, but there are remnants of two
additional toes which may be seen only in the bony structure underlying
the skin. Most of their ancestral relatives were three-toed as far back
as the Oligocene period. During Eocene time, however, there was a stage
which may be regarded as four-toed although it was evidently a temporary
condition, linking known horses with more remote forms having five toes.

    [Illustration: Oligocene Mammals From Weld County, Colorado

    The giant pigs (_Archaeotherium mortoni_) at the left of the group,
    and the rhinoceros (_Trigonias osborni_) were common animals of the
    western plains region at one time.]

_Eohippus_, the “dawn horse” as it has been called, is one of the oldest
and best known of the American horses. Its relation to existing members
of the family can be traced by means of changes in tooth structure as
well as in the gradual reduction in the number of toes that is seen
among intermediate forms. Its ancestors some day may be positively
identified in that group of generalized, primitive, five-toed, hoofed
mammals which are known to have lived at the beginning of the mammalian
era, but such identification has not yet been established. Even
_Eohippus_ bore little resemblance to the familiar horse of today. Its
height was only eleven inches, and in body form it had much of the
appearance of a modern dog. There were four toes on the front foot, one
of them decidedly shorter than the others but complete in all its parts,
and evidently capable of service in carrying a portion of the animal’s
weight. The hind foot had three complete toes and a tiny remnant of a
fourth which could not have been apparent externally.

As changes in the structure of the feet progressed, the central toe of
the original five continued to increase in size while the adjacent
digits became relatively shorter and eventually so reduced in length
that they could touch the ground no longer. The smaller bones at the
extremities, corresponding to the joints of our fingers and toes,
eventually disappeared from the side toes. Then the longer bones of the
outer digits lost the broadened supporting surface, where the missing
toes had been attached, and became reduced to pointed remnants known as
splints. Extreme shortening of the splint bones eventually leaves only a
small knob which is often referred to as a rudimentary toe. In the
skeleton of a large horse the splints are readily seen, but in some of
the earlier species they are so small that they may easily be destroyed
or overlooked by the collector who removes the fossilized material from
the surrounding rocks. Even then, the bones of the wrist and ankle may
indicate in an unmistakable manner that an additional toe once was
present, for each bone is supported by another, and at the point of
attachment there is a characteristic surface whose purpose is usually
obvious.

Throughout the Cenozoic era the changes continued. Among the horses of
the North American Oligocene were _Mesohippus_, approximately the size
of a collie dog, and _Miohippus_ which was slightly larger. Both were
three-toed, but the rudimentary splint of a fourth toe was still present
in the front foot. _Parahippus_ and _Merychippus_ carried on during the
Miocene period, the latter being characteristic of the time, and
showing, in addition to other progress, a decided trend toward the
modern structure of molar teeth. There was some increase in size but the
largest horse of that period was hardly more than a small pony.

    [Illustration: A Pleistocene Horse of the Texas Plains (_Equus
    scotti_)]

_Hipparion_ and _Protohippus_, living during Upper Miocene and Pliocene
time, represent later stages of the three-toed condition. The side toes
were completely formed but greatly shortened, only the central toe
touching the ground. In some of the species the outer toes had also
become very slender, approaching the splint condition. By this time the
molar teeth were longer and better adapted for feeding on grasses which
were becoming sufficiently abundant to attract some of the forest
dwellers into the open country.

During the Pliocene period, in the genus _Pliohippus_ and also in
_Hipparion_, the feet were far advanced in structure, with most of the
species single-toed, the side digits having reached the splint stage.
Pleistocene horses of the genus _Equus_, like living species of that
genus, were strictly one-toed animals, ranging over grassy areas and
highly specialized for a life in that kind of environment.

Specialization is to be noted partly in the foot and leg structures
where the modifications have contributed to greater speed and travelling
ability. This is of great service to an animal of the plains where food
and water are often scarce, and great distances frequently have to be
covered in order to obtain sustenance. The horse, as we know it, is
built for speed, its limbs and feet being elongated to permit a greater
stride, and also modified to decrease the weight without loss of
strength. The ordinary ball-and-socket joint is replaced by a
pulley-like construction which limits the direction of movement but
provides an excellent mechanism for locomotion, especially over flat,
open ground. Flexibility in other directions is sacrificed for greater
strength, and the foot incidentally becomes less suited for other
purposes.

This is what is meant by “specialization”—a departure from
“generalization.” The study of fossils provides numerous illustrations
of specialized development which contributes greatly to an interest in
prehistoric life. Any specialized structure or habit which increases
fitness for a particular way of living is also known as an “adaptation.”
Quite in line with the idea of specialization and adaptation is the
change which occurred in the construction of the horses’ teeth, for the
dental equipment of the modern grazing animals differs widely from that
of the browsing creatures which lived on the soft leaves and other plant
substances of the forests.

    [Illustration: The Structure of Molar Teeth

    The large lower molar of a long-jawed mastodon shows worn and unworn
    cusps, with the enamel layer forming a heavy border around the
    central dentine where the surface covering has been worn through. In
    the grinding teeth of rhinoceroses (illustrated at the right) the
    crown pattern is quite different, but both types are adapted for
    softer foods and are similar in having the protective enamel on the
    outside only. The central tooth shows the condition after the
    shallow surface depressions have been removed by wear.]

The cheek teeth or grinding equipment of the horses underwent as
complete a change as the feet. Modification resulted in a new type of
tooth which enabled herbivorous animals to take advantage of a kind of
vegetation which was late in arriving and has since become the principal
diet of the ungulates. The grasses are coarse and harsh as compared with
the leaves of forest shrubbery, requiring more thorough grinding to make
them digestible. In addition they contain minute particles of silica,
which is a highly abrasive mineral that quickly wears down the tooth
substance, especially the softer materials found in tooth construction.
An increase in the length of the tooth would offset the excessive wear
but would not necessarily produce a better mechanism for grinding.

The fulfillment of the new requirements is to be seen in the change from
what is known as the low-crowned, browsing type of molar, to the
high-crowned, grazing type. Details of the changes that may be traced
through millions of years of gradual adjustment become apparent only
from the examination of a great deal of fossil material. As compared
with earlier types of construction, a modern molar tooth may appear
extremely complicated, but the process which brought about the improved
quality is very simple. A little discussion of tooth structure, however,
is required to make this clear.

A tooth, as everyone knows, is partly imbedded in the jaw, partly
exposed outside the gum. In a short-crowned tooth the exposed portion is
known as the crown, and the part imbedded in the jaw consists of one or
more roots which are comparatively long. The crown is nearly always
protected by a thin layer of hard enamel. In a grinding tooth, the
working surface has a number of more or less prominent elevations known
as cusps. The enamel layer completely covers this surface until wear
begins. As the tooth goes into service the signs of use begin to appear;
the enamel is soon worn from the tops of the cusps, and the underlying
substance, called dentine, becomes exposed. This is far less resistant
to wear, and as the enamel continues to be reduced the tooth becomes
less efficient as a grinding device, partly because of the smoothing off
of the surface, partly because of the relative softness of the inner
material which is being exposed in increasing quantity. A very old molar
tooth of the low-crowned type has a smooth surface from which almost the
last trace of the enamel has been removed. In many prehistoric animals
the enamel is of a darker color than the dentine or cement, this
difference in color enabling one to see at a glance how the teeth are
constructed.

    [Illustration: Grazing Type of Molar Teeth

    The side view of the bison’s molar and premolar equipment
    illustrates the elongated construction which is common among grazing
    animals. In the pattern of the grinding surface may be seen a cross
    section of the enamel layers. One layer surrounds each tooth while
    two folded “cylinders” of the same material occupy the interior.]

In a long-crowned tooth the roots are usually very short, for much of
the crown itself is imbedded in jaw bone, and the longer roots are not
required. Growth of the tooth is usually completed after a few years;
then as it is gradually worn away it is continuously moved upward by the
production of new bone under the roots, which slowly fills the bottom of
the socket and continues to provide the necessary support. An equally
important difference between the two types of teeth, however, is to be
seen in the arrangement of the enamel, the long-crowned type being
provided with this durable substance on the inside of the crown instead
of having a mere protective cap on the outside.

The more complicated structure was developed from the simpler form by
the easy method of deepening certain depressions located between cusps
at the top of the tooth. As the crown of the tooth increased its length
these depressions remained tucked in, and eventually became deep pits
roughly cylindrical in shape. In addition to the enamel and dentine, a
third tooth substance, known as the cement, made its appearance at about
this time, and we find that quantities of this new material were
deposited outside the crown enamel and also inside the enamel walls of
the pit, in this way producing a firmly consolidated structure otherwise
weakened by deep channels and hollow pockets. The cement differs only
slightly from the dentine but is deposited while the uncut tooth is in
the gum tissues of the mouth, the enamel and dentine elements being
formed earlier in the embryonic tooth before it emerges from the jaw
bone.

A tooth constructed by such a process, if cross-sectioned through the
crown, will be found to consist of successive layers of hard and softer
materials. In living animals the top of the tooth soon wears off and the
enamel layers stand in higher relief because of their greater resistance
to wear. A roughened surface of excellent grinding quality is thus
provided, and as long as the wear continues there remains the same
relative amount of enamel to retain the roughness, and resist abrasion.

Among the various types of grazing animals there is a marked difference
in the arrangement and form of the enamel layers. Within a species of
genus, however, the complicated enamel patterns of the molar teeth are
consistently similar. In the case of horses especially, these patterns
provide a most helpful key to the identification of extinct forms. The
general pattern, in any of the more modern horses, may be understood
more readily if the wavy enamel layers be regarded as forming a set of
cylinders with deeply crinkled walls. Near the outer border of the
tooth, surrounded by a thin layer of cement, is the enclosing cylinder
which represents the enamel cap of the old-fashioned, low-crowned tooth.
Inside of this is the central mass of dentine which has been penetrated
by two of the deep pits previously mentioned. The original enamel cap
has been depressed into these pits, forming two inner cylinders which
are filled with cement. Instead of being circular in outline, when the
cap is worn through at the grinding surface these inner cylinder walls
are seen to be wrinkled and folded so as to produce a most irregular
pattern. However, if several teeth of the same kind of horse are
compared, it will be found that the edges of these cylinders produce
figures which are remarkably uniform and characteristic for that
species.

    [Illustration: American Mastodon (_Mastodon americanus_)
    A true mastodon of the short-jawed type.]


MASTODONS AND MAMMOTHS

Elephant-like mammals both living and extinct are classed together in a
single order bearing the name Proboscidea. Living members of the group
are the elephants, of which the large Indian and African species are
best known. Among prehistoric representatives the most frequently
mentioned in the popular literature of North American animals are the
following:

The American Mastodon, an immigrant from Siberia which ranged over
nearly all of the United States and Canada. It was principally a forest
dweller, rarely found in plains regions, was abundant during the
Pleistocene period and may have been known to the early American
Indians;

The Woolly Mammoth, which was about nine feet tall. It ranged over
British Columbia into the United States and across to the Atlantic,
disappearing in late Pleistocene time;

The Columbian Mammoth, about eleven feet tall, lived in the early half
of the Pleistocene period, ranging over the warmer portions of North
America, including practically all of the United States and much of
Mexico;

The Imperial Mammoth, reaching a height of more than thirteen feet, and
becoming extinct in the Middle Pleistocene. It was a western form,
remains being found from Nebraska to Mexico City.

Originally placed in the genus _Elephas_, the mammoths are referred to
commonly as elephants, though technically they should not be regarded as
such. Recent explorations and researches have added greatly to our
knowledge of these animals but have also caused much confusion with
regard to scientific names, for many new subdivisions of the larger
group are now recognized, and it has become necessary to change some of
the older nomenclature.

    [Illustration: A Long-Jawed Mastodont (_Trilophodon phippsi_).
    One of the Early American Proboscideans]

The large mammoth exhibited by the Museum bears the impressive name of
_Archidiskodon meridionalis nebrascensis_. Fifty years ago it might have
been identified simply as a specimen of the imperial elephant and in
such case would have received the old name of that species, which was
_Elephas imperator_. But late in the last century it was proposed that
the mammoths be recognized by some other name to distinguish them more
sharply from living elephants. The name suggested for the new genus thus
established was _Archidiskodon_, in recognition of the more archaic or
primitive construction of the enamel plates in the mammoths molar teeth.
The specific name, _meridionalis_, had been given to a kind of mammoth
which is well known from the southern part of Europe, and the Latin
name, signifying “southern,” had been applied to differentiate this
species from the northern or woolly mammoth.

This mammoth, however, had disappeared from southern Europe and for many
years its subsequent history remained a mystery. The late Dr. Henry
Fairfield Osborn had been engaged in an extensive study of the subject,
and when the nearly perfect skeleton from Angus, Nebraska, was brought
to his attention he recognized it as being closely related to
_meridionalis_, and considered it to be a record of the migration of
that species into North America. Because of minor variations from the
typical mammoth of southern Europe he regarded it as a variety or
subspecies which had descended from the latter, and the subspecific
name, _nebrascensis_, was added to take care of this situation, using a
Latinized form of the name of the State in which the skeleton was found.
With the knowledge we now have of these mammoths it becomes apparent
that _Archidiskodon meridionalis nebrascensis_ is an ancestor of the
imperial mammoth, currently known as _Archidiskodon imperator_, and not
identical with it.

This instance is typical of the manner in which prehistoric animals
obtain their names. Although given a Latin form, these technical names
are derived from many languages, and the root words are applied with
reference to anything that happens to appeal to the author as
significant. Consequently there is seldom a name of this kind which may
be translated directly into natural history or science. It is a mistake
to believe that these strange phrases conceal important technical
information which is available only to those who are familiar with dead
and foreign languages. Actually they contain nothing of the sort, and
the most enlightened of the Greeks and Romans could not find it there.
When a name is needed there is none better than the one provided by the
specialist who is skilled in the business of naming things. Some
technical ability is required, to apply the name where it properly
belongs, but technical knowledge is not obtained from such sources.
Names, in any form, have another purpose to serve. There is no magic in
them and there need be no mystery about them.

    [Illustration: Molar Tooth of Mammoth

    This type of tooth is constructed for long continued use and will
    withstand the wear of more abrasive foods. The position of the white
    enamel plates is seen in this view of the grinding surface. These
    plates extend all the way to the base of the tooth, which is of the
    long-crowned variety and not to be destroyed by the wearing away of
    a single outside layer of enamel.]

Other specimens in the Museum collection are the long-jawed mastodonts,
so named because of the elongated jaws and protruding chin which is
often mistaken for a tusk. Early members of this group had more cheek
teeth than later types of mastodons, and longer jaws were required for
their accommodation. Some of them had flattened lower tusks which
evidently were used for digging purposes. These are popularly known as
“shovel tuskers.” The more modern American mastodon had shorter jaws
and, like the mammoths and elephants, only one pair of tusks. Both the
long-jawed and short-jawed types are represented by complete skeletons,
and also by tusks, jaws, and teeth of many individuals. The American
mastodons and mastodonts were of about the same size as the smaller
mammoths.

The difference between mastodons and mammoths is most readily recognized
in the structure of the grinding teeth, the molars and pre-molars. In
the mastodon these teeth are of the short-crowned type, while in the
mammoths, as in the modern elephants, they are long-crowned. The
difference between these two types of molars has been described with
reference to horses, and the change from the older to the modern form
may be regarded as coming about in the same general way, through a
series of gradual modifications. In both horse and mammoth the final
development shows internal enamel extending from the grinding surface
nearly to the roots. Otherwise, however, there is almost no resemblance,
for the mammoth tooth is made up of flattened enamel plates, the number
of which is variable for different species. In the jaws of a very young
individual these plates may be seen as separate parts. As the tooth
continues to grow, the plates become cemented together, and when the
ends of the plate are worn down it may be observed that each consists of
a layer of enamel surrounding a flat central core of dentine. The type
of construction is rather more obvious in the mammoth tooth than in that
of a horse, partly because of the larger size, and partly because of the
relative simplicity of construction.

The earlier history of the Proboscidea is not recorded in the rocks of
North America, for the group was of African origin and its migrations
did not extend as far as the New World until middle Cenozoic times. The
mastodons and mammoths were the largest of land animals since the Age of
Reptiles, but their Old World ancestors were not conspicuous because of
their bulk. Many of these ancient forms, even in the earliest stages,
reveal some of the prominent characters that dominate the entire group.
None of them, however, should be regarded as a miniature mammoth or
mastodon, for these highly specialized types were perfected only at a
comparatively recent date, and by a process that works very slowly.
Among the earlier forms there were also some oddities which failed to
survive or to produce a successful branch of the stock such as the
elephants.

    [Illustration: Nebraska Mammoth
    (_Archidiskodon meridionalis nebrascensis_)]

The earliest known member of the order was _Moeritherium_, an animal of
the size of a tapir, living in Egypt during the late Eocene and early
Oligocene time. At this stage the characteristic specializations leading
to the mastodons and mammoths were apparent but not far advanced. The
proboscis was probably much like the flexible snout of modern tapirs,
for the need of a long trunk had not yet arrived. In upper and lower
jaws the second pair of incisor teeth were becoming large and prominent.
The enormous tusks of the mammoths later developed from the enlargement
of the same pair of upper incisors, and in some of the long-jawed
mastodonts the lower pair also produced large tusks, though frequently
the lower tusks were not prominent.

_Dinotherium_ had downward-growing tusks in the lower jaws, none in the
upper. This genus was fairly common in the Miocene of Europe, Asia, and
Africa. In the tropics it survived throughout the Pliocene and possibly
into the Pleistocene. Some of the species acquired the size of
elephants, but it is apparent that they were not ancestral to any of the
more progressive types. They are to be regarded rather as an offshoot
from the main line of descent.

In 1859 only ten species of the elephant-like mammals were known, and
all were referred to a single genus. At the present time eleven genera
appear to be well founded, and the number of recognized species has
reached a hundred, if it has not already passed that figure. New
discoveries are expected to add to the existing total. With this mass of
material before us we note certain definite trends among the more
progressive types. The increasing weight was accompanied by the
development of strong, upright limbs in which the bones have a columnar
position instead of the angular assembly which prevails among most of
the mammals. As the tusks increased in size there was a shortening of
both skull and neck to bring the weight closer to the point of support.
The front teeth disappeared except the second pair of upper incisors
which remain as tusks in the modern elephant. The cheek teeth present in
the shortened jaws of the mammoth were reduced to one pair at a time in
the upper set and another pair below. From a simple, low-crowned origin
these grinding teeth developed into the more successful high-crowned
pattern with numerous plates of enamel inside. A prehensile upper lip
acquired the length and usefulness of the elephants trunk.

    [Illustration: Rancho la Brea Fossils

    One of the most unusual of the many animals that have been taken
    from the tar pits is the large ground sloth, seen at the left in
    this group. Such sloths were very abundant during Pleistocene time,
    and some may have lived up to a few thousand years ago.
    Archaeologists have found indications that these creatures may have
    been hunted by cave-dwelling peoples of the American Southwest.

    Other skeletons include the saber-tooth tiger, characterized by the
    long curved upper canine teeth which undoubtedly were used for
    stabbing and slashing, and the dire wolf, the smaller of the two
    which are facing the sloth. The artist’s reconstruction of this
    scene also shows the great vulture, Teratornis, which is the largest
    known bird of flight.]

Over-specialization in the production of tusks appears to have been the
principal factor in the downfall of the mammoths. The large size of the
animals and the difficulties of finding sufficient food to sustain life
must have been a serious handicap at times, but their ability and
inclination to travel over long distances enabled some of them to find
tolerable living conditions until the end of the Glacial Period. They
are now extinct and the nearest living relatives are the elephants,
somewhat reduced in size of tusks and body but otherwise very similar.

There are many other tribes of mammals whose ancient history is
partially known though broken by periods of time for which there is no
fossil evidence. All have undergone changes in which various forms and
degrees of specialization are featured; this general process is best
revealed by the horses and elephant-like animals which have left a
clearer record. For other groups the story would differ but little
except as to names and specific details.


THE RANCHO LA BREA FOSSIL PITS

The La Brea tar pits, as they are often called, provide a remarkable
record of Pleistocene life in southwestern North America. Scattered over
an area of about thirty acres just off Wilshire Boulevard in Los
Angeles, these bone deposits were known, as far back as 1875, to contain
the remains of prehistoric animals. It was not until 1905, however, that
their value was recognized by paleontologists. In that year the
University of California began an investigation, and excavations were
carried on at intervals by various institutions during the next ten
years. A great deal of material was acquired by the Los Angeles Museum
of History, Science, and Art, where many skeletons, skulls, and other
interesting specimens have been placed on exhibition.

The pits have the form of small craters formed by the seeping of oil
from the underlying rocks. The seeps appear to have been active during
part of the Pleistocene period but apparently not at the beginning. The
oil is rich in asphalt which has served as a preservative for the bones,
and owing to its sticky properties has been an effective animal trap for
thousands of years.

The fossil beds at present are of oil-soaked earth and sand. In past
times there must have been a greater percentage of oil, often concealed
by a layer of dust or pools of water. The large number of carnivorous
animals found in the deposits suggests that they were attracted by the
cries and struggles of creatures wandering carelessly into the asphalt
and serving as live bait to keep the traps in continuous operation.

Animals found there include many species still living in the locality,
some that have migrated to other territory, and a large number that have
become extinct. Among the latter may be mentioned species that differ
but slightly from living relatives, others that have left no
descendants. Horses, bison, and wolves, though extinct species, were of
relatively modern types. On the other hand the large sloths and
saber-tooth cats seem rather out of place. True cats are represented by
the mountain-lion, bob-cat, and a species of lion which is nearly
one-fourth larger than any of the great cats of the Old World. A
long-legged camel, with a height of approximately eight feet to the top
of the head, was among the native animals of the district. Skunks,
weasels, badgers, squirrels, rabbits, bear, deer, and antelope were more
or less abundant.

The La Brea group exhibited by the Denver Museum of Natural History
includes the following species: horse (_Equus occidentalis_), bison
(_Bison antiquus_), wolf (_Aenocyon dirus_), saber tooth (_Smilodon
californicus_), sloth (_Mylodon harlani_). Horses had entirely
disappeared from the North American continent by the time the first
white man arrived. _Equus occidentalis_ was one of the several species
living during the Pleistocene period, this one apparently being
restricted to California and perhaps adjacent states. _Bison antiquus_
was slightly larger than the plains bison of recent times and had it
horns set at a characteristic different angle. The species was first
described from Kentucky and appears to have had a wide distribution.

The wolves in this group are about the size of timber wolves, but have
heavier skulls with less brain capacity, massive teeth especially
adapted to biting and crushing large bones, and limbs of rather light
construction. They probably assembled in packs where meat was abundant
and, hunting in this fashion, were able to attack and overcome the
larger ungulates and edentates. To most visitors the large ground sloth
is the most interesting animal of the group. This edentate animal is
shown at the edge of the pool with one foot stuck in the “tar.”

The edentates are a group of primitive animals with very simple teeth,
if any. Teeth are usually lacking in the front part of the mouth,
sometimes entirely absent, as among anteaters. Better known living
representatives of the group are the tree sloths, armadillos, and
anteaters of South America. Ground-sloths were prominent among South
American mammals during much of Cenozoic time. During Pliocene and
Miocene time there was a marked tendency to large size, and it was
principally during these two periods that they appeared in the United
States area.

_Mylodon_ was one of the larger North American ground-sloths. Its teeth,
without the protective enamel which is present among higher mammals, are
restricted to the cheek region, and have the form of simple pegs;
instead of being specialized they stand close to the extreme of
generalization. The construction of the entire skeleton is massive,
suggesting great strength with slow movements. The hands are well
developed, provided with stout claws, and must have served the creature
well as protection against attacks by predatory neighbors. We have some
idea as to what caused the extermination of the ground-sloths in this
particular region, but the complete disappearance of such a large and
widely distributed group at the close of the Pleistocene period is a
mystery that may never be explained.

    [Illustration: The Folsom, New Mexico, Bison (_Bison taylori_)]

The saber-tooth cat, sometimes referred to as a tiger, was specialized
as a meat eater though hardly as a hunting animal. In the La Brea region
its principal food was probably the flesh of the sluggish ground-sloths.
The size was equal to that of the African lion, with hind limbs slightly
longer and the front legs more powerfully developed. The most remarkable
characteristic is to be found in the development of the upper canine
teeth and modifications of the skull which were necessary to enable the
animal to use these teeth as weapons.

In order to make the “sabers” effective it was necessary to get the
lower jaws out of the way, and this was provided for in an unusual type
of hinge which enabled the mouth to open wider than is possible in the
case of the less specialized carnivores. Judging by all the structural
features of the skeleton, _Smilodon_ could not have lived well on small
animals, for it was not equipped to capture that kind of prey. It is
evident that large mammals were preferred, and that the method of attack
was to spring upon the victim and cling there with the powerfully
developed fore limbs until the kill was completed by stabbing into a
vulnerable spot. That the position of the large sabers near the front of
the mouth interfered with normal feeding, is a reasonable conclusion.
There are also anatomical features which lead to the belief that this
carnivore was a blood sucker, perhaps more than it was meat-eater.

If most of these conclusions are correct we have here another case of
over-specialization and a possible explanation of the extinction of two
species. Such evidence as we have is far from conclusive, for there is
no proof that Rancho La Brea was the last stand of either the
saber-tooth or the ground-sloth. Both races were widely distributed and
their living conditions could not have been exactly duplicated in other
localities. It has been suggested, however, that _Smilodon_ ate the last
of _Mylodon_, and starved soon afterward because it had become unable to
partake of other foods. The conjecture is offered for what it is worth,
together with the facts on which the story has been based.

The geological record for Pleistocene time is not as complete as one
might imagine. Numerous localities have produced representative fossils
but the yield is rarely large enough to solve many of the riddles which
are constantly arising as investigation proceeds. Aside from those areas
which bordered the retreating ice cap and where living conditions were
far from favorable, the sedimentary deposits of this period are not
continuous over large areas. Many Pleistocene fossils are found in
stream channel beds which are always subject to removal by subsequent
floods.

    [Illustration: Early Man in North America

    There is abundant evidence to indicate that the great elephants of
    Pleistocene time were hunted by primitive Americans whose only
    weapons were darts or spears tipped with points of stone. A skull
    and the lower jaws of several mammoths are shown here.]

Isolated patches of fossil-bearing sediments frequently record the
migration of animals in unmistakable terms, but the details of the
wanderings and the conditions encountered in the newly established
habitats are often left in doubt. To correlate the facts revealed at one
locality with findings at other places and, if possible, to date all
prehistoric events with a greater degree of accuracy are among the major
tasks of current investigations.



                             THE AGE OF MAN


The Pleistocene or “Ice Age,” and the Recent period in which we are
living at the present moment are not sharply separated by any event
readily recognized or dated, and the two combined are of very short
duration as compared with other periods more clearly established by the
passing of centuries. Together they comprise the Age of Man as commonly
recognized, with about a million years representing the Pleistocene
period, some ten to twenty thousand years the Recent. When geologists of
the nineteenth century suggested that the coming of man should be
regarded as the beginning of a new era, the name Psychozoic was
proposed, and to some extent this term has been applied to the present
period. More in keeping with other period names is Holocene, meaning
_entirely recent_. Common usage, however, applies the simple term Recent
to this unfinished chapter which is also without a clear-cut beginning.

Zoologically, man is merely one of the creatures that arrived in the
course of time, along with other mammals. Just when he arrived and how
he looked at the time of his coming cannot be determined from a study of
fossils. Perhaps it is of no importance. There is nothing to indicate
his existence before the Cenozoic, no completely satisfactory proof of
existence before the Pleistocene period. As with other inhabitants of
the earth, it is probable that he became prominent only after a great
deal of competition with other creatures which kept his ancestors
submerged for thousands of years. The Ice Age, with its check upon the
progress of competing animals, undoubtedly gave him an advantage. His
superior mentality enabled him to overcome adversity by methods not
available to other mammals; his inventive and mechanical genius must
have been greatly strengthened by his experience during this interval.

At about this point, where prehistory begins to merge into history, the
geologist and paleontologist must let other interpreters carry on.
Archeologists and anthropologists take up the work, and through their
efforts many details have been added to our knowledge of the human race.
The study of biology, which is the science of life, has provided an
instructive viewpoint that enables us to see ourselves against the vast
background built up by investigations into the nature of the earth and
its ancient inhabitants. This science deals with living creatures as
_organisms_—plants and animals so organized as to be capable of
existence only in an environment which provides exact life requirements.

The Age of Man has been variously characterized as an age of soul, of
higher intelligence, of culture, and finally, of civilization, freedom
and democracy. The “crowning glory” of the organic world is pictured in
history as a creature who has busied himself for thousands of years with
the building up and tearing down of civilizations. Prehistory reveals
this habit as something unique in the human character, for there is no
other organism that has specialized so persistently in the creation of
its own environment, no other that has had the combined power and talent
to produce so much change.

More than anything else, the prehistoric record is a lesson in
adaptation, which in its broadest sense means fitness for life under
particular conditions, and always subject to organic law. Man’s efforts
to bring about an adjustment between himself and his civilization have
centered largely on the method of forcing himself into the mold that
happens to be present, one pattern today, another tomorrow. No creature
of the past has had to adapt itself to anything so radically new or so
thoroughly revolutionary. The vital problem now is whether this man-made
environment will prove helpful or disastrous.

Though one of its names is “culture,” it has grown sporadically and
unevenly, with little evidence of the cultivation that is implied and
required. Parts have been expanded to extraordinary proportions while
others equally essential have been retarded in their growth. A more
intelligent handling of this environment factor seems to be possible,
and the present mania for “organization” may become tempered with an
awakening consciousness of organic requirements where organism and
environment are involved. Once we grasp the idea that “culture” results
from man’s effort to improve his living, by putting into his environment
something that was not there before—then, surely, this history of a
billion years of living, and as many “ways of life,” should teach us
something we ought to know as we go into an all-out endeavor to teach a
whole world how to obtain a one-and-only way.

We may stand at the beginning of an era for which an appropriate name
has not yet been suggested. Civilization, on the other hand, may provide
only a minor epoch to be added in some remote time to the story of
fossils.



                         SUPPLEMENTARY READING


The literature pertaining to fossils is widely scattered and usually too
technical for the layman. It is better to use the resources of the
nearest library than to feel that a specified list of books is
necessary.

Any textbook on geology, zoology, or botany will provide helpful
information. Most books of this type will be found interesting and
readable if used to solve definite problems suggested by the student’s
immediate curiosity. Very few can be read from beginning to end without
a great deal of effort and discouragement.

The following have been prominent among the books consulted by the
author:

_Textbook of Geology_; by Pirsson and Schuchert. This work has undergone
several revisions and currently appears in two volumes: _Physical
Geology_ by Longwell, Knopf, and Flint; _Historical Geology_ by C. O.
Dunbar. Published by John Wiley & Sons. (Historical geology covers the
entire range of prehistoric life—plant, invertebrate, and vertebrate.)

_Historical Geology_ (The Geologic History of North America); by Russell
C. Hussey. Published by McGraw-Hill. Concise, interesting, and
informative.

_Geology and Natural Resources of Colorado_; by R. D. George. Published
by the University of Colorado. Contains an excellent summary of the
historical geology and sedimentary formations of Colorado.

_Vertebrate Paleontology_; by Alfred Sherwood Romer. Published by the
University of Chicago Press. This is one of the most comprehensive and
up-to-date treatments of the subject for students desiring to go beyond
the elementary stage.

_A History of Land Mammals in the Western Hemisphere_; by William
Berryman Scott. Published by The Macmillan Company. This well-known
account of living and extinct mammals is one of the favorites among
students.

_The Age of Mammals_; by Henry Fairfield Osborn. A classic in this field
of literature, but for advanced reading. The book is now out of print.

_The Dinosaur Book_; by Edwin H. Colbert. Published by the American
Museum of Natural History, New York. An illustrated story of amphibian
and reptilian evolution.

_Down to Earth_; by Carey Croneis and William C. Krumbein. Published by
the University of Chicago Press. An excellent popularization of the
earth sciences—geology and paleontology.

_Lexicon of Geologic Names of the United States_; compiled by M. Grace
Wilmarth. Bulletin 896 (in two parts) of the United States Geological
Survey. A rich source of information concerning the age, character, and
distribution of geologic formations, with numerous references to
fossil-bearing beds.

_Bibliography of North American Geology_ (including paleontology);
various bulletins of the United States Geological Survey. Where library
facilities provide access to the technical literature of museums,
universities, and scientific societies, this is a valuable aid in
locating publications dealing with original work in paleontology.
Bulletins 746 and 747 cover the years between 1785 and 1918; Bul. 823
(1918-1928); Bul. 937 (1929-1939); Bul. 938 (1940-1941); Bul. 949
(1942-1943); Bul. 952 (1944-1945); Bul. 958 (1940-1947); Bul. 968
(1948); Bul 977 (1949). Preparation is a continuous process with recent
bulletins appearing at one or two year intervals.

_Ancient Man in North America and Prehistoric Indians of the Southwest_;
by H. M. Wormington. Published by Denver Museum of Natural History, City
Park, Denver 6, Colorado. Both volumes contain authentic and up-to-date
accounts of early American cultures.


MAPS

_Geologic Maps._ United States Geological Survey: map of the United
States (1932); map of Colorado (1935). Geologic maps of a few other
states are available; information regarding these may be obtained from
state universities or state geological surveys.

  Note: Bulletins of the U.S.G.S. are purchasable from the
  Superintendent of Documents, Washington, D. C. Maps are sold by the
  Director of the Geological Survey, Washington, D. C.



                          Transcriber’s Notes


—Silently corrected a few typos

—Restored one accidental omission in the Table of Illustrations

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
  _underscores_.





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