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Title: Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae
Author: Russell, Robert J.
Language: English
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* * * * *
UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Vol. 16, No. 6, pp. 473-579, 9 figures in text
August 5, 1968
Evolution and Classification
of the Pocket Gophers of the
Subfamily Geomyinae
BY
ROBERT J. RUSSELL
UNIVERSITY OF KANSAS
LAWRENCE
1968
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Frank B. Cross, J. Knox Jones, Jr.
Volume 16, No. 6, pp. 473-579, 9 figs.
Published August 5, 1968
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED BY
ROBERT R. (BOB) SANDERS, STATE PRINTER
TOPEKA, KANSAS
1968
[Illustration: Look for the Union label]
31-4628
Evolution and Classification
of the Pocket Gophers of the
Subfamily Geomyinae
BY
ROBERT J. RUSSELL
CONTENTS
PAGE
INTRODUCTION 477
MATERIALS AND ACKNOWLEDGMENTS 477
TAXONOMIC CHARACTERS 478
Prismatic character of molars 478
Character of enamel patterns 479
Grooving of incisors 480
Masseteric ridge and fossa 480
Basitemporal fossa 481
Specializations of skull 481
FOSSIL RECORD 484
Miocene 485
Pliocene 486
Pleistocene 490
Thomomys 492
Zygogeomys 496
Geomys 496
Pappogeomys 503
Orthogeomys 504
HISTORY OF CLASSIFICATIONS 505
CLASSIFICATION 512
Family Geomyidae 512
Subfamily *Entoptychinae 513
Genus *_Pleurolicus_ 514
Genus *_Gregorymys_ 514
Genus *_Grangerimus_ 514
Genus *_Entoptychus_ 514
Subfamily Geomyinae 514
Tribe *Dikkomyini 515
Genus *_Dikkomys_ 516
Genus *_Pliosaccomys_ 517
Tribe Thomomyini 518
Genus _Thomomys_ 518
Subgenus *_Pleisothomomys_ 519
Subgenus _Thomomys_ 520
Tribe Geomyini 521
Genus *_Pliogeomys_ 522
Genus _Zygogeomys_ 523
Genus _Geomys_ 525
Genus _Orthogeomys_ 528
Subgenus _Orthogeomys_ 529
Subgenus _Heterogeomys_ 530
Subgenus _Macrogeomys_ 531
Genus Pappogeomys 532
Subgenus _Pappogeomys_ 534
Subgenus _Cratogeomys_ 535
PHYLOGENY OF THE GEOMYIDAE 536
Primitive Morphotype 537
Entoptychid Radiation 540
Phyletic Trends in Subfamily Geomyinae 542
Plio-Pleistocene Radiation of Geomyini 558
Morphotype 559
Specializations in Genera 560
Zygogeomys 564
Geomys 565
Orthogeomys 568
Pappogeomys 569
LITERATURE CITED 572
INTRODUCTION
When C. Hart Merriam wrote his monograph of the subfamily Geomyinae in
1895, he had no opportunity to examine fossil specimens. No doubt his
phylogenetic conclusions and classification would have been greatly
influenced had he enjoyed that opportunity because study of fossil
geomyids reveals the historic sequence of phyletic development, and
this sequence provides a firm basis for distinguishing specialized
from primitive characters. The history of the Geomyinae has been
characterized by the evolution of specializations. These evolutionary
trends begin, as we presently know them, with a generalized ancestral
stock in the early Miocene. The direction, degree, and rate of change,
beginning with the primitive morphotype of the subfamily, has not been
the same in the various lineages. The classification within the
subfamily is based upon the phyletic interpretations of available data
and the relationships they disclose. In turn, a new, and I hope more
realistic, phylogeny and classification is offered.
MATERIALS AND ACKNOWLEDGMENTS
Recent specimens were studied of all the known genera, subgenera and
29 of the 36 living species. Most of the species not studied are
monotypic and have restricted geographic ranges. They are: _Geomys
colonus_, _G. fontanelus_, and _G. cumberlandius_, _Orthogeomys
cuniculus_ and _O. pygacanthus_ of the subgenus _Orthogeomys_, and
_O. dariensis_ and _O. matagalpae_ of the subgenus _Macrogeomys_.
Examination of these modern species would not radically change the
estimation of the degree of phyletic development of the genera and
subgenera involved. All of the major polytypic and widespread species
were studied.
Specimens of the extinct genera _Dikkomys_, _Pliosaccomys_,
_Pliogeomys_, _Nerterogeomys_, and _Parageomys_ also were studied,
as were examples of the extinct species _Geomys quinni_, _Geomys
tobinensis_, and _Orthogeomys onerosus_. Considerable fossil
material of living species, especially of the genera _Geomys_
and _Pappogeomys_, was used.
Inasmuch as the present account concerns mainly structural changes in
the subfamily Geomyinae at the level of subgenera and above, and the
temporal sequence of those changes, no attempt is made in the present
account to revise taxonomy below the level of subgenera. Considerable
modification of the classification below that level (for species and
subspecies) is to be expected in _Orthogeomys_ and Pleistocene taxa of
_Geomys_ when available specimens are studied.
I thank Prof. Robert W. Wilson for his assistance in securing fossil
geomyids for study, and those in charge of the paleontological
collections at the California Institute of Technology, Prof. Bryan
Patterson, formerly of the Field Museum of Natural History, and Prof.
Claude W. Hibbard of the University of Michigan, Museum of Zoology.
For their kindness in lending Recent species, I thank Mr. Hobart M.
Van Duesen of the American Museum of Natural History, Dr. David H.
Johnson of the U. S. National Museum, and Dr. Oliver P. Pearson of the
California Museum of Vertebrate Zoology, the late Colin C. Sanborn of
the Field Museum of Natural History, and Profs. Emmet T. Hooper and
William H. Burt of the University of Michigan Museum of Zoology.
I am especially grateful to Prof. E. Raymond Hall for his guidance
and helpful criticisms with the manuscript. For assistance with
paleontological problems, I thank Drs. Robert W. Wilson and William
A. Clemens. Several persons have offered helpful suggestions and
encouragement in the course of my study. For assistance of various
sorts I especially thank Drs. J. Knox Jones, Jr., Rollin H. Baker,
A. Byron Leonard, Sydney Anderson, James S. Findley, Robert L.
Packard, and Robert G. Anderson. Advice concerning the drawings of the
dentitions was generously given by Mr. Victor Hogg, and the drawings
were done by Mrs. Lorna Cordonnier under his direction and by Mr.
Thomas H. Swearingen. For assistance with secretarial tasks I thank
Valerie Stallings, Violet Gourd, Ann Machin, Toni Ward, Sheila Miller,
and my wife, Danna Russell.
TAXONOMIC CHARACTERS
Morphological features of the fossils and their stratigraphic
provenience provide the information upon which phylogenetic
interpretations are based. Although the most critical sequences of
the fossil record are lacking, and although the existing fossils are
mostly fragmentary and therefore seldom furnish ideally suitable data
for the interpretations that have been made, phylogenetic conclusions
drawn from fossil materials are superior to those drawn on other
bases. The especially relevant characters are those disclosing
primary trends in the evolution of the modern assemblages. The higher
systematic categories recognized in the following account are based
primarily upon such characters.
The most important characters found are in the teeth, although several
structural changes in the lower jaw, especially those associated with
the insertion of cranial musculature, are almost as important.
_Prismatic Character of Molars_
In primitive geomyines the molar consisted of two columns united at
their mid-points and forming a figure 8 or H-pattern (see Fig. 4B).
Both labial and lingual re-entrant folds were formed between the two
columns. The primitive pattern is retained in the premolars of all
known Geomyinae. Therefore, in the earliest (Miocene) members of the
subfamily, the pattern of the molars was essentially like that of the
premolars.
In Pliocene Geomyinae the two columns of the molars tend to merge into
one. This is evident on the worn occlusal surface of the teeth; the
lateral re-entrant folds are shallow vertically and progressively
recede laterally until only a slight inflection remains. In the final
stages of attrition, the inflection disappears and the tooth is a
simple elliptical column. In the Pleistocene the monoprismatic pattern
appears at earlier stages of wear owing to the decrease in depth of
the re-entrant folds, and in Geomyinae of Recent time the initial
stages of wear on the enamel cap of infants erase the last vestiges of
two columns in the molar teeth.
The general trend in evolution, therefore, has been from a bicolumnar
to a monocolumnar pattern. The particular patterns of wear
characterizing each genus are described in detail beyond.
The third upper molar has evolved less rapidly than the first and
second and in one of the modern lineages (tribe Geomyini) tends to
retain at least a vestige of the primitive bicolumnar pattern in
the final stage of wear. Therefore, the loss of any trace of the
bicolumnar pattern in M3 is considered to be a much specialized
condition. Unfortunately, the fossil record of the third upper molar
is less complete than that for the first molar and second molar; the
tooth drops out of its alveolus more often than does any one of the
other molariform teeth and is seldom recovered.
_Character of Enamel Patterns_
In the primitive genera the enamel pattern is bilophate and the
enamel loop (see Fig. 4B) is continuous on the occlusal surface of
a worn molar. Concomitant with the union of the double columns, the
bilophodont pattern is reduced to a single loph, but the enamel still
completely encircles the dentine.
In the molars of modern geomyines, the enamel loop is not continuous
but is interrupted on the sides of the crown by vertical tracts of
dentine that are exposed at the occlusal surface of the tooth during
early stages of wear. Therefore, a continuous enamel band is to be
found only in a juvenal individual whose teeth have been subjected to
only slight attrition on the enamel cap. In molars lacking enamel on
the labial and lingual sides, anterior and posterior enamel plates, or
blades, are found on each molar. The premolar also has an enamel plate
on the anterior surface and another on the posterior surface, and in
addition both re-entrant angles are protected by a V-shaped investment
of enamel. One or the other of the various plates can be reduced or
lost accounting for the several distinctive tooth-patterns of the
modern geomyines. If loss occurs, it usually is the anterior plate in
the lower dentition and the posterior plate in the upper dentition,
including the upper premolar. When reduction of the posterior plate of
the upper cheek teeth occurs, enamel is first lost from the labial
side of the tooth, thus leaving only a short vestigial plate on the
lingual end of the crown.
_Grooving of Incisors_
The incisors are smooth with no trace of a groove in the ancestral
lineage. In the specialized assemblage (tribe Geomyini) pronounced
grooves are always developed on the anterior face of the upper
incisor. The pattern of grooving is constant in each species and thus
provides characters of taxonomic worth for grouping species into
genera. The only inconstancy noted was an incisor of _Geomys_ from the
Tobin local fauna of the middle Pleistocene which has three grooves
rather than the normal two (No. 6718 KU). The extra groove is an
obvious abnormality, and the tooth was associated with others of the
same species from the same quarry that were normally grooved.
Grooves on the lower incisors are unknown. The functional significance
of grooving has been debated on numerous occasions in the literature.
Grooves appear in a number of only distantly related rodents and in
lagomorphs. The grooving occurs always in small herbivorous mammals,
and in some way may be related to feeding habits.
The grooves provide a serrated cutting edge on the occlusal edge of
the upper incisor. In the genus _Geomys_, for example, the two
incisors, including the slight space between them, present a total of
five serrations, which may facilitate cutting and piercing tuberous
and fibrous roots upon which _Geomys_ feeds. Also the sulci would
perform the same function as the longitudinal groove on the side of a
bayonet, and would aid the animal in extracting its upper incisors
from coarse, fibrous material. In gathering food, the gopher sinks its
upper incisors into a root, and then, with the upper incisors firmly
anchored, slices off small chunks by means of the lower incisors.
Therefore, in pocket gophers, grooving may be an adaptation for
feeding on fibrous or woody material. Finally, grooves increase the
enamel surface of the incisor without additional broadening of the
tooth itself. There could be a selective advantage for sulcation if
the extra enamel and the serrate pattern strengthen the incisors,
which are under heavy stress while penetrating or prying off pieces of
coarse material. Few broken incisors of pocket gophers are found.
_Masseteric Ridge and Fossa_
This ridge and fossa are on the lateral surface of the ramus. The
crest on the ridge begins at the base of the angular process and
terminates slightly anterior to the plane of the lower premolar. The
masseteric fossa receives the insertion of the rostral or superficial
division of the masseter muscle. The mental foramen lies immediately
anterior, or anteroventral, to the fossa.
In the ancestral lineage, the ridge is distinct but relatively low;
the masseteric fossa is shallow and is a poorly developed area for
attachment of the superficial masseter muscle. In modern Geomyinae the
ridge is massive and forms a high crest, especially anteriorly, and
the masseteric fossa is a deep, prominent cup along the dorsal side
of the crest. The elaboration of the crest and fossa evidently is
associated with an increase in size of the superficial masseter
muscle, which enlarges and provides increased power for the propalinal
type of mastication. A high crest has evolved independently in both
modern lineages, Thomomyini and Geomyini.
_Basitemporal Fossa_
The name basitemporal fossa is suggested here to denote the deep pit
that lies between the lingual base of the coronoid process and the
third lower molar. The basitemporal fossa receives the insertion of
the temporal muscle. The fossa, which until now has not been named,
is a unique feature in advanced Geomyinae, being unknown in either
primitive Geomyinae or in other rodents.
The temporal is one of several muscles holding the occlusal surface of
the lower molariform dentition firmly against the upper cheek teeth
during mastication. In primitive geomyines that masticate food
by a planing action, the temporal muscle also moves the mandible
posteriorly and food is ground between the enamel plates when the
lower jaw is retracted as well as when it is moved forward.
The basitemporal fossa appears in late Pliocene geomyines and
increases the attachment surface of the temporal muscles that powers
the planing action important in utilizing woody and fibrous foods. The
basitemporal fossa developed in only one of the modern lineages (tribe
Geomyini), the same lineage in which grooved incisors evolved.
Both features probably are adaptations for feeding on coarse food.
The fossa is not greatly developed in either the ancestral tribe
Dikkomyini or the modern tribe Thomomyini, although in some specimens
a slight depression marks the site of the basitemporal fossa.
[Illustration: FIG. 1. Types of skulls in the subfamily
Geomyinae. × 1.
A. and B. Generalized type of skull. _Geomys bursarius lutescens_,
adult, male, No. 77955 KU, 10 mi. N Springview, Keya
Paha Co., Nebraska.
A. Dorsal view of skull.
B. Ventral view of lower jaw.
C. and D. Dolichocephalic type of skull. _Orthogeomys_ (_Orthogeomys_)
_grandis guerrerensis_, adult, female, No. 39807 KU,
1/2 mi. E La Mira, 300 ft., Michoacán, México.
C. Dorsal view of skull.
D. Ventral view of lower jaw.
E. and F. Platycephalic type of skull. _Pappogeomys_ (_Cratogeomys_)
_gymnurus tellus_, adult, female, No. 33454 KU, 3 mi. W
Tala, 4300 ft., Jalisco, México.
E. Dorsal view of skull.
F. Ventral view of lower jaw.
]
_Specializations of Skull_
The skull in most geomyines is generalized, being neither extremely
long and narrow nor short, broad and flat as in specialized skulls
(see Fig. 1). In Pleistocene lineages of the modern tribe Geomyini,
long skulls and broad skulls evolved and have been termed
dolichocephalic and platycephalic specializations, respectively by
Merriam (1895:88-101). He correlated them with two diametrically
different mechanical methods of mastication.
In animals with dolichocephalic skulls the principal movements of the
mandible in the masticatory process are anteroposterior. The resulting
propalinal action of enamel plates in opposition to each other
characterizes also animals with a generalized skull, and evidently is
the method of mastication in the primitive geomyines, but in animals
with a dolichocephalic skull the method is developed to a high degree
by elongation of the cranium, mandible, and teeth. Both the mandibular
and maxillary tooth-rows are relatively longer than in the generalized
skull, providing a longer block for the planing action of the lower
molariform teeth. All teeth, especially P4 and M3, are longer. In M3
the heel (posterior loph) in particular is elongated. Both the
anterior and posterior enamel plates usually are retained in M1 and
M2.
The superficial (or rostral) masseter muscle, originates on the
side of the rostrum and inserts in the masseteric fossa and on the
masseteric ridge. The deep masseter, especially the zygomatic part
having its origin along the zygomatic arch, inserts on the angular
process of the lower jaw. These two divisions of the masseter muscle
have a longer pull (forward) in the dolichocephalic skull than in a
non-dolichocephalic skull. The temporal and diagastric muscles retract
the lower jaws.
Other, secondary, modifications of the dolichocephalic skull are
shortening of the angular process of the mandible, broadening of the
rostrum, and narrowing of the cranium and zygomata. Depth of the
posterior part of the skull is unchanged. The skull appears to be deep
and of nearly equal breadth from nasals to occiput. A good example of
a dolichocephalic skull is that of _Orthogeomys_ (see Fig. 1, C and
D).
In the platycephalic skull, the principal masticatory movement of the
mandible is anterooblique, to one side and then to the other. The
oblique passage of the enamel blades of the lower teeth across those
of the upper teeth produces a shearing rather than planing action
(Fig. 1E, F). The anterooblique movement of the lower jaw is possible
because of major architectural changes in the cranium and mandible.
These changes include: (1) Broadening of the postrostral part of the
skull, especially the occiput (mastoidal breadth equals or exceeds
zygomatic breadth in skulls of some taxa); (2) flattening of the
skull; (3) anteroposterior compression of the molariform teeth,
especially the molars. Therefore, the entire maxillary tooth-row is
relatively shorter than in the dolichocephalic skull. Only a vestige
of the heel ordinarily remains on M3. The loss of the posterior enamel
blades of P4, M1, and M2 eliminates unnecessary friction, and each of
these teeth is wider than long. The distance between the posterior
ends of the lower jaws is increased approximately in proportion to the
extent that the occiput is widened. As a result of the flattening of
the skull the angular processes of the lower jaws are lateral to the
zygomatic arches, and approximately on the same vertical level with
them. Consequently the insertions of masticatory muscles are shifted
laterally. This is especially true of the zygomatic division of the
deep masseter, which inserts on the angular process. Contraction of
that muscle division of one side of the skull moves the lower jaws
obliquely forward. The diagastric and temporal muscles of course
retract the lower jaws.
The platycephalic skull is the most specialized skull in the Geomyinae
and is a result of the new (for the Geomyinae) method of mastication.
The subgenus _Cratogeomys_ (see Fig. 1, E and F) has a platycephalic
skull. The trend toward platycephalic specialization has been the
major feature of evolution in _Cratogeomys_.
FOSSIL RECORD
The fossil record of the subfamily Geomyinae begins in the early
Miocene of western North America. No geomyids have been recovered from
beds of the late Miocene age. Beginning with the early Pliocene the
fossil record becomes progressively more complete, and geomyines are
relatively abundant in deposits of late Pliocene and Pleistocene age.
Although pocket gophers of the subfamily Geomyinae are rare in
lower Miocene deposits, members of the subfamily Entoptychinae are
relatively common and highly diversified. Four genera and a number of
species have been described (see Wood, 1936:4-25), and the subfamily
ranged widely in western North America. I interpret this to mean that
the geomyines were indeed uncommon in the early Miocene and their
distribution restricted since so few of their remains have been
recovered in comparison with entoptychines and the known records are
only from the northern part of the Great Plains. On the other hand,
entoptychines enjoyed a widespread distribution in western North
America (see discussion beyond). Probably the geographic range of the
geomyines was largely allopatric to that of the more specialized
entoptychines. The zone of fossoral adaptation for herbivorous
rodents is ecologically narrow, and as a result competition is severe.
As a rule, the outcome of episodes of intergroup competition is
geographic exclusion. If these rodents were fossorial in the
early Miocene--their morphology suggests they were at least
semi-fossorial--mutually exclusive patterns of distribution are
to be expected.
Miocene
_Dikkomys_ is the only genus of the Geomyinae known from the early
and middle Miocene. _Dikkomys matthewi_ was described by Wood (1936)
on the basis of isolated teeth from lower Harrison deposits
(Arikareean in age) near Agate, Sioux County, Nebraska. Later,
Galbreath (1948:316-317) described the features of an almost complete
mandible recovered from the younger upper Rosebud deposits, now
considered by MacDonald (1963:149-150) to be middle Miocene, near
Wounded Knee, Shannon County, South Dakota. More recently Black
(1961:13) has described a new species, _Dikkomys woodi_, from the
Deep River Formation, Meagher County, Montana. The Deep River Formation
is late Hemingfordian (middle Miocene) in age. No remains of _Dikkomys_
have been identified in the extensive rodent fauna of the John Day beds
of the lower Miocene of Oregon, although entoptychines are abundant in
these deposits.
In the present account, _Dikkomys_ is regarded as the ancestor from
which the Pliocene and modern geomyines were derived. These probably
did not evolve from the subfamily Entoptychinae because the dentition
of entoptychines, especially the premolars and third molars, was
already highly specialized by Miocene time.
The numerous records of _Thomomys_ and especially _Geomys_ reported
from supposed Miocene or Pliocene deposits are without foundation (see
Matthew, 1899:66; 1909:114, 116, 119; 1910:67, 72; 1923a:369; 1924:66;
Matthew and Cook, 1909:382; Cook and Cook, 1933:49; and Simpson,
1945:80). Most of the records of _Geomys_ date back to the description
of _Geomys bisculcatus_ Marsh (1871:121) from the Loup Fork beds of
Nebraska (near Camp Thomas on the Middle Loup River). At first Marsh
and other investigators thought these beds were of the late Miocene
age. Subsequently the Loup Fork fauna was determined by Matthew
(1923b) to be mostly early Pliocene (Clarendonian), but with a later
Pleistocene element. Recently, Schultz and Stout (1948:560) have shown
that the various Loup River faunas and also those from along the
Niobrara River (Hay Springs, Rushville, Gordon local faunas) are of
middle Pleistocene age, the fossil-bearing beds occurring just below
the Pearlette Ash. These beds are those termed the Loup Fork or North
Prong of Middle Loup by the earlier workers who supposed them to be of
Miocene or Pliocene age. Both _Geomys_ and _Thomomys_ have been
recovered from most of these deposits, but they are no older than
middle Pleistocene. This is not surprising in view of the primitive
structure of the geomyids known from Miocene and Pliocene beds, but
the supposed early appearance of _Geomys_ and _Thomomys_ led to much
confusion concerning geomyid evolution in the late Tertiary.
The dearth of geomyines in the Miocene is counterbalanced by the
relatively abundant and highly differentiated gophers of the subfamily
Entoptychinae. They reached the zenith of their development in this
period. Four genera and a number of species are known from the western
part of the United States, mostly from beds along the Pacific
Coast and in the northern part of the Great Plains. The great
diversification of the group in a relatively short period suggests
prior movement into a new adaptive zone and subsequent specialization
in different subzones and therefore an episode of radial adaptation.
The radiation of the entoptychines is discussed elsewhere in the
account of geomyid phylogeny, but it should be noted here that both
the Geomyinae and the Entoptychinae appear in the fossil record at
about the same time in the early Miocene. The principal distinguishing
features of each of the two lineages were well developed at the time
of their first occurrence, and the entoptychines were the more
successful in early Miocene. The Entoptychinae are known only from
the early and middle Miocene, unless the earlier deposits of the
John Day Formation of Oregon from which mammals have been recovered
are considered to be latest Whitneyian (latest Oligocene); for
correlations, see Wilson (1949:75). Both lineages likely had an
earlier history extending back to their divergence in the Oligocene.
Pliocene
The oldest and most primitive Pliocene geomyine is _Pliosaccomys
dubius_ Wilson (1936:20) from the Smith Valley local fauna of middle
Pliocene (Hemphillian) age in Nevada. According to Wilson (_op.
cit._:15) the beds probably were deposited near the middle of
Hemphillian time. Shotwell (1956:730) recorded _Pliosaccomys dubius_
from the McKay Reservoir and from the Otis Basin (1963:73) local
faunas of the middle Pliocene (Hemphillian) of Oregon, and Green
(1956:155) has recovered remains of _Pliosaccomys_ (cf. _dubius_) from
the Wolf Creek local fauna, uppermost part of the lower Pliocene (late
Clarendonian in age), of Shannon County, South Dakota. Recently, James
(1963:101) has described a second species, _Pliosaccomys wilsoni_, of
this primitive genus. The new species was found in early Pliocene
deposits (late Clarendonian) from the Nettle Spring local fauna
(Apache Canyon), in the Cuyama Valley, Ventura County, California.
_Pliosaccomys wilsoni_ does not differ greatly from _P. dubius_;
however, the few differences in dental characters seem to warrant
specific recognition. The reduction of cusps on the metalophid of p4
from three (_dubius_) to two (_wilsoni_) and the lack of accessory
cuspules on the protolophid of p4 in _wilsoni_ are probably
specializations, suggesting that _P. dubius_ even though the more
recent in age is the less advanced of the two. _P. wilsoni_ is known
only from a lower jaw of a young individual that had dp4 in place,
along with m1 and m2. The permanent premolar was in the process of
erupting, and the deciduous tooth was removed so that the unworn
surface of p4 could be examined.
_Pliosaccomys_ occurred geographically in the area that the
Entoptychinae had occupied in the early Miocene. The Smith Valley
material includes dentitions in almost all stages of wear and the
chronological sequences in the development of the patterns of wear can
be reconstructed. An understanding of the dental patterns of the
primitive geomyines is based mostly on the interpretation of the
stages of wear in _Pliosaccomys_.
No other pocket gopher is known from the area in which _Pliosaccomys_
occurred, and it is unknown after middle Hemphillian age.
_Pliosaccomys_ has closer affinities with _Dikkomys_ of the early
Miocene than with any geomyid of the modern assemblage and gives no
clue to the origin of the lineage culminating in the modern pocket
gophers of the tribe Geomyini.
_Pliogeomys buisi_ Hibbard (1954:353) was found in the Buis Ranch
local fauna, of latest middle Pliocene, on the west side of Buckshot
Arroyo, Beaver County, Oklahoma. The original material included a
right ramus bearing the premolar and first two molars (the holotype)
and five isolated premolars and molars. One of the molars is slightly
worn and from an immature individual. One premolar is a deciduous
tooth. Hibbard (_op. cit._:342) identified the beds from which he
obtained the Buis Ranch local fauna as from the lowermost part of the
Upper Pliocene. Moreover, he judged the Buis Ranch local fauna to be
only slightly older than the Saw Rock Canyon local fauna of Seward
County in southwestern Kansas. Previously (Hibbard, 1953:408-410),
the Saw Rock Canyon local fauna had been assessed as older than
the Rexroad local faunas (latest late Pliocene) and, therefore,
representative of the early part of the late Pliocene. More recently,
Hibbard (1956:164) identified the Buis Ranch beds as part of the
Ogallala Formation, which here occurs unconformably just beneath the
Rexroad Formation (composed of strata nearly all of late Pliocene
age). Therefore, he regarded the Buis Ranch beds as latest middle
Pliocene in age. Hibbard (1954:356) suggested that pocket gopher
remains from the Saw Rock Canyon local fauna were referable to
_Pliogeomys buisi_, and, in effect, tentatively assigned them to
_Pliogeomys_ (in his description of the genus Hibbard remarked that
the upper incisor is bisulcate as in _Geomys_, and the only upper
incisor that he mentions was one of the Saw Rock Canyon fossils
and not part of the Buis Ranch material). _Pliogeomys_ has closer
affinities with modern pocket gophers of the tribe Geomyini than it
does with the middle Pliocene genus _Pliosaccomys_.
The pocket gopher fauna known from the late Pliocene was more varied
than the faunas known from any earlier time. In addition to the
extinct _Pliogeomys_, which occurs in early late Pliocene (see
discussion above), the living genera _Zygogeomys_, _Geomys_,
_Pappogeomys_ (in the sense used on p. 534), and _Thomomys_
first appear in the late Pliocene. The only other living genus,
_Orthogeomys_, makes its first appearance in the late Pleistocene.
The earliest record of the genus _Thomomys_ is based on a fragment of
a left mandibular ramus bearing p4 and m1, _Thomomys gidleyi_ Wilson
(1933b:122), from the Hagerman local fauna of Twin Falls County,
Idaho. Wilson (_loc. cit._) was uncertain as to age (late Pliocene or
early Pleistocene) but subsequently (1937:38 and 67-70) settled on the
middle part of the late Pliocene. Hibbard (1958:11) later considered
the age as early Pleistocene (suggesting that the deposits accumulated
in the Aftonian interglacial interval) but subsequently (Hibbard _et
al._, 1965:512), on the basis of potassium argon age determinations,
also settled on late Pliocene.
Remains of _Nerterogeomys_ [=_Zygogeomys_] have been found in the
Benson local fauna, Cochise County, Arizona, and the Rexroad local
fauna of Kansas. This early Blancan gopher first was described as
_Geomys minor_ by Gidley (1922:123), and was later referred by Gazin
(1942:487) to his new genus _Nerterogeomys_. Hibbard (1950:138)
identified specimens from the Fox Canyon locality, one of the
localities of Meade County, Kansas, where the Rexroad local fauna is
preserved, as _Nerterogeomys_, and tentatively referred them to the
species _N. minor_. _Nerterogeomys_ cf. _minor_ has been recovered
also from Locality 3 of the Rexroad local fauna (Hibbard, 1950:171)
of Meade County, Kansas. Apparently these are also the small gophers
about which Franzen (1947:58) wrote. She assigned them to the genus
_Geomys_, and they may actually be a primitive form of _Geomys_ that
represents an intermediate stage in the development of the enamel
pattern from the uninterrupted loops of the ancestor to the
discontinuous pattern of modern _Geomys_. I favor this interpretation;
the evidence, however, is inconclusive, and I have, therefore,
reluctantly allocated them, along with the other specimens of
_Nerterogeomys_, to the genus _Zygogeomys_. In an early paper,
Hibbard (1938:244) erroneously referred the same specimens, two upper
premolars of a young individual, to the genus _Thomomys_, and the same
material was identified with the genus _Geomys_, also without specific
assignment, in a later paper (Hibbard, 1941b:278). _Thomomys_ is
unknown from the late Pliocene of the Great Plains. The specimens
previously referred to _Nerterogeomys_ are assigned to the genus
_Zygogeomys_ for the first time in this report; for a discussion of
the systematic arrangement see the accounts beyond. The type and
paratype of _Nerterogeomys_ from the Benson local fauna of Arizona
have no indication of enamel reduction.
Specimens of the genus _Geomys_ from the late Pliocene were referred
to the large _Geomys quinni_ McGrew, first by Franzen (1947:55) and
later by Hibbard and Riggs (1949:835) and Hibbard (1950:171). _Geomys
quinni_ has been obtained from the Fox Canyon locality and Locality 3
of the Rexroad local fauna. At Locality 3, both _Zygogeomys_ (cf.
_minor_) and _Geomys quinni_ have been found together, but _Geomys
quinni_ can be distinguished by its much larger size and the advanced
enamel pattern of the cheek teeth (see systematic accounts beyond).
All age classes are represented among the specimens of _Geomys
quinni_; therefore, it seems unlikely that the smaller gophers
referred to _Zygogeomys_ are actually the young of _Geomys quinni_.
Hibbard (personal communication, May, 1966) informed me that specimens
of _Geomys_ from the late Pliocene (Fox Canyon and Rexroad Locality 3)
are erroneously referred to _G. quinni_. According to Hibbard, this
material represents instead two distinct undescribed species,
descriptions of which have been submitted by him for publication.
Allocation of late Pliocene specimens of _Geomys quinni_ to other
species will restrict _quinni_ to the early Pleistocene.
_Cratogeomys bensoni_ Gidley (1922:123) was of medium size. The name
was based on an upper incisor bearing a single median sulcus and an
associated lower jaw containing all of the cheek teeth from the Benson
local fauna, Cochise County, Arizona. Additional lower jaws carrying
various teeth also were recovered. The specimens might just as well
have been assigned to the genus _Pappogeomys_ since the lower
dentitions of all the genera of the tribe Geomyini have the same
enamel pattern, and the subgenera _Pappogeomys_ and _Cratogeomys_
have upper incisors with median grooves. The specimens are too
fragmentary to warrant more than generic identification. Mainly
because of their late Pliocene age and primitive traits the specimens
are here regarded as early representatives of the subgenus
_Pappogeomys_. Discovery of the upper molariform dentition would make
a more precise assignment possible.
Pleistocene
Numerous specimens of geomyids from many localities and horizons are
available from the Pleistocene of North America. Specimens of the
genera _Geomys_ and _Thomomys_ are especially common. Few specimens
are known of the genera _Orthogeomys_ and _Pappogeomys_, especially
from the early and middle Pleistocene, owing, probably, to slight
knowledge of the early Pleistocene of México where these two genera
are thought to have evolved (see map, Figure 2). This lack of
knowledge about early Pleistocene deposits in México is a handicap in
the present instance since the center of differentiation for several
of the modern genera is judged to have been in México, probably on,
and at the edge of, the Central Plateau. The relative abundance of the
remains of _Geomys_ and _Thomomys_ from Pleistocene deposits farther
north, and the marked absence of other genera, may mean that
_Orthogeomys_ and _Pappogeomys_ did not range northward from southern
and central México in most of the Pleistocene. One species of
_Pappogeomys_ eventually ranged into the southwestern United States in
the late Pleistocene (toward the end of the Wisconsin) and it occurs
there today, but the genus is essentially Mexican.
The fossil record of _Zygogeomys_, as the genus is here understood,
evidently continued in the United States will into the Middle
Pleistocene, depending upon the stratigraphic interpretation of the
age of the Curtis Ranch local fauna from southeastern Arizona. Hibbard
(1958:25) regarded the Curtis Ranch local fauna as Irvingtonian in
age, a local fauna that lived either in the late Kansan glacial or the
Yarmouthian interglacial, and his correlation is tentatively followed
here. In deposits laid down later than those of Irvingtonian age no
remains of _Zygogeomys_ have been found. Today a single species exists
as a relic in the mountains of central México and _Zygogeomys_ may
have retreated southward to its present refugium in the late
Pleistocene. Perhaps, _Zygogeomys_ occurred in northern México and the
southwestern United States in the early and middle Pleistocene (see
Fig. 2), occupying the area between the ranges of _Pappogeomys_ to the
south and _Geomys_ to the north. Competition with _Pappogeomys_, and
especially _Geomys_, during Irvingtonian time may have extirpated
_Zygogeomys_ over most of this area, and by late Pleistocene
(Sangamon) much of the former range of _Zygogeomys_ came to be
occupied by one or the other of its competitors. The occurrence of
_Geomys garbanii_ in southern California (see White and Downs, 1961)
and the unidentified species of _Geomys_ in Aguascalientes (Mooser,
1959; for faunal correlation, see Hibbard and Mooser, 1963), both
from deposits of Irvingtonian age, supports this suggestion.
[Illustration: FIG. 2. Probable distribution of the Subfamily
Geomyinae in the early Pleistocene (late Blancan), depicting
major areas of differentiation of the modern genera.
1. _Thomomys_
2. _Geomys_
3. _Zygogeomys_
4. _Pappogeomys_
5. _Orthogeomys_
]
_Thomomys_
The earliest Pleistocene records of _Thomomys_ are mostly isolated
teeth. Although they can be identified as genus _Thomomys_, most of
the materials are too fragmentary to be identified to species. In
_Thomomys_ two distinct patterns of occlusal surfaces of the molars
can be recognized: the generalized elliptical pattern in the subgenus
_Pleisothomomys_, not unlike the pattern in other geomyids, and the
pear-shaped pattern in the subgenus _Thomomys_, which results from
constriction of the upper molars on the labial side and constriction
of the lower molars on the lingual side. Some fossils assigned to
_Thomomys_ were not examined with this distinction in mind by
the persons who made the assignments. Consequently some of the
identifications now in the literature may be subject to change.
Three occurrences of _Thomomys_ are from the early and middle
Pleistocene, with a possible fourth (depending upon the age of the Hay
Springs local fauna of Nebraska). The earliest Pleistocene record is
from the Broadwater-Lisco beds along the North Platte River in Morrill
County, western Nebraska. Possibly the specimen from there was
misidentified. Those beds are Lower Pleistocene, and are regarded by
Schultz and Stout (1948:560-561, 573) and by Hibbard (1958:11), as
having been deposited mostly during the Aftonian interglacial. There
is also some indication that some of the strata were deposited late
in the Nebraskan glaciation. There are no other early Pleistocene
records of _Thomomys_. Savage (1951:228) reported the genus from
the Irvington local fauna, Alameda County, California. The specimens
were not identified to species, although they were described as
indistinguishable from _Thomomys bottae_. Paulson (1961:137) recorded
specimens from the Cudahy local fauna, Meade County, Kansas. These
fragmentary specimens are referable to the subgenus _Thomomys_, owing
to the strong constriction of the molars, but have not been identified
to species. The Cudahy is an Irvingtonian local fauna, and is
considered to have been deposited during the late Kansan glaciation.
The stratum containing the Cudahy local fauna immediately underlies
the Pearlette Ash. The Cudahy material includes five isolated molars
and a fragmentary ramus bearing only the premolar. The genus
_Thomomys_ has been recovered also from the Hay Springs local fauna
in Sheridan County, northwestern Nebraska, by Shultz and Tanner
(1957:71). The Hay Springs local fauna is considered to have been
deposited in late Kansan glaciation or in early Yarmouth interglacial
by Shultz and Tanner (_op. cit._:69), or of Irvingtonian age; however,
Hibbard (1958:25) regarded the beds containing this fauna as Illinoian
(thus post-Irvingtonian in age), and equivalent in age to the Berends
local fauna of Oklahoma and the Butler Springs and Mt. Scott local
faunas of Kansas. The _Thomomys_ from Hay Springs local fauna has not
been referred to species.
The relative abundance of _Geomys_, and rarity of _Thomomys_, in Great
Plains fossil beds of early and middle Pleistocene is probably due to
allopatric distributions of the two genera. The Great Plains area was
evidently the center of distribution and differentiation of _Geomys_.
Perhaps _Thomomys_ evolved earlier to the west, in the Great Basin and
Pacific Coastal regions, and not on the Great Plains.
Upper Pleistocene records of _Thomomys_ are more common. The genus
was widespread in beds identified with the Illinoian and Sangamon
and extended its range eastward to the Atlantic Coast. Stephens
(1960:1961) reported _Thomomys_ from the Doby Springs local fauna,
Harper County, northwestern Oklahoma. The material (34 isolated teeth)
was too fragmentary to permit assignment to species. The molars are
constricted on one side, indicative of the subgenus _Thomomys_, like
the Cudahy specimens reported by Paulson (see discussion above).
Stephens erroneously mentioned that the enamel plate on the posterior
face of the upper premolar is unique in _Thomomys_; this plate occurs
also in _Zygogeomys_. The Doby Springs local fauna was recovered from
beds that have been identified as Illinoian deposits, and it is
correlated with the Berends local fauna in Beaver County, Oklahoma,
and the Butler Springs local fauna in Meade County, Kansas (see
Stephens, _op. cit._: 1700).
Local faunas in Maryland and Florida of Rancholabrean age
include _Thomomys_, in every instance referable to the subgenus
_Pleisothomomys_ on the basis of unconstricted molars. _Thomomys
potomacensis_ (Gidley and Gazin, 1933), from Cumberland Cave local
fauna, Allegany County in western Maryland, is the type of the genus
_Pleisothomomys_ Gidley and Gazin (1933:354). _Pleisothomomys_ is here
regarded as a subgenus. The material used in the original description
included four lower jaws, one with a complete dentition. Hibbard
(1958:25) pointed out that the Cumberland Cave assemblage is a
composite fauna including both glacial and interglacial forms. He
placed the stratigraphic position of the fauna as definitely Upper
Pleistocene, probably deposited in both Illinoian glaciation and
during the Sangamon interglacial. _T. potomacensis_ is significantly
larger than _T. orientalis_ Simpson (1928:6), from the Saber-tooth
Cave local fauna, Citrus County, Florida. Simpson's material included
a rostral fragment with an incisor, premolar, and first molar. The
Saber-tooth Cave local fauna is regarded by Kurten (1965:219) as
having been recovered from Sangamon deposits. _Thomomys_ is unknown
from Wisconsin deposits in the eastern United States, and today the
genus does not occur east of the Great Plains.
_Thomomys_ of Rancholabrean provincial age from the western United
States and México is known only from Wisconsin beds.
Three extinct species of _Thomomys_, all referable to the subgenus
_Thomomys_, have been described. _Thomomys microdon_ Sinclair
(1905:146), based on the rostral portion of a skull without a
mandible, is from the Potter Creek Cave local fauna, Shasta County,
California, and has been recovered also from Samwel Cave, Shasta
County, California. _T. microdon_ closely resembles _Thomomys
monticola_ that lives in the area today. _Thomomys scudderi_ Hay
(1921:614) is from the Fossil Lake (or Christmas Lake) local fauna in
central Oregon. Elftman (1931:10-11) referred these specimens to
_Thomomys townsendii_, and he considered _T. scudderi_ to be a synonym
of _T. townsendii_. Davis (1937:156-158) disagreed with Elftman
concerning the taxonomic status of _T. scudderi_, which he regarded as
a valid species. According to Davis, _T. scudderi_ is more closely
allied to _Thomomys bottae_ than to _T. townsendii_. Cope (1878:389;
1889:160-165) had referred the same specimens to _Thomomys clusius_
(now _Thomomys talpoides clusius_). Cope considered the beds to be
Pliocene in age. In all accounts of the Fossil Lake local fauna up to
Hay (1921), the specimens of _Thomomys_ were referred to the species
_clusius_, _talpoides_, or _bulbivorus_ (see Elftman, _loc. cit._).
The Fossil Lake local fauna is currently considered as being of
Rancholabrean provincial age, probably dating from the Wisconsin
glacial maximum when the lake reached its greatest size. The third
extinct species described from the Wisconsin is _Thomomys vetus_ Davis
(1937:156), also from the Fossil Lake local fauna in Lake County,
Oregon. Davis pointed out that _T. vetus_ differs from _T. scudderi_
Hay, of the same fauna, in larger size and other cranial details, and
that it is closely allied to the living species _Thomomys townsendii_,
and not to _Thomomys talpoides_, which is the only species of
_Thomomys_ living in the area today.
_Thomomys townsendii_ was recovered by Gazin (1935:299) from the
American Falls beds (probably Wisconsin deposits) in Idaho.
_Thomomys talpoides_ is reported from the Howard Ranch local fauna in
Hardeman County, western Texas, by Dalquest (1965:69-70), who referred
the isolated teeth to _T. talpoides_ on geographic grounds, apparently
on the erroneous assumption that _T. talpoides_ was the species of
_Thomomys_ nearest geographically to Hardeman County. Hay (1927:259)
reported _Thomomys fuscus_ [= _Thomomys talpoides_] from late
Pleistocene beds near Wenatchee, Chelan County, Washington. Hibbard
(1951:229) recorded _Thomomys talpoides_ from late Pleistocene
deposits in Greeley County, Kansas, and Walters (1957:540) reported
the same species from late Pleistocene deposits in Clark County,
Kansas. According to Hibbard (1958:14) other remains reported as _T.
talpoides_ have been recovered from numerous areas of Wisconsin
glacial drift in western North America.
_Thomomys bottae_ has been identified from Wisconsin age deposits in
western North America, as follows: Burnet Cave, Gaudalupe Mt., New
Mexico (Schultz and Howard, 1935:280); Carpinteria Asphalt, California
(Wilson, 1933a:70); McKittrick Asphalt, Kern County, California (J. R.
Schultz, 1938:206); Rancho La Brea, Los Angeles County, California
(Dice, 1925:125--specimens described as a new subspecies, _T. b.
occipitalis_); Papago Springs Cave, Santa Cruz County, Arizona
(Skinner, 1942:150 and 158--probably _bottae_, but possibly _umbrinus_
on the assumption that the two are specifically instead of
subspecifically distinct); Isleta Cave, Bernalillo County, New Mexico
(Harris and Findley, 1964:115--some of these fossils may be
post-Wisconsin in age); Potter Creek Cave and Samwel Cave, Shasta
County, California (Sinclair, 1905:146--identified as _T. leucodon_,
now a subspecies of _T. bottae_; also see Hay, 1927:214-215).
_Thomomys umbrinus_ has been reported from San Josecito Cave, Nuevo
León, México (Russell, 1960:542); Upper Bercerra, México (Hibbard,
1955a:51--identified only as _Thomomys_ sp., but undoubtedly referable
to _T. umbrinus_). Post-Wisconsin remains of _Thomomys umbrinus_ are
reported by Alvarez (1964:6) from capa II and capa III of the Cueva La
Nopalera, southwestern Hidalgo. Hay (1927:222-223) reported specimens
of the genus _Thomomys_ from Wisconsin deposits in Hawver Cave,
Eldorado County, California, but did not assign them to species.
Gilmore (1947:158) found the remains of _Thomomys umbrinus_ in cave
deposits near Quatro Ciénegas in central Coahuila. These cave deposits
may have been laid down during the Wisconsin, but more likely
accumulated in the post-Wisconsin.
_Zygogeomys_
Remains found in the Curtis Ranch local fauna, Cochise County, in
southeastern Arizona are regarded as of middle Pleistocene age. See
Gazin (1942:481-484), Wilson (1937:39-40), Hibbard (1958:25), and
Hibbard _et al._ (1965:510-511). Although some question as to the
exact age of the Curtis Ranch local fauna still seems to exist, most
authorities on the Pleistocene agree that the age is not Pliocene and
that it is older than Rancholabrean. Gidley (1922:122) described the
pocket gopher found in the Curtis Ranch beds as _Geomys parvidens_,
which is preoccupied by _Geomys parvidens_ Brown (1908:194), a name
proposed for the pocket gopher from the Conard Fissure of Arkansas;
therefore, Hay (1927:136) proposed the name _Geomys persimilis_ for
the Curtis Ranch species to replace _Geomys parvidens_ Gidley.
_Geomys persimilis_ Hay became the type species of Gazin's genus
_Nerterogeomys_ (1942:507). In this paper, _Nerterogeomys_ is
considered to be a junior synonym of _Zygogeomys_.
_Zygogeomys persimilis_ is represented by a rostral fragment bearing
all the cheek teeth on the left side and the upper incisors. In
addition, two lower jaws, one with the first three cheek teeth,
are referred to the species (see Gazin, 1942:507). The fossils
identified as _Geomys_ from the Arroyo San Francisco, Cedazo fauna,
in Aguascalientes, México, by Mooser (1959:413) may be referable
instead to _Zygogeomys_. I have not seen the specimens and no figures
are available; Mooser states that a cranium was recovered. If either
the upper premolar or third molar is in place, generic identification
could be made with reasonable certainty. No other fossils of
_Zygogeomys_ have been uncovered in late Pleistocene deposits and the
significance of the absence of _Zygogeomys_ has been discussed in an
earlier paragraph of this section. _Geomys_ has not been found so far
south as Aguascalientes, but _Zygogeomys_ occurs farther south now and
presumably had a more extensive range on the plateau to the north in
the Pleistocene.
_Geomys_
_Geomys_ is common in Pleistocene deposits, especially on the Great
Plains. Certainly the center of differentiation for _Geomys_ was
in this region, although at times, probably when conditions were
favorable, _Geomys_ expanded its range into adjacent areas, reaching
the Pacific Coast in Irvingtonian times and the Atlantic Coast at the
time of the Illinoian glaciation. The earliest Pleistocene records
of the genus are from the Great Plains. McGrew (1944:49) described
_Geomys quinni_ from the Sand Draw local fauna, Brown County,
Nebraska, considered by Hibbard (1958:11) to be Nebraskan in age. As
mentioned in the account of Pliocene geomyids, _Geomys quinni_ occurs
also in the late Pliocene deposits of southwestern Kansas. Also,
_Geomys quinni_ occurs in the Broadwater-Lisco local fauna of Morrill
and Garden counties, western Nebraska (Barbour and Schultz, 1937:3;
Schultz and Stout, 1948:560-563; Schultz _et al._, 1951: table 1). The
Broadwater-Lisco is currently regarded as Aftonian deposits (Schultz
and Stout, _loc. cit._; Hibbard, 1958:11). Hibbard (1956:174)
identified _Geomys quinni_ from the Deer Park local fauna, probably
deposited during the early Aftonian interglacial, of Meade County,
Kansas. Strain (1966:36) described _Geomys paenebursarius_ on the
basis of fossils obtained from early Pleistocene deposits of the
Hudspeth local fauna from western Hudspeth County in the Trans-Pecos
of Texas. The Hudspeth fossils were probably deposited during the
Aftonian interglacial. From Kingman County, Kansas, Hibbard (_op.
cit._: 164) recovered isolated teeth of _Geomys_ from the Dixon local
fauna, regarded by him (_op. cit._:153-154) as deposited during the
latest Nebraskan glaciation, and correlated by him with the Sand Draw
local fauna of Nebraska. Hibbard (1958:11) later regarded the Dixon as
a transitional fauna between Nebraskan and Aftonian. The remains of
_Geomys_ from the Dixon are known only from isolated teeth. The teeth
are small, and suggest that a smaller species of _Geomys_ may have
occurred along with the more common and larger _G. quinni_ during the
early Pleistocene (see discussion beyond of the Saunders _Geomys_).
_Geomys quinni_ was widespread and common throughout the central Great
Plains from the late Pliocene (Rexroad fauna) through the early
Pleistocene (Nebraskan and Aftonian deposits).
Hibbard (1956:179) referred the pocket gopher remains taken from the
Saunders local fauna in Meade County, Kansas, to _Geomys tobinensis_,
a small species having continuous enamel bands around the lower
premolar in younger specimens. The Saunders local fauna was deposited
in the late Aftonian and is younger than the Deer Park local fauna
discussed above. Paulson (1961:138) later pointed out that the
Saunders _Geomys_ is distinct from _Geomys tobinensis_; hence, the
small pocket gopher from the Saunders local fauna is probably an
unnamed species, perhaps more closely allied to _paenebursarius_
than to _quinni_. The small _Geomys_ reported from the Aftonian
Broadwater-Lisco local fauna of Nebraska (Schultz and Stout, 1948:563)
may also be the same as the Saunders pocket gopher, but the smaller
adult specimens occurring in the same bed with larger specimens
probably are females and the larger specimens males. In all living
Geomyini females have smaller skulls than males.
The Irvingtonian provincial age is currently regarded as Middle
Pleistocene and includes the late Kansan glaciation (that part
occurring after the glacial maximum) and the Yarmouthian interglacial
(see Hibbard _et al._, 1965:512-514). The Irvingtonian provincial
age, therefore, follows the late Blancan provincial age of the early
Pleistocene and is succeeded by the Rancholabrean provincial age of
the late Pleistocene. No specimen of an Irvingtonian _Geomys_ is
referable to any living species. Two Irvingtonian species have been
described. Hibbard (1944:735) named _Parageomys tobinensis_ [= _Geomys
tobinensis_] from the Tobin local fauna of Russell County, Kansas.
This species since has been reported from the Cudahy local fauna of
Meade County, Kansas (Paulson, 1961:137). Hibbard (1956:183) also
identified as _Geomys tobinensis_ the pocket gopher recovered from the
Saunders local fauna, a late Aftonian deposit of Meade County, Kansas,
and reduced the technical name _Parageomys_ from generic to subgeneric
rank. Paulson (_op. cit._:138) pointed out that the Saunders specimens
differ from _G. tobinensis_, and he, therefore, restricted the name
to the small _Geomys_ of the Cudahy and Tobin local faunas of
Irvingtonian provincial age. _G. tobinensis_ is markedly smaller than
the Blancan _G. quinni_. The Cudahy and Tobin local faunas are of
approximately the same age, and presently both are included in one
unit, the Cudahy fauna. The Cudahy fauna is considered to have been
deposited in late Kansan as it occurs in strata immediately below the
Pearlette ash.
Recently, White and Downs (1961:8) described a new Irvingtonian
species, _Geomys garbanii_, from the middle Pleistocene Vallecito
Creek local fauna of San Diego County, California. Many well preserved
fossils of the new species were recovered. _Geomys garbanii_ is of
medium size (approximately the size of one of the larger subspecies of
_G. bursarius_), and significantly larger than the Irvingtonian
_Geomys tobinensis_ of the Great Plains. The Vallecito Creek
occurrence of _Geomys_ is the first authenticated record from the
Pacific Coast region. Matthew (1902:320) erroneously referred remains
of _Thomomys_ to the genus _Geomys_ in his revised list of Cope's
earlier report on the Fossil Lake (or Silver Lake) fauna (see
discussion of _Thomomys_ above).
A number of Irvingtonian fossil remains of _Geomys_ have not been
identified with particular species. Hibbard (1941a:206) found _Geomys_
in the Borchers local fauna (deposited in the time of the Yarmouthian
interglacial) of Meade County, Kansas. Also, _Geomys_ has been
reported from several sites in Nebraska. Schultz and Tanner (1957:67)
reported _Geomys_ from the Angus fossil quarry in Nuckolls County,
south-central Nebraska. The Angus fossils were found in sediments
of the Sappa Formation considered by Schultz and Tanner to be a
Yarmouthian deposit. Fossil quarries (Hay Springs, Rushville, and
Gordon) along the south side of the Niobrara River Valley in Sheridan
County, Nebraska, have also provided records of geomyids. Both a large
and small species of _Geomys_ have been reported from the more
recently excavated Rushville and Gordon sites (Schultz and Stout,
1948:562-567, and table 3). In view of the great disparity in size
owing to sex, these may actually be males and females of the same
species, as mentioned above. The name Hay Springs has been used in
reference to all three sites. The ages of the Hay Springs sites are
approximately the same, but their correlation is presently under
debate. Schultz and Tanner (1957:68-71) maintain that the fossils are
distinctly middle Pleistocene, and that they were deposited during
late Kansan glaciation, or perhaps from early Yarmouthian into early
Illinoian, with the largest concentration coming from the Sappa sands
of pre-Illinoian (Yarmouth) age. Hibbard (1958:25), basing his opinion
on the presence of _Microtus pennsylvanicus_, and the stage of
evolution of other species in the assemblage, regards the Hay Springs
sites as probably Illinoian deposits, but certainly no older than
that.
Mooser (1959:413) identified as _Geomys_ the pocket gopher from
Irvingtonian deposits in Arroyo San Francisco (loc. no. 5) near the
city of Aguascalientes, México. As suggested elsewhere in this
account, these fossils may be referable to _Zygogeomys_ rather
than _Geomys_. The Irvingtonian provincial age of this fauna was
established by Hibbard and Mooser (1963:245-250). Other alleged
occurrences have recently been compiled by Alvarez (1965:19-20).
Maldonado-Koerdell (1948:20) noted four fossil occurrences of the
genus _Geomys_ in México. Two of these from San Josecito Cave in
Nuevo León have since been identified with the genera _Orthogeomys_
and _Pappogeomys_ (Russell, 1960:543-548); the third listed by
Maldonado-Koerdell from "near Ameca, Jalisco," was based on Brown's
(1912:167) mention of some bones supposedly of the family "Geomyidae,"
and the fourth refers to pocket gopher remains from the "Hochtals von
Mexiko" listed as _Geomys_ by Freudenberg (1921:139). His generic
identification is doubtful and the specimens should be compared with
Mexican genera of the Geomyinae.
Upper Pleistocene records of _Geomys_ also are common. Upper
Pleistocene is here understood to include late Illinoian,
Sangamon and Wisconsin deposits; all are considered to be of
Rancholabrean provincial age (see Hibbard _et al._, 1965:512-515) and
post-Irvingtonian. The presence of remains of _Bison_ and/or _Microtus
pennsylvanicus_ are currently considered mammalian index fossils of
Rancholabrean faunas. In the Illinoian, _Geomys_ extended its range to
the Atlantic Coast in the southeastern United States. The eastern and
western species-groups evidently were isolated throughout much of the
late Pleistocene, and, therefore, evolved separately. Of the two, the
eastern, or _pinetis_, species-group seems to have remained somewhat
more generalized, and the western, or _bursarius_, species-group has
become more specialized. The Rancholabrean _Geomys_ from deposits in
the southeastern United States are referable (see Ray, 1963:325) to
_Geomys pinetis_.
Marsh (1871:121) described _Geomys bisulcatus_ from the North Prong of
the Loup River (near Camp Thomas), Nebraska. These beds are also
termed the Loup Fork or Loup River fossil beds (see discussion on p.
485), and they lie along the upper reaches of the Middle Loup River
in Thomas County (near Senea), Hooker County (near Mullen), and
southeastern Cherry County (probably the North Prong beds northwest
of Mullen). These beds were at first thought to be of Miocene age,
but later were regarded as early Pliocene (see Schultz and Stout,
1948:562-566 for a historical account of expeditions to these fossil
sites). Schultz and Tanner (1957:71-72) pointed out that the principal
fossiliferous beds in the Middle Loup region are of middle to late
Pleistocene age, with most of the fossils coming from the Crete sand
and silt beds which are probably early Illinoian deposits, and,
therefore, younger than the Hay Springs faunas. Some fossils may have
come from the Sappa deposits dated by Schultz and Tanner (_loc. cit._)
as mostly Yarmouthian deposits. _Geomys bisulcatus_, judging from
the original description and Hibbard's discussion of the cotypes
(1954:357), does not differ significantly from _Geomys bursarius_.
However, _Geomys bisulcatus_ is tentatively retained as a valid
species. Based on the evidence cited above it seems unlikely that
_Geomys bisulcatus_ occurred in pre-Irvingtonian times as often
suggested in the literature.
The genus _Geomys_ has been identified in several faunas of Illinoian
age, all from the Great Plains. Stephens (1960:1961) reported the
genus from the Doby Springs local fauna in Harper County, Oklahoma,
and Starrett (1956:1188) reported it from the Berends local fauna in
Beaver County, Oklahoma. Schultz (1965:249) assigned 21 isolated
teeth, including six incisors, from Butler Springs local fauna
(considered by him to be late Illinoian, following the glacial
maximum) to _Geomys_ cf. _bursarius_. Hibbard and Taylor (1960:167)
reported a baculum tentatively identified as that of _Geomys_ from the
early Illinoian Butler Springs local fauna (including the Adams fauna)
of Meade County, Kansas. Hibbard (1963:206) recorded the genus
_Geomys_ from the Mt. Scott local fauna (late Illinoian deposits) of
Meade County, Kansas; the specimens probably are referable to the
living species _bursarius_. From McPherson County, Kansas, Hibbard
(1952:7) reported the genus _Geomys_ from the Kentuck Assemblage,
which he (1958:25) regarded as a composite of Illinoian and Sangamon
species. Specific identification of the Illinoian pocket gophers is
uncertain, primarily due to the fragmentary nature of the material. On
the basis of dental characters alone most specimens could be referred
to _G. bursarius_; however the taxonomic status of _G. bisulcatus_
is in doubt, and more complete material may indicate that the
Illinoian gophers are specifically distinct from the living species.
Consequently, most authors, including myself, have made no attempt to
refer these specimens to species. Nevertheless, the Illinoian _Geomys_
from the Great Plains is more closely allied to the living species of
_Geomys_ than it is to the earlier Irvingtonian species.
_Geomys bursarius_ has been collected from a number of Sangamon fossil
sites on the Great Plains. Although specific identification of
specimens of _Geomys_ from Illinoian faunas is uncertain, the Great
Plains _Geomys_ from Sangamon and later deposits probably is referable
to the living species as Hibbard and Taylor (1960:165) pointed out.
They found no difference between _Geomys_ recovered from the Cragin
Quarry local fauna (early Sangamon) of Meade County, Kansas, and the
living species _Geomys bursarius_. Isolated teeth of the same species
were collected from the Jinglebob local fauna of Meade County, Kansas
(Hibbard, 1955b:206), a fauna of the late Sangamon. Hibbard (1943:240)
also recorded the genus _Geomys_ (referable to _G. bursarius_) from
the Rezabek local fauna of Lincoln County, Kansas. According to
Schultz _et al._ (1951:6 and table 1) the genus _Geomys_ occurs
in buried or "fossil" soils of Sangamon age, lying just above the
Loveland Loess, in Nebraska. No specific localities were given by
them, nor were any particular specimens mentioned. Dalquest reported
_Geomys bursarius_ from two Sangamon faunas in northern Texas. The
species is represented in the Ward Quarry local fauna of Cooke County,
Texas (1962a:42), and the Good Creek local fauna of Foard County,
Texas (1962b:575).
_Geomys bursarius_ has been reported from Wisconsin fossil deposits
of the Great Plains and adjacent areas as follows: Jones local fauna,
Meade County, Kansas (Hibbard and Taylor, 1960:64-66); Two Creeks
Forest beds of the third interstadial soils formed between Cary and
Mankato glaciations, late Wisconsin (Schultz _et al._, 1951:8 and
table 1); Cita Canyon local fauna in the northern part of the
Panhandle of Texas (Johnson and Savage, 1955:39); Howard Ranch local
fauna of Hardeman County in northwestern Texas (Dalquest, 1965:70);
Quitaque local fauna of Motley County, Texas (Dalquest, 1964:501);
Clear Creek local fauna of Denton County in north-central Texas
(Slaughter and Ritchie, 1963:120); Ben Franklin local fauna, of late
Wisconsin beds along the North Sulphur River in Delta County, NE Texas
(Slaughter and Hoover, 1963:137); Bulverde Cave (Hay, 1920:140;
1924:247) and Friesenhahn Cave (Tamsitt, 1957:321), both in Bexar
County, south-central Texas; Alton, Illinois (Hay, 1923:338-339);
Wisconsin drift of Illinois, without mention of specific locality
(Bader and Techter, 1959:172); Wisconsin drift of southwestern
Wisconsin and northeastern Iowa (Hay, _op. cit._:343); Wisconsin drift
near Galena, Illinois, and mouth of Platte River in eastern Nebraska
(Leidy, 1869:406).
Brown (1908:194) described _Geomys parvidens_ from the Conard Fissure,
in northern Arkansas. Hibbard (1958:25) concluded that the Conard
Fissure fauna represents a glacial stage, probably the Illinoian, and
Hibbard _et al._ (1965:510-511) regarded the fauna as a composite
including both Irvingtonian and Rancholabrean elements. White and
Downs (1961:21) considered _G. parvidens_ to be a subspecies of
_Geomys bursarius_.
The first Pleistocene occurrence of _Geomys_ in the southeastern
United States is from the Reddick I deposits reported by Gut and Ray
(1963:325), who found the remains of _Geomys pinetis_ among the
fossils comprising the "rodent beds" of Marion County, Florida. Gut
and Ray tentatively identified the beds as Illinoian, but Kurten
(1965:219) regarded the Reddick I fauna as early Sangamon. Simpson
(1928:2) reported _Geomys floridanus_ [= _pinetis_] from Saber-tooth
Cave deposits of Citrus County, Florida. The Saber-tooth Cave (or
Lecanto Cave) local fauna is considered by Kurten (_op. cit._:219)
also to be a Sangamon deposit. _Geomys floridanus_ [= _pinetis_] was
reported from the Seminole Field deposits by Simpson (1929:563); both
Simpson and Kurten (_op. cit._:221) agreed that the Seminole Field
fauna is mainly late Wisconsin, although sub-Recent fossils occur at
the tops of the beds. Ray (1958:430) collected remains of _Geomys
pinetis_ from the Melbourne Bone Bed of Brevard County, Florida. The
Melbourne local fauna is considered to be from Wisconsin deposits by
Kurten (_op. cit._:220). The eastern species of _Geomys_ were probably
derived from Great Plains stock that reached the southeastern Coastal
Plains in early Rancholabrean (Illinoian) time. Presently there is no
contact between the eastern and western populations of the genus, and
it is assumed that disjunction occurred as a result of Wisconsin
glaciation. It is interesting to note that the genus _Thomomys_
occurred in this region at approximately the same time; both genera
occur in Saber-tooth Cave deposits.
_Pappogeomys_
The genus _Pappogeomys_ is not known from Pleistocene deposits older
than the Wisconsin glaciation, but a pre-Pleistocene occurrence in the
Benson beds of Arizona (see discussion of the Pliocene above) shows
that _Pappogeomys_ had been differentiated by late Pliocene time.
The absence of _Pappogeomys_, beginning in the early Pleistocene and
continuing well into the late Pleistocene, is attributed to the
southern distribution of the genus, where its range probably was
centered on the Central Plateau of México. The paucity of early and
middle Pleistocene deposits from this critical region prevents any
definite statements about phyletic development within the genus. All
of the late Pleistocene records pertain to the subgenus _Cratogeomys_
(long in use as a generic name but in the present paper reduced to
subgeneric rank in the genus _Pappogeomys_). Schultz and Howard
(1935:280) found _Cratogeomys_ [= _Pappogeomys_] _castanops_ in
Burnett Cave in the Guadalupe Mountains of south-central New Mexico.
The Burnett deposits are probably late Wisconsin (see Schultz and
Tanner, 1957:75, for discussion of the age of these deposits based
on carbon-14 tests). These writers (_loc. cit._) also referred
the mandible of a small pocket gopher to the genus _Pappogeomys_
[= subgenus _Pappogeomys_]. However, neither genera nor subgenera of
the tribe Geomyini can be distinguished on the basis of their inferior
dentitions. Judging from the distribution of the modern geomyines, it
seems unlikely that the subgenus _Pappogeomys_ has occurred beyond its
present range in the late Pleistocene; therefore the small mandible is
most likely that of a young individual of _Pappogeomys castanops_.
Russell (1960:543) referred specimens collected at San Josecito Cave
in Nuevo León, México, to the group of small subspecies _Cratogeomys_
[= _Pappogeomys_] _castanops_. Also, Russell (_loc. cit._) identified
a rostral fragment as of the genus _Cratogeomys_ [= subgenus
_Cratogeomys_] although the fragment had a combination of features
different than in any named species of the genus; he did not name the
fragment as a new species, preferring to wait for additional material
that could clarify its taxonomic relationships.
Hibbard (1955a:52-53) identified _Cratogeomys_ [= _Pappogeomys_]
_tylorhinus_ from the Becerra Superior deposits in the valley of
Tequixquic in the northern part of the state of México. The Wisconsin
age of these beds suggests an earlier Pleistocene derivation of the
_gymnurus_-group of species.
Several specimens of the subgenus _Cratogeomys_ have been reported
from beds of latest Wisconsin (certainly after the glacial maximum)
or post-Wisconsin age. Gilmore (1947:158) found fossil remains of
_Cratogeomys_ [= _Pappogeomys_] _castanops_ commonly in Quaternary
cave deposits on the mountain slopes in the vicinity of Cuatro
Ciénegas, in central Coahuila. These deposits actually may be of
post-Wisconsin origin (see discussion above). Alvarez (1964:8)
obtained fragments of _Cratogeomys_ [= _Pappogeomys_] _tylorhinus_
from sub-Recent deposits of Capa III in the Cueva La Nopalera in
southwestern Hidalgo, México. _Pappogeomys merriami_ lives in the
area today. Mayer-Oakes (1959:373) reported remains of _Cratogeomys_
[= _Pappogeomys_] _merriami_ from levels eight and eleven of the
excavations at El Risco II, in the northern part of Mexico City. The
ages of these deposits are unknown to me, but they probably are no
older than late Wisconsin with most of the beds dating from the
post-Wisconsin.
_Orthogeomys_
This genus is not known from the Pleistocene, except for its
occurrence in the San Josecito cave deposits of southwestern Nuevo
León, México (Russell, 1960:544). Although _Orthogeomys_ does not
occur in the immediate vicinity of the cave at the present time, the
northern limits of its range is nearby in southern Tamaulipas. The
_Orthogeomys_ from San Josecito Cave differs from living species,
and has been named _Heterogeomys_ [= _Orthogeomys_] _onerosus_
Russell (_loc. cit._), and is evidently referable to the subgenus
_Heterogeomys_. As mentioned before, the San Josecito Cave local
fauna represents deposits of Wisconsin glaciation.
HISTORY OF CLASSIFICATION
The account of the Tucan or Indian mole by Hernandez (sometimes listed
as Fernandez) in 1651 probably is the earliest published one of a
geomyid (see Merriam, 1895:201; Coues, 1877:607-608). Linnaeus in 1758
did not mention geomyids. In 1772, Kerr described Hernandez's Tucan
under the name _Sorex mexicana_ on the basis of Hernandez's account
without having seen any specimens. Lichtenstein in 1827 applied the
technical name _Ascomys mexicana_ to three specimens collected by
Deppe from unknown localities on the tableland of México. Merriam
(_loc. cit._) pointed out that the name _mexicanus_ of Lichtenstein in
1827 is a _nomen nudum_, and that it is preoccupied by _mexicanus_
used by Kerr in 1792. The latter can not be technically identified
with any particular species of geomyid.
Bartram in 1791 wrote of the pocket gopher of Florida, without
formally describing it. The first available technical name is _Mus
bursarius_ of Shaw in 1800. Rafinesque in 1817 proposed the first
generic names for the geomyids when he described _Geomys_ and
_Diplostoma_. In 1839, Waterhouse referred the genus _Geomys_ to his
family Arvicolidae, considered by him to be a subgroup of muroids. In
1841, he suggested that _Geomys_ was related to _Bathyergus_ and
_Spalax_. Waterhouse in 1848 (p. 8) treated the pocket gophers as a
subgroup of rodents under the group name Saccomyina, in which he
included the genera _Heteromys_, _Saccomys_, _Perognathus_, and
_Dipodomys_. Hence, Waterhouse was the first to recognize the
relationship between the heteromyids and geomyids. In the next year
Gervais erected the family Pseudostomidae for a group of specialized
squirrels to include _Geomys_ and _Thomomys_ and the same genera (at
least in part) of heteromyids that Waterhouse classified in the
"family" Saccomyina.
In 1839 the name _Thomomys_ was proposed by Maximilian (Wied-Neuwied).
All of the generic names previously proposed for pocket gophers were
considered by subsequent authors to be synonyms of _Geomys_.
A third family name, Sciurospalacoides, was proposed by Brandt
(1855:188) who referred _Geomys_ and _Thomomys_ to that family. He
placed his new family phylogenetically between the family Sciuridae
and the family Spalacoides (a group in which Brandt included the
genera _Spalax_, _Sipheus_, and _Ellobius_). Brandt took exception
to the classification of Waterhouse (1848), who united the geomyids
and heteromyids in one family. Brandt placed the heteromyid genera
in other groups: _Perognathus_ in the Muridae, and _Macrocolus_
[= _Dipodomys_] in the Macrolini, a subfamily of the family Dipodoides.
Modern classification of the pocket gophers begins with Baird in 1858.
The important classifications are summarized in Table 1; a few that do
not depart essentially from those listed have been omitted owing to
limited space for the tabular arrangement, but are discussed in the
following account.
Baird probably was strongly influenced by the arrangement proposed by
Waterhouse in 1848, but was opposed to separating geomyids from
heteromyids as was done by Brandt. Baird was convinced of the close
relationship of the geomyids and heteromyids, and referred both groups
to one family, the Saccomyidae, as Waterhouse had done earlier. In
order to recognize the morphological specializations he used two
subfamilies, Geomyinae and the Saccomyinae. In the 20 years that
followed, some authors followed Brandt and others followed Baird.
Gill, in 1872 (p. 71), proposed a classification essentially like
Baird's of 1858, but Gill raised Baird's subfamilies to the rank
of family (see Table 1). In referring all pocket gophers to the
Geomyidae, Gill used that name as a family term for the first time.
Also he established the superfamily Saccomyoidea to include his two
families, Geomyidae and Saccomyidae; therefore, the Saccomyoidea was
equivalent to the group Saccomyina of Waterhouse (1848) and the
Saccomyidae of Baird (1858). Coues (1877), in his classic monograph of
the Geomyidae followed the arrangement proposed by Gill in treating
the pocket gophers as a family. Alston in 1876 proposed another
classification based on Baird (1858), with two subfamilies, the
Geomyinae and the Heteromyinae, united together in the family
Geomyidae; thus, he recognized that the genus _Saccomys_ Frédéric
Cuvier, 1823, was a synonym of _Heteromys_ Desmarest, 1817, as had
been pointed out by Gray (1868:201) and Peters (1874:356). Coues
(1877:487-490) acknowledged the invalidity of the genus _Saccomys_,
but refused to give up the name in supergeneric classification. Winge,
first in 1887 and subsequently in 1924, classified the geomyids and
heteromyids together in the family Saccomyidae as did Baird in 1858,
and like Coues, Winge too ignored the synonymy of _Saccomys_ with
_Heteromys_ and insisted on retaining the technical terms Saccomyidae
and Saccomyini.
Up to the time of Merriam's classic revision of the Recent Geomyidae
in 1895 all the known species of living pocket gophers were referred
to two genera, _Geomys_ and _Thomomys_. Merriam described much new
material, especially from México and Central America, and proposed
seven new genera (see Table 1). His complete and detailed study of the
dentitions and osteology of the skull remains today as the definitive
work on this subject, and is the point where most studies of the
Geomyidae must begin. His treatment of the Recent genera survived for
52 years without change until Hooper (1946:397) arranged _Platygeomys_
as a synonym of _Cratogeomys_. However, Merriam's genera have been
recognized in all subsequent classifications except for the current
review (see Table 1).
Cope described the first known fossil geomyids in 1878, and published
an excellent review of the two genera, _Pleurolicus_ and _Entoptycus_,
in 1884 (pp. 855-870, pl. 64, figs. 1-9). Both genera were recovered
from the John Day Miocene deposits of Oregon. Cope did not propose a
new systematic arrangement of these geomyids, but referred them to the
family Saccomyidae and mentioned that the Saccomyidae was equivalent
to the family Geomyidae of Alston. Winge, in 1887, followed Cope in
referring _Pleurolicus_ and _Entoptycus_ to the Saccomyidae along with
the living genera _Thomomys_ and _Geomys_. Miller and Gidley (1918),
in their synopsis of the supergeneric groups of rodents, proposed a
new subfamily, Entoptychinae, to include the divergent Miocene pocket
gophers. Miller and Gidley also revived the old subfamily Geomyinae of
Baird (1858), but restricted its application to the modern pocket
gophers and their immediate ancestors. In 1936, A. E. Wood revised the
taxa of the subfamily Entoptychinae, and described the first Miocene
genus, _Dikkomys_, of the Geomyinae. He followed the supergeneric
classification of Miller and Gidley (1918).
The recent classifications of Simpson (1945) and Wood (1955) have
combined the classifications of Merriam (1895) and Wood (1936). Wood
(1955) brought up to date the list of genera, including those that
were described after the publication of Simpson's classification
(1945). In Table 1, the list of genera is principally from Simpson
(1945) but generic names used by Wood (1955) are included. This is the
currently accepted classification.
The new classification proposed in this paper (see Table 1) includes
three tribes proposed as vertical units; they are intended to stress
the phyletic trends in the known evolutionary sequences by placing
immediate ancestors together with their descendants.
_Pliogeomys_ is placed in the same tribe (Geomyini) as _Zygogeomys_,
_Geomys_, _Orthogeomys_, and _Pappogeomys_. That tribe includes the
most specialized Geomyinae. _Zygogeomys_, _Geomys_, _Orthogeomys_, and
_Pappogeomys_ are lineages resulting from a Pleistocene radiation in
which all the lineages diverged from a common Pliocene ancestor. The
radiation of the Geomyini was well under way by the close of the late
Pliocene. Although _Pliogeomys_ may not be the actual ancestor, it
closely resembles the primitive morphotype.
TABLE 1.--History of the classification of the Superfamily Geomyoidea
===============+==============+==================+================
Baird 1858 | Gill 1872 | Winge 1887 | Merriam 1895
| Coues 1877 | and 1924 | Ellerman 1940
---------------+--------------+------------------+----------------
Family | Family | Family | Family
Saccomyidae | Geomyidae | Saccomyidae | Geomyidae
---------------+--------------+------------------+----------------
Subfamily | | "Group" |
Geomyinae | | Geomyini |
-- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- -
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | |
_Thomomys_ | _Thomomys_ | _Thomomys_ | _Thomomys_
| | |
| | |
| | |
| | |
| | |
| | | _Zygogeomys_
| | |
| | |
_Geomys_ | _Geomys_ | _Geomys_ | _Geomys_
| | |
| | | _Orthogeomys_
| | | _Heterogeomys_
| | | _Macrogeomys_
| | |
| | | _Pappogeomys_
| | | _Cratogeomys_
| | | _Platygeomys_
-- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- -
| | |
| | |
| | |
| | *_Pleurolicus_ |
| | |
| | |
| | *_Entoptychus_ |
---------------+--------------+------------------+----------------
| | |
| | |
| | |
| | "Group" |
| | Gymnoptychine** |
| | _Gymnoptychus_ |
---------------+--------------+------------------+----------------
Subfamily | Family | "Group" |
Saccomyinae | Saccomyidae | Saccomyini |
-------------+-------------+------------------+----------------
=====================+=====================+===================
Wood 1935 | Simpson 1945 | Names used in
Wood 1936 | Wood 1955 | present paper
---------------------+---------------------+-------------------
Family | Family | Family
Geomyidae | Geomyidae | Geomyidae
---------------------+---------------------+-------------------
Subfamily | Subfamily | Subfamily
Geomyinae | Geomyinae | Geomyinae
-- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- -
| | Tribe
| | Dikkomyini
| |
*_Dikkomys_ | *_Dikkomys_ | *_Dikkomys_
| *_Pliosaccomys_ | *_Pliosaccomys_
| |
| | Tribe
| | Thomomyini
| |
*_Pleisothomomys_ | *_Pleisothomomys_ | }
_Thomomys_ | _Thomomys_ | } _Thomomys_
| |
| | Tribe
| | Geomyini
| |
| *_Pliogeomys_ | *_Pliogeomys_
_Zygogeomys_ | _Zygogeomys_ | }
| *_Nerterogeomys_ | } _Zygogeomys_
| |
_Geomys_ | _Geomys_ | }
| *_Parageomys_ | } _Geomys_
_Orthogeomys_ | _Orthogeomys_ | }
_Heterogeomys_ | _Heterogeomys_ | } _Orthogeomys_
_Macrogeomys_ | _Macrogeomys_ | }
| |
_Pappogeomys_ | _Pappogeomys_ | }
_Cratogeomys_ | _Cratogeomys_ | } _Pappogeomys_
_Platygeomys_ | _Platygeomys_ | }
-- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- -
Subfamily | Subfamily | Subfamily
Entoptychinae | Entoptychinae | Entoptychinae
| |
*_Pleurolicus_ | *_Pleurolicus_ | *_Pleurolicus_
*_Gregorymys_ | *_Gregorymys_ | *_Gregorymys_
*_Grangerimus_ | *_Grangerimus_ | *_Grangerimus_
*_Entoptychus_ | *_Entoptychus_ | *_Entoptychus_
---------------------+---------------------+-------------------
| Geomyidae | Geomyidae
| _incertae sedis_ | _incertae sedis_
| |
| |
*_Gidleumys_ | *_Diplolophus_ | *_Diplolophus_
| *_Griphomys_ | *_Griphomys_
---------------------+---------------------+-------------------
Family | Family | Family
Heteromyidae | Heteromyidae | Heteromyidae
---------------------+---------------------+-------------------
* Denotes extinct genera.
** Winge included in his family Saccomyidae the "group"
Gymnoptychine and the contained genus _Gymnoptychus_ Cope, 1873,
which genus currently is placed in the family Eomyidae. The type
of _Gymnoptychus_ Cope, 1873, is synonymous with _Ischyromys_
Leidy, 1856, and the valid name for the genus is _Adjidaumo_
Hay, 1899.
_Pliosaccomys_, on the other hand, represents the terminal stages of a
long trend that began with the _Dikkomys_-like Geomyinae of the early
Miocene. In this lineage, the rate of evolution in the dentition and
the skull was slow; therefore, the differences between early Miocene
(_Dikkomys_) and middle Pliocene (_Pliosaccomys_) are not great and
the two are united into the tribe Dikkomyini. The Dikkomyini is the
ancestral geomyinen trunk from which the modern groups have diverged.
The Pliocene ancestor of _Thomomys_ is unknown but probably resembled
_Pliosaccomys_, with which it may have been a contemporary. _Thomomys_
is the least specialized of the modern Geomyinae, and, consequently,
shows the most resemblance to the ancestral tribe. The specializations
of _Thomomys_, however, clearly preclude its reference to the tribe
Dikkomyini; therefore, it is set apart in the monotypic tribe
Thomomyini. That tribe has not undergone an adaptive radiation
comparable to that of the tribe Geomyini or that of the Entoptychinae
in the early Miocene. Here, for the first time, _Thomomys_ is set
apart in classification from the other living pocket gophers.
Merriam's genera _Orthogeomys_, _Heterogeomys_, and _Macrogeomys_ are
closely related. Each of these taxa is retained as a subgenus of a
single genus, _Orthogeomys_. Some species of _Macrogeomys_ seem to be
more closely allied to the subgenus _Orthogeomys_ and others to the
subgenus _Heterogeomys_. A revision of the genus is needed; it might
show that the currently recognized subgenera are artificial, and that
a different arrangement of the species would more clearly express
their evolutionary relationships. The subgenus _Heterogeomys_ seems to
be the most nearly uniform of the subgenera, and it is the least
specialized. Radiation within the genus may have begun relatively
recently, but the many special adaptations for tropical environments
suggest that the genus has been in the Neotropical Zone a long time.
Therefore, discovery of an early dichotomy from the common ancestral
stock of the tribe would come as no surprise.
_Nerterogeomys_ Gazin here is arranged as a junior synonym of
_Zygogeomys_. Both are less specialized than any of the other
Geomyini, except _Pliogeomys_. The single living species (_Zygogeomys
tricopus_) is obviously a relic. Its range is small. The two
subspecies differ only in minor features. The living species does
have a few unique characteristics, only to be expected in the
surviving species of a long phyletic lineage. Some of these are
specializations. Otherwise, _Zygogeomys_ and _Nerterogeomys_ are
closely related and the latter is best placed as a synonym of the
former. Both are admittedly closely related to _Geomys_. _Zygogeomys_
and _Geomys_ share several characters, particularly primitive ones;
there is considerable parallelism, especially marked in Irvingtonian
species of _Geomys_. Nevertheless, _Geomys_ is more specialized,
particularly in the dentition, and it has developed some
_Pappogeomys_-like specializations. _Zygogeomys_ has retained more of
the primitive characters of the tribe. A strong case could be made for
recognizing only one genus, _Geomys_, containing _Zygogeomys_ as one
of two subgenera. Nevertheless, the characters separating _Zygogeomys_
and _Geomys_ are of considerable importance and I consider the two
kinds to be distinct genera.
The species of _Geomys_, both living and extinct, form a distinct and
well-marked group. The genus is less primitive in most respects than
_Zygogeomys_ and _Orthogeomys_ and it is less specialized than
_Pappogeomys_, excluding the ancestral stock (subgenus _Pappogeomys_).
Some specimens of species of Irvingtonian age (_Geomys tobinensis_ and
_Geomys garbanii_, especially the former) retain primitive enamel
plates as does _Zygogeomys_; but this is true of only a small
percentage of the individuals. Also the adult dental pattern developed
somewhat later in ontogeny in these middle Pleistocene species of
_Geomys_ than in either Recent or late Pliocene and early Pleistocene
representatives (_Geomys paenebursarius_, _Geomys quinni_) of the
genus. Whether these features represent a stage in the evolution of
the late Pleistocene and Recent species or a terminal stage in members
of a sterile and primitive branch of the main line of evolution of
_Geomys_ is uncertain. At present I favor the latter explanation, and
view _G. paenebursarius_ and _G. quinni_ as early progressive species
that evolved dental specializations that were maintained in the main
line of phylogeny.
Hibbard proposed the generic name _Parageomys_ (1944:55), but later
regarded it as a subgenus of _Geomys_ (1956:182) that includes those
species retaining continuous enamel bands until relatively late in
ontogeny; no other differences have been noted. When the early
phylogeny of _Geomys_ is better understood, _Parageomys_ may serve as
a subgeneric taxon in which the primitive species of _Geomys_ can be
grouped, but as of now _Parageomys_ is arranged as a synonym of
_Geomys_.
_Pappogeomys_ and _Cratogeomys_ also form a natural group. Their close
relationship is best reflected in formal taxonomy by including them in
the same genus. Their dissimilarities are of the sort that separate a
primitive ancestral lineage from a divergent and progressively more
specialized assemblage. The fossil record is inadequate, and I
can only speculate that _Cratogeomys_ diverged from primitive
_Pappogeomys_-stock in the earlier Pleistocene, at least before the
end of the Irvingtonian. _Cratogeomys_ probably originated on the
Mexican Plateau and probably underwent its subsequent evolution there.
The living species of the subgenus _Pappogeomys_ are evidently relics
of the ancestral stock of the genus. Hooper (1946:397), I think
correctly, considered _Platygeomys_ as congeneric with _Cratogeomys_,
although the highest degree of specialization of the genus is attained
in those species formerly classed in the genus _Platygeomys_. Even so,
in my opinion, the differences are insufficient to warrant even
subgeneric recognition.
CLASSIFICATION
Family GEOMYIDAE Gill, 1872
Rodents of the superfamily Geomyoidea specialized for completely
fossorial life (early Pliocene to Recent); specialized earlier (late?
Oligocene and early Miocene) for semi-fossorial habits; body thickset,
fusiform without apparent neck (in modern geomyids); legs short;
forelegs especially stout; eyes and ears small (pinna reduced to
inconspicuous crest concealed beneath pelage); tail tactile, shorter
than head and body; lips closing behind incisors; cheek pouches
external, fur-lined; baculum rodlike, arched, having expanded
quadriform platelike base; pelage long, soft without underfur,
covering body in thick coat (in some species of _Orthogeomys_ scant,
harsh or scattered bristles); color varying from pale tints of buffy
(almost white) to metallic black.
Skull thick-walled, massive, angular, relatively broad, and flattened;
distinctly murine form, but having zygomasseteric structure of
advanced sciuromorphs, including small infraorbital canal (that
transmits no part of masseter muscle) and well-developed, broad
zygomatic plate; zygomata massive and widely flaring, jugals stout;
rostrum robust, relatively broad and deep, and without evidence of
transverse canal (as in Heteromyidae); anterior projection of nasals
only slightly exceeding that of upper incisors; interorbital region
usually constricted, narrower than rostrum; anterior opening of
infraorbital canal far forward on side of rostrum, about half
way between zygomatic plate and upper incisor and just behind
premaxillary-maxillary suture, its opening countersunk in oblique
sulcus (for protection from muscle contraction); postorbital process
lacking, except for rudimentary knoblike projection in subgenus
_Macrogeomys_; palate relatively narrow, its deeply sculptured surface
sloping steeply downward posteriorly causing region supporting
maxillary tooth-row to be markedly depressed; palatine bone reduced,
forming, on two abruptly different levels, posterior margin of hard
palate behind tooth-rows; parietals compressed and narrow, and most of
cerebral cavity roofed by squamosals (in some species squamosals
overlap lateral parts of parietals); tympanic bullae completely
inferior in position and fully ossified, external meatus being
developed laterally as elongated tube; mastoid not inflated, but
broadly exposed at posterolateral margin of the skull; occiput large,
its surface usually rugose, and paroccipital processes large and
flangelike, at least in advanced groups (early Pliocene to Recent);
ramus relatively short and stout, having distinct crest and ridges
for muscle attachments; coronoid process well developed, erect;
articular condyle prominent; angular process prominent, reflected
laterally, and in modern groups lateral extension protruding from
posterior border of ramus nearly at right angle; capsule for root of
lower incisor, prominent between angular process and articular
condyle.
Anterior surface of incisors broad and flat, always smooth on lower
teeth, but either smooth or grooved on upper teeth depending on taxon;
cheek teeth hypsodont, becoming progressively higher crowned in modern
groups, rooted in primitive groups (late? Oligocene to middle
Pliocene), rootless and ever-growing in modern groups (late Pliocene
to Recent); upper and lower premolars persistently bicolumnar; upper
and lower molars bicolumnar only in primitive groups (late? Oligocene
and early Miocene), becoming progressively monocolumnar in advanced
groups (early Pliocene to Recent), primitive bicolumnar pattern being
retained on occlusal surface only in early stages of ontogeny and in
third molar throughout life; enamel pattern of occlusal surface of
cheek teeth based on sextituberculate prototype (see Wood and Wilson,
1936:388-391), having cusps arranged in two transverse rows of three
cusps each, excepting three anterior cusps of premolars that are
arranged in trefoil, especially on p4 (sometimes only one or two,
rather than three, cusps develop in a particular set, especially in
p4), conules absent; protostyle and endostyle in upper teeth and
protostylid and hypostylid in lower teeth formed from cingulum; cusps
of each row uniting with wear into transverse enamel lophs (or
lophids), each tooth having two lophs, one on anterior column,
protoloph and protolophid, and one on posterior column, hypoloph and
hypolophid, that unite with additional wear forming continuous enamel
band; enamel lacking on sides of each column in advanced lineages,
thereby restricting enamel to anterior and posterior walls; with
extreme reduction, posterior plates of upper teeth and, more commonly,
anterior plates of lower molars, missing. Dental formula: 1/1, 0/0,
1/1, 3/3.
Key to the Subfamilies of Geomyidae
A Angular process of ramus mostly below alveolar level of
mandibular tooth-row; pattern of premolar like that of molars,
consisting of two subequal crests united at one or both margins
of tooth; molars persistently bicolumnar; molariform teeth
always rooted. Subfamily Entoptychinae p. 513
A´ Angular process of ramus mostly above level of mandibular
tooth-row; pattern of permolar unlike that of molars,
consisting of two prisms differing in size and united at their
mid-points but never at either margin; molars progressively
monocolumnar, except for early Miocene forms; molariform teeth
rooted only in primitive genera (late? Oligocene to middle
Pliocene), and rootless and ever-growing in later genera (late
Pliocene to Recent). Subfamily Geomyinae p. 514
Subfamily ENTOPTYCHINAE Miller and Gidley, 1918
Anterior face of upper incisor usually smooth, sometimes bearing faint
groove in center or near medial margin of tooth, at least in
_Gregorymys_; cheek teeth hypsodont, medium to high crowned, and
rooted in all but _Entoptychus_ (has rootless, ever-growing teeth);
cheek teeth identical in form, premolars resembling molars and lower
cheek teeth mirror images of upper teeth; crowns biprismatic, having
two columns joined at edge of protomeres (for description of term, see
discussion of primitive morphotype on page 537) and with persistent
lateral fissure between them; lateral re-entrant fold deep,
penetrating at least half width of crown, from external side in upper
teeth and internal side in lower teeth (in specialized genus
_Entoptychus_ lophs, upon additional wear, join also at edge of
parameres, thus uniting columns at both ends and thereby enclosing
interior part of lateral fissure as a transverse fossette in center of
tooth); enamel investment of prisms usually complete, including
inflection bordering re-entrant folds, occlusal pattern becoming
interrupted with wear only in _Entoptychus_, where enamel disappears
first from sides of crowns (following union of anterior and posterior
columns at both sides) and later, in final stages of attrition, from
anterior wall of lower molars and posterior wall of upper molars.
Maxillary bone without pronounced vertical depth in part supporting
cheek teeth, its inferior border only slightly lower than inferior
border of premaxillary and alveolar lips of molariform teeth
consequently approximately level with, or slightly below, alveolar lip
of upper incisor; squamosal without lateral expansion, therefore,
meatal tube of auditory bulla separated from zygomatic process of
squamosal by deep, well-developed postglenoid notch; angular part of
mandible below alveolar level of mandibular cheek teeth; angular
process only slightly reflected laterally; coronoid process low, tip
only slightly above condyle.
For information concerning the structure and relationships of the
known genera, and for accounts of species, see Wood (1936). A list
of the named genera in order of specialization is as follows:
*_Pleurolicus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:66.
*_Gregorymys_ Wood, 1936. Amer. Mus. Novit., 866:9.
*_Grangerimus_ Wood, 1936. Amer. Mus. Novit., 866:13.
*_Entoptychus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:64.
Five new species have been described since Wood's (1936) revision.
They are: _Pleurolicus clasoni_ MacDonald (1963:180); _Gregorymys
kayi_ Wood (1950:335); _Gregorymys montanensis_ Hibbard and Keenmon
(1950:198); _Grangerimus dakotensis_ MacDonald (1963:182);
_Grangerimus sellardsi_ Hibbard and Wilson (1950:623).
Subfamily GEOMYINAE Baird, 1858
Anterior face of upper incisor primitively smooth, grooves
consistently developed only in one modern lineage (Geomyini); cheek
teeth hypsodont, primitively rooted and having crown of medium height
(late Oligocene to middle Pliocene), being higher crowned, rootless
and ever-growing in modern lineages (late Pliocene to Recent);
primitively crowns of cheek teeth biprismatic, having two columns
joined at mid-points by narrow isthmus and entire crown sheathed in
continuous band of enamel; premolars retaining primitive biprismatic
form, anterior and posterior columns never uniting at edge of
protomeres or parameres, and with both lateral re-entrant folds
persistent throughout life; primitive biprismatic pattern becoming
decidedly modified in molars (except in M3), having two prisms
progressively uniting into one column by reduction and loss of lateral
inflections, primitive biprismatic patterns being retained only in
early stages of ontogeny; third upper molars retaining, at least
partially, primitive bicolumnar pattern (except in Thomomyini), with
relatively broad isthmus and horizontally shallow re-entrant folds,
lingual fold sometimes wanting; enamel pattern becoming discontinuous
(late Pliocene to Recent) owing to loss of enamel from sides of each
column; remaining enamel restricted to anterior and posterior plates,
or cutting blades, and enamel bordering lateral inflections in
premolars (considering both sides together, these plates constitute
essentially two transverse cutting blades); enamel pattern of M3
varying, depending on taxon; with specialization, anterior plates of
lower molars and posterior plates of upper premolar and molars may be
reduced or lost; except in primitive species (early Miocene), no
enamel fossettes retained in adult dentitions.
Maxillary bone having pronounced vertical depth in part supporting
cheek teeth, inferior border arching downward well below inferior
border of premaxillary; consequently, alveolar lips of molariform
teeth decidedly below level of alveolar lip of upper incisor;
squamosal with marked lateral expansion at expense of postglenoid
notch; notch compressed and reduced between meatal tube of auditory
bulla and zygomatic process of squamosal; angular part of mandible
mostly above alveolar level of mandibular cheek teeth; angular process
reflected laterally at right angles to axis of ramus and developed
into heavy knoblike projection; coronoid process well developed, tip
decidedly higher than condyle; fossorial specializations remarkably
well developed in advanced lineages, degree of specialization of
primitive Miocene species unknown but probably only semi-fossorial as
in Entoptychinae.
Key to the Tribes of the Geomyinae
A Enamel investment complete and uninterrupted, even in final
(adult) stages of wear; cheek teeth rooted, with crowns of medium
height; third lower molar biprismatic, the two columns separated
by inner and outer re-entrant folds as in lower premolar.
Tribe Dikkomyini p. 515
A´ Enamel investment incomplete and discontinuous, reduced, at least
in final (adult) stages of wear, to interrupted enamel plates;
cheek teeth rootless and ever-growing (except in extinct genus
_Pliogeomys_), crowns of maximum height; third lower molar
monoprismatic, without trace of inner and outer re-entrant folds
as in first and second lower molars.
B Upper incisors smooth, occasionally with a fine indistinct
groove near inner margin of tooth; form of third upper molar
same as M1 and M2, monoprismatic, anteroposteriorly compressed,
and having transverse enamel plates on both anterior and
posterior faces, and without suggestion of either labial or
lingual re-entrant folds; basitemporal fossa absent (except
for a shallow depression in one Recent species, _T. townsendii_);
forefoot small and narrow with claws not elongated for digging.
Tribe Thomomyini p. 518
B´ Upper incisors grooved, bearing either one or two sulci; form of
third upper molar distinctly different from M1 and M2, fully or
partially biprismatic (with a few exceptions discussed beyond),
without marked anteroposterior compression (either subtriangular,
elongated, suborbicular or quadriform in cross-section, but not
elliptical as in M1 and M2), and having typical transverse
anterior plate and two lateral plates (varying in their
development, depending on taxa), but no posterior plate, and with
lateral re-entrant folds usually developed, especially labial
inflection (although sometimes minute in a few species, as
described beyond); basitemporal fossa well-developed, although
occasionally shallow or absent (primitive species of _Zygogeomys_);
forefoot large and broad, with elongated claws for digging.
Tribe Geomyini p. 521
Tribe DIKKOMYINI, new tribe
_Genotype._--_Dikkomys_ Wood, 1936.
_Chronologic and geographic range._--Early to Middle Pliocene (early
Arikareean to mid-Hemphillian) in western United States. Known from
Miocene fossil sites in Montana, South Dakota, and Nebraska and
Pliocene sites in South Dakota, Oregon, Nevada, and southern
California. For precise localities see accounts of _Dikkomys_ and
_Pliosaccomys_ beyond.
_Diagnosis._--Small Geomyinae; lacking specializations of more
advanced tribes; upper incisors smooth, at least in _Pliosaccomys_;
molariform teeth always rooted and having crowns of medium height;
enamel investment of cheek teeth complete and uninterrupted in all
stages of wear; crowns of molars primitively biprismatic, having two
columns united at mid-points, thus forming narrow isthmus separating
lateral re-entrant folds as in premolars, and, with wear, also uniting
secondarily at protomeres (with exception of third lower molars),
consequently, isolating remnant of that inflection as shallow fossette
(columns uniting first at protomeres in _Pliosaccomys_); anterior and
posterior columns of first and second molars, both above and below,
becoming progressively united into one column in advanced Dikkomyini
(early and middle Pliocene), but m3 (M3 unknown) retaining primitive
biprismatic pattern, with columns joined at centers but never at
protomeres (for details of dentition see generic accounts); mandible
stout, its angle mostly above mandibular tooth-row; masseteric ridge
low; basitemporal fossa barely discernable in some fragments of
_Pliosaccomys_; postcranial skeleton unknown.
Key to the Genera of the Tribe Dikkomyini
A Molars biprismatic throughout life; anterior and posterior
lophs of first and second molars in pre-final stages of wear
uniting first at their mid-points and later at edge of
protomeres; anterior lophid of lower premolar having distinct
anteroexternal inflection. Genus _Dikkomys_ p. 516
A' First and second molars becoming monoprismatic in final
(adult?) stages of wear, biprismatic only in pre-final stages
of wear; third molars persistently biprismatic; anterior and
posterior lophs of first and second molars uniting first at
edge of protomeres; anterior lophid of lower premolar lacking
anteroexternal inflection. Genus _Pliosaccomys_ p. 517
Genus =Dikkomys= Wood
1936. _Dikkomys_ Wood, Amer. Mus. Novit., 866:26, July 2.
_Type._--_Dikkomys matthewi_ Wood, 1936, from Lower Harrison deposits
near Agate, Sioux County, Nebraska.
_Chronologic range._--Early Miocene, from early Arikareean (Lower
Harrison local fauna of Nebraska) to middle Miocene, late
Hemingfordian (Upper Rosebud local fauna, South Dakota, and the Deep
River Formation, Montana). According to MacDonald (1963:149-150), the
Upper Rosebud is middle Miocene rather than early Miocene.
_Description._--Size small, about as in small kinds of _Thomomys_;
known only from fragmentary mandible, including molariform dentition
in place, and isolated cheek teeth, including M1 (see Wood, 1936:26-28
and fig. 32; Galbreath, 1948:316-317 and fig. 1; and Black, 1961:13-14
and fig. 58); upper incisors unknown; cheek teeth hyposodont,
persistently rooted, and having crowns of medium height compared with
Recent geomyids; enamel investment complete and uninterrupted in all
molariform teeth in all stages of wear; P4 unknown, but probably
formed like p4; p4 persistently biprismatic, two crowns joined at
mid-points by relatively narrow isthmus separating lateral re-entrant
folds; anterior lophid of p4 having distinct anteroexternal
inflection; molars also biprismatic throughout life; two lophids of
lower molars first uniting at mid-points as in p4, and, with
additional wear, m1 and m2 secondarily uniting at edge of protomeres
and forming isolated enamel fossette between point of connection
(detailed description of stages of wear discussed in account of
phylogeny of subfamily); m3 permanently joined at mid-point only,
without lateral union at edge of protomeres; upper molars, judging by
M1 (M2 and M3 unknown), having same pattern as lower molars, but first
union of lophs decidedly on lingual side of center, consequently,
lingual re-entrant fold small; M1 probably developing U-pattern in
advanced stages of wear by union of protomeres, with minute lingual
fossette developing in transition as lophs secondarily become united
at lingual edge of columns; mandible stout and geomyidlike; masseteric
ridge weakly developed; basitemporal fossa absent.
Evidently, _Dikkomys matthewi_ is more primitive than _Dikkomys
woodi_. The modified H-pattern in m1 and m2, with the metalophid and
hypolophid joined at both their mid-points and also at their
protomeres (by union of the protostylid and hypostylid in the lower
dentition), is persistent throughout life. Therefore, the enclosed
enamel fossette is not eradicated with wear. In m1 and m2 of _Dikkomys
woodi_, the fossette is shallower, and, at least in advanced stages of
wear, it would disappear, therefore, forming a U-pattern on the
occlusal surface, as in M1 and M2, but lateral inflection horizontally
shallow rather than deep as in entoptychines.
Specimen (No. P 26284 FMNH) reported as _Dikkomys matthewi_ by
Galbreath (1948:316) is referable to the recently described species
_Dikkomys woodi_ Black, 1961.
_Specimens examined._--One, no. P 26284, Field Mus. Nat. Hist., from
upper Rosebud, Shannon Co., South Dakota.
_Referred species._--two:
_Dikkomys matthewi_ Wood, 1936. Amer. Mus. Novit., 866:26, July.
Type from early Arikareean Lower Harrison deposits (early Miocene)
near Agate, Sioux County, Nebraska.
_Dikkomys woodi_ Black, 1961. Postilla, Yale Peabody Museum, 48:13,
January 16. Type from Deep River Formation, late Hemingfordian
(middle Miocene), Meagher County, Montana; also known from Upper
Rosebud deposits (middle Miocene) near Wounded Knee, Shannon
County, South Dakota.
Genus =Pliosaccomys= Wilson
1936. _Pliosaccomys_ Wilson, Carnegie Inst. Washington Publ.,
473:20, May 21.
_Type._--_Pliosaccomys dubius_ Wilson, 1936, from Smiths Valley local
fauna in Lyon County, Nevada.
_Chronologic range._--Early Pliocene, late Clarendonian (Wolf Creek
local fauna, South Dakota, and Nettle Springs local fauna, California)
to Middle Pliocene, middle part of Hemphillian (Smiths Valley local
fauna, Nevada, and McKay Reservoir and Otis Basin local faunas,
Oregon).
_Description._--Size small (alveolar length of mandibular tooth-row
measuring 6.0 in holotype), about as in _Thomomys monticola_; upper
incisor relatively broad and flat, having anterior face smooth,
without trace of grooving; crowns of cheek teeth of medium height and
rooted; enamel investment continuous and uninterrupted in all stages
of wear; premolars permanently, biprismatic; P4 having anterior prism
subtriangular and decidedly smaller that sub-crescentic posterior
prism, and joined near centers by narrow, obliquely oriented isthmus;
p4 having anterior prism subovate, posterior prism strongly compressed
anteroposteriorly, and joined at mid-points by relatively broad and
straight isthmus; first and second molars, both above and below,
monoprismatic in final (?adult) stage of wear, derived ontogenetically
from primitive bilophate pattern by coalescence of two columns into
one; M1 and M2 mirror images of m1 and m2 in pre-final stages of wear,
two columns first uniting at edge of protomeres forming U-pattern, and
primitive H-pattern never developing in either series (for detailed
description of stages of wear, see account of phylogeny, p. 546); m3
(M3 unknown, but probably with same form as in Geomyini, see p. 552)
persistently biprismatic, two columns joined by relatively broad
isthmus at centers, consequently, forming H-pattern of primitive
ancestors; rostrum heavy and broad as in modern geomyids; palate
narrow and strongly ribbed; mandible stout; masseteric ridge and fossa
well developed; basitemporal fossa absent.
_Specimens examined._--Six, nos. 1796 (holotype)--1799, 1804 and 1806
(CIT) now in the Los Angeles County Museum, all from Smiths Valley
local fauna, Middle Pliocene, Nevada.
_Referred species._--two:
*_Pliosaccomys dubius_ Wilson, 1936. Carnegie Inst. Washington Publ.,
743:20, May 21. Known from early and middle Pliocene faunas
including Wolf Creek local fauna (late Clarendonian), Shannon
County, South Dakota; McKay Reservoir local fauna and Otis Basin
local fauna (Hemphillian), Oregon; type from Smiths Valley local
fauna (probably middle Hemphillian), Lyon County, Nevada.
*_Pliosaccomys wilsoni_ James, 1963. Univ. California Publ. Geol. Sci.,
45:101, June 26. Type from Nettle Springs local fauna of late
Clarendonian (early Pliocene), Ventura County, California.
Tribe THOMOMYINI, new tribe
_Type._--_Thomomys_ Wied-Neuwied, 1839.
_Chronologic and geographic range._--Known from late Pliocene (early
Blancan) to Recent. Known primarily from western North America from
southern Canada south to Central México in Pliocene, Pleistocene and
Recent and in middle and late Pleistocene of Maryland and Florida.
_Diagnosis._--Size small to medium (basilar length exclusive of _T.
bulbivorus_, measuring from approximately 24 to 45, including both
males and females); upper incisors without grooving, excepting fine,
indistinct sulcus rarely near inner margin (grooving more common in
_T. monticola_ than in other Recent species); crowns of cheek teeth
high, rooted and ever-growing; all molars, including M3, monoprismatic
and anteroposteriorly compressed, sometimes (especially in subadults)
having slight inflection on labial side in upper teeth and lingual
side in lower teeth; molars bicolumnar in pre-final stages of wear
(seen in juvenal teeth only), patterns of wear in both upper and lower
molars resembling those of _Pliosaccomys_, except that crowns of m3
and M3 unite into single column in final stages of wear; enamel
pattern interrupted in all cheek teeth, loss occurring only at sides
of each column; transverse enamel blade completely covering posterior
face of both P4 and p4; all upper and lower molars with two transverse
enamel blades, one on anterior surface and one on posterior surface,
of each tooth, including M3; small third plate sometimes persistent on
broad side of tooth, labial side in upper molars and lingual side in
lower molars (_T. bulbivorus_); skull generalized, neither unusually
narrow and deep or broad and flat; usually without marked cresting or
rugosity; masseteric ridge well developed and massive; basitemporal
fossa absent, sometimes shallow depression forming in _T. townsendii_;
pelage soft, never harsh or hispid, covering body with thick coat of
hair; forefoot exceptionally small for fossorial mammal, claws not
especially long; body form remarkably fossorial.
The tribe Thomomyini is monotypic, including only the genus
_Thomomys_.
Genus =Thomomys= Wied-Neuwied
1839. _Thomomys_ Wied-Neuwied, Nova Acta Phys. Med. Acad. Caesar.
Leop.-Carol., 19(1):377.
1836. _Oryctomys_ Eydoux and Gervais (in part), Mag. de Zool., 6:20,
pl. 21. Type: _Oryctomys_ (_Saccophorus_) _bottae_, from
coast of California, probably near Monterey.
1903. _Megascapheus_ Elliot, Field Columb. Mus., Publ. 76, Zool.
Ser., 3(11):190, July 25. Type: _Diplostoma bulbivorum_
Richardson, from Columbia River, probably near Portland, Ore.
1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm. 14:354. Type:
_Pleisothomomys potomacensis_ Gidley and Gazin, from
Pleistocene, Cumberland Cave local fauna, Allegany County,
Maryland.
_Chronologic range._--Known from late Pliocene to Recent.
_Description._--Same as that given for the tribe Thomomyini above.
_Discussion._--Features characterizing _Thomomys_ and the tribe
Thomomyini are more advanced than those characterizing the tribe
Dikkomyini. Also, the Thomomyini retain more of the primitive
features of the Geomyinae than do the more specialized tribe Geomyini.
Specializations are few, but include the third molar being a single
column both above and below, enamel plates, and a masseteric ridge.
Key to the Subgenera of _Thomomys_
A Molars sub-crescent or ovate in cross-section, not
becoming abruptly narrower at one end of tooth.
Subgenus _Pleisothomomys_ p. 519
A´ Molars pear-shaped, not sub-crescent or ovate, in
cross-section, crown becoming abruptly narrow at one
end of tooth. Subgenus _Thomomys_ p. 520
Subgenus =Pleisothomomys= Gidley and Gazin
1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm., 14:354,
November 13.
_Type._--_Pleisothomomys potomacensis_ Gidley and Gazin, 1933.
_Chronologic range._--Late Pliocene (Hagerman local fauna, Idaho) to
late Pleistocene. The latest records are from the fauna of Saber-tooth
Cave, Florida, a late Pleistocene assemblage that probably was
deposited in the Sangamon. The middle and late Pleistocene records are
from the eastern United States, suggesting that the subgenus
_Pleisothomomys_ was restricted to that region while the subgenus
_Thomomys_ occupied the western United States and parts of Canada and
México as it does today.
_Description and Comparison._--Separated from subgenus _Thomomys_ only
on basis of sub-crescentic shaped molars (only jaw fragments and
isolated teeth known), seemingly a primitive feature of the genus.
This dental structure continued into the late Pleistocene; none of the
Recent species expresses this feature of the molars, although the
molars of _Thomomys vetus_ of the late Pleistocene (Wisconsin
deposits), referred to the subgenus _Thomomys_ on the basis of
its alleged relationship to _Thomomys townsendii_ (see Davis,
1937:156-158), are less distinctly pear-shaped, and are more
sub-crescentic, than in any other known species of the subgenus
_Thomomys_. _Pleisothomomys_ Gidley and Gazin (_loc. cit._) was
proposed as a genus but is here considered as of no more than
subgeneric worth, and is recognized because of the apparent constancy
of the sub-crescentic molars in the earlier members of the genus and
in those populations of _Thomomys_ occurring in Pleistocene times in
the eastern United States.
_Referred species._--Three (all extinct):
*_Thomomys gidleyi_ Wilson, 1933. Carnegie Inst. Washington Publ.
440:122, December. Type from Hagerman beds, late Pliocene,
Idaho.
*_Thomomys potomacensis_ Gidley and Gazin, 1933. Jour. Mamm.,
14:354, November 13. Type from Cumberland Cave, middle and late
Pleistocene, Maryland.
*_Thomomys orientalis_ Simpson, 1928. Amer. Mus. Novit., 328:6,
October 26. Type from Saber-tooth Cave, late Pleistocene,
Florida.
Subgenus =Thomomys= Wied-Neuwied
1839. _Thomomys_ Wied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar.
Leop. Carol., 19(1):377.
1903. _Megascapheus_ Elliot, Field Columb. Mus., Publ. 76, Zool.
Ser., 3 (11):190, July 25. Type: _Diplostoma bulbivorum_
Richardson, from Columbia River, probably near Portland, Oregon.
_Type._--_Thomomys rufescens_ Wied-Neuwied, 1839.
_Chronologic range._--Early Pleistocene (Broadwater-Lisco local fauna,
Nebraska) to Recent. Numerous records, mostly isolated teeth, from
nearly all stratigraphic levels of the Pleistocene (for details, see
account of fossil record).
_Description._--Molars pear-shaped in cross-section, becoming abruptly
narrow at one end of the tooth. The teeth of the late Pleistocene
species _Thomomys vetus_ are less distinctly pear-shaped than other
referred species (see remarks in the description of the subgenus
_Pleisothomomys_).
Essentially on the basis of its significantly larger size and details
of the skull, Elliott (1903:190) proposed subgeneric recognition of
_Thomomys bulbivorus_ and described the subgenus _Megascapheus_ to
include it. Also the molars of _Thomomys bulbivorus_ usually have a
small enamel plate, both above and below, bordering the persistent
inflection on the protomere end of the tooth; each lateral plate is
isolated from the transverse plates on the anterior and posterior
walls of the tooth. In my opinion these features do not warrant
subgeneric recognition; however, these characters do distinctly
separate _Thomomys bulbivorus_ from other groups of species, and the
character of the molars suggests retention of a primitive trait.
Therefore, I propose that the unique structure of this species be
recognized by setting it apart in the _bulbivorus_ species-group.
_Referred species._--Ten species, three extinct, placed in three
species-groups (the numerous subspecies of this genus are listed in
Miller and Kellogg, 1955:276-332, and Hall and Kelson, 1959:412-447).
_bulbivorus_ species-group
_Thomomys bulbivorus_ (Richardson, 1829). Fauna Boreali-Americana,
1:206. Type from Columbia River, probably near Portland,
Oregon.
_umbrinus_ species-group
*_Thomomys scudderi_ Hay, 1921. Proc. U. S. Nat. Mus., 49:614.
Type from Fossil Lake beds, late Pleistocene, Oregon.
_Thomomys umbrinus_ (Richardson, 1829). Fauna Boreali-Americana,
1:202. Type from southern México, probably near Boca de Monte,
Veracruz.
_Thomomys bottae_ (Eydoux and Gervais, 1836). Mag. de Zool., Paris,
6:23. Type from coast of California, probably near Monterey.
*_Thomomys vetus_ Davis, 1937. Jour. Mamm., 18:156, May 12. Type
from Fossil Lake beds, late Pleistocene, Oregon.
_Thomomys townsendii_ (Bachman, 1839). Jour. Acad. Nat. Sci.
Philadelphia, 8:105. Type probably from near Nampa, Canyon Co.,
Idaho (erroneously given as "Columbia River").
_talpoides_ species-group
*_Thomomys microdon_ Sinclair, 1905. Bull. Dept. Geol. Univ.
California, 4:145-161. Type from Potter Creek Cave, late
Pleistocene, California.
_Thomomys monticola_ J. A. Allen, 1893. Bull. Amer. Mus. Nat. Hist.,
5:48, April 28. Type from Mt. Tallac, 7500 ft., El Dorado Co.,
California.
_Thomomys talpoides_ (Richardson, 1828). Zool. Jour., 3:518. Type
locality fixed at near Fort Carlton (Carlton House),
Saskatchewan River, Saskatchewan, Canada.
_Thomomys mazama_ Merriam, 1897. Proc. Biol. Soc. Washington,
11:214, July 15. Type from Anna Creek, 6000 ft., near Crater
Lake, Mt. Mazama, Klamath Co., Washington.
Tribe GEOMYINI, new tribe
_Genotype._--_Geomys_ Rafinesque, 1817.
_Chronologic and geographic range._--Known from late middle Pliocene
deposits to Recent. The range of living members extends from extreme
southern Manitoba and the southeastern United States south to southern
Panamá, and probably northern Colombia, South America.
_Diagnosis._--Size small to large (condylobasal length of skull 33.0
to 73.0 in adults, including both sexes); sexual dimorphism marked,
sometimes strongly, females being smaller than males, especially in
cranial dimensions; upper incisors invariably grooved, number and
position of grooves varying according to genus; cheek teeth
high-crowned and ever-growing, except in one primitive genus
(_Pliogeomys_); all three lower molars and M1 and M2 monoprismatic,
and elliptical in cross-section in final stages of wear (teeth of
young, subadult, and adult animals); primitive biprismatic patterns
(as known from Recent specimens) occurring only in pre-final stages of
wear (teeth of juveniles only); biprismatic patterns of lower molars
as in _Dikkomys_, and upper molars as in _Pliosaccomys_ (for detailed
description of these patterns, see account beyond of the phylogeny of
the Geomyinae); m3 becoming monoprismatic, anteroposteriorly
compressed and elliptical in cross-section like m1 and m2, but M3
remaining, with rare exceptions (see accounts of _Geomys_ and
_Pappogeomys_ beyond), at least partially biprismatic throughout life,
having one or both lateral inflections usually persisting (with
exceptions) and developing various occlusal shapes (subtriangular,
elongate, obcordate, suborbiculate, or quadriform) but never
elliptical.
Enamel of cheek teeth reduced to interrupted plates, with exception of
p4 in _Pliogeomys_; plate on posterior wall of P4 variable, occurring
completely across posterior surface in primitive members, but
progressively reduced to lingual side only or completely lost in
modern genera (see generic accounts beyond for detailed description);
both anterior and posterior plates usually retained in M1 and M2,
posterior plate sometimes reduced to lingual side or completely lost
(as in _Pappogeomys_) but anterior plate always completely retained;
M3 usually having three plates, one anterior and two lateral;
posterior plate wanting (sometimes lingual plate moved to posterior
position); plates retained completely across posterior walls of all
lower cheek teeth with no reduction, but anterior plates of m1-3
always lacking, except in primitive genus _Pliogeomys_ (only Geomyini
having both anterior and posterior enamel plates on lower molars).
Skull primitively generalized, but becoming specialized towards either
dolichocephaly (_Orthogeomys_) or platycephaly (_Pappogeomys_) in two
modern genera; skull highly specialized for fossorial life; mandible
stout and deep, angular process being high and diverging laterally at
right angles to ramus; masseteric ridge and fossa weakly developed in
primitive members, becoming well developed and massive in modern
genera; basitemporal fossa absent in primitive forms (_Pliogeomys_ and
early members of _Zygogeomys_); pelage usually soft, but harsh and
hispid in some genera; forefeet broad and massive, claws long and
stout for digging; body form remarkably fossorial.
The tribe Geomyini includes the most highly specialized members of the
subfamily Geomyinae.
Key to the Genera of the Tribe Geomyini
A Cheek teeth rooted; p4 with uninterrupted enamel loop; enamel
plates on both anterior and posterior walls of m1 and m2;
masseteric ridge weakly developed, low, not massive.
Genus _Pliogeomys_ p. 522
A´ Cheek teeth rootless, ever-growing; p4 with enamel investment
interrupted at ends of columns, consequently, forming four
isloted plates; enamel plate retained only on posterior wall
of m1 and m2, anterior wall without trace of enamel (except
rarely in pre-final stage of wear in _Geomys tobinensis_ of
middle Pleistocene); masseteric crest strongly developed and
massive.
B Enamel plate on posterior wall of P4, but usually
restricted to lingual end of tooth (usually absent in
subgenus _Orthogeomys_ of genus _Orthogeomys_);
M3 conspicuously bicolumnar, longer than wide owing to
elongation of posterior loph.
C Upper incisor bisulcate; skull generalized; rostrum
relatively narrow; length of labial enamel plate of
M3 decidedly less than length of lingual plate;
pelage soft and thick. Genus _Zygogeomys_ p. 523
C´ Upper incisor unisulcate; skull strongly
dolichocephalic; rostrum remarkably broad and massive;
length of lingual plate of M3 approximately equal to,
or greater than, length of labial plate; pelage harsh,
often hispid and scant. Genus _Orthogeomys_ p. 528
B´ Posterior wall of P4 without trace of enamel; M3 not
strongly bicolumnar, having shallow re-entrant fold on
labial side, and crown no longer than wide owing to
shortness of posterior loph.
D Upper incisor bisulcate; skull generalized; both
anterior and posterior walls of M1 and M2 having
complete enamel plates. Genus _Geomys_ p. 525
D´ Upper incisor unisulcate; skull generalized or
tending towards platycephaly; enamel plate on
posterior wall of M1 usually reduced to lingual
side or absent (complete only in one species,
_Pappogeomys bulleri_); enamel plate on posterior
wall of M2 also absent in advanced species
(subgenus _Cratogeomys_). Genus _Pappogeomys_ p. 532
Genus =Pliogeomys= Hibbard
1954. _Pliogeomys_ Hibbard, Michigan Acad. Sci., Arts and
Letters, 39:353.
_Genotype._--_Pliogeomys buisi_ Hibbard, 1954, from Buis Ranch local
fauna (middle Pliocene), Beaver County, Oklahoma.
_Chronologic range._--Latest Middle Pliocene, known only from the
highest part of the Hemphillian mammalian fauna (Buis Ranch local
fauna, Oklahoma). Professor Hibbard informs me (personal
communication) that he found the type, a right ramus, lying on the
surface near the base of the fossil beds. The isolated teeth of small
geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392)
may also be referable to this genus. The Saw Rock Canyon local fauna
may also be middle Pliocene in age but is considered to be from the
later part of the late Pliocene, and, therefore, somewhat younger than
the Buis Ranch local fauna (Hibbard, _op. cit._:342).
_Description and discussion._--The size of members of this small genus
of the Geomyinae is about the same as in smaller adults of _Geomys
bursarius_. According to Hibbard (_op. cit._:353), the holotype is
smaller than specimens from the Rexroad local fauna referred to
_Geomys quinni_ and larger than specimens referred to _Zygogeomys_ cf.
_minor_. The cheek teeth are rooted, and the crowns are as high as
those of living geomyids. The upper incisor is bisulcate, and the
inner groove is fine and indistinct in places.
Of the molariform dentition only the lower premolar and first two
lower molars are known. The enamel investment of p4 is complete, and
would not be subject to interruption at any stage of wear; the two
prisms are joined at their mid-points, and the isthmus of dentine is
relatively broad (as in _Pliosaccomys_) when compared with modern
pocket gophers of this tribe. Also, the re-entrant folds, rather than
having parallel sides, diverge broadly to the sides. The divergence is
especially noticeable in the labial fold. The lower deciduous premolar
would have formed essentially the same enamel pattern with wear as
observed in _Nerterogeomys_ [= _Zygogeomys_] cf. _minor_ (see Hibbard,
1954:fig. 5, A and B) and _Pliosaccomys dubius_ (see Wilson, 1936; pl.
1, fig. 1). Each molar is a single column in the final stages of wear;
pre-final stages are unknown. Anterior and posterior enamel plates are
present on m1 and m2 (m3 has not been recovered). The dentine tracts
of m1 are exposed over a relatively wide surface; therefore, the
enamel plates are distinctly separated. The tracts of dentine of m2
are much narrower than in m1 and the enamel plates are barely
separated at the anterolateral margin of the tooth. Possibly the
enamel band of m2 was continuous in an earlier stage of wear.
The mandible is stout and its general construction not unlike that in
modern geomyines. The capsule at the base of the angular process that
receives the terminal end of the lower incisor is well developed. The
base of the angular processes is preserved, and suggests that the
process was short and decidedly smaller than in living examples of the
tribe. The masseteric ridge is distinct but weakly developed, and not
at all massive as in living pocket gophers. The mental foramen is
immediately anterior, and slightly ventral, to the anterior extension
of the crest. The basitemporal fossa is absent as such, but its
position is marked by a slight depression.
_Specimens examined._--Two rami; nos. 29147 (holotype) and 33446;
several isolated teeth 30194 and 30195, including an upper incisor and
a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo.
_Referred species._--One.
*_Pliogeomys buisi_ Hibbard, 1954. Papers Michigan Acad. Sci.,
Arts, and Letters, 39:353. Type from Buis local fauna, latest
middle Pliocene, Beaver County, Oklahoma.
Genus =Zygogeomys= Merriam
1895. _Zygogeomys_ Merriam, N. Amer. Fauna, 8:195, January 31.
1942. _Nerterogeomys_ Gazin, Proc. U. S. Nat. Mus., 92:507
(type, _Geomys persimilis_ Hay, 1927).
_Type._--_Zygogeomys trichopus_ Merriam, 1895, from Nahuatzen,
Michoacán.
_Chronologic range._--Late Pliocene (Benson and Curtis Ranch local
faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent.
_Description and discussion._--The size is small to medium for the
subfamily Geomyinae. This genus is distinguished principally by the
retention of primitive features. In the living species, the skull is
generalized, rather than specialized toward either extreme
dolichocephaly or platycephaly. The angular process is short, barely
exceeding the lateral extensions of the mastoid process of the
squamosal. The rostrum is remarkably narrow in relation to its length.
The jugal is reduced and displaced ventrally, causing the maxillary
arm of the zygomata to articulate with the squamosal arm of the
zygomata along the dorsal border of the zygomatic arch (a feature
observed also in _Orthogeomys cherriei costaricensis_).
The upper incisor, recovered in material from the late Pliocene and
middle Pleistocene, is bisulcate as in the genus _Geomys_ and the
primitive genus _Pliogeomys_. The enamel plate across the posterior
wall of P4 is either complete (late Pliocene to late Pleistocene) or
restricted to the lingual half of the tooth (always restricted in
living species). The Pliocene specimens of the Rexroad local fauna
referred to _Nerterogeomys_ cf. _minor_ by Hibbard (1950:138-139) are
exceptional. In these specimens the length and position of the
posterior enamel plate is variable; however, all but one specimen had
persistant enamel. Evidently, in approximately 43 per cent of the
specimens, a complete enamel blade was present (see Paulson,
1961:139), and in the others (except the one without any enamel) the
plate was restricted to a small area of the ventral surface, usually
on the lingual side of the loph. Hibbard suggested that the decrease
in size of the plate, and its restriction to the lingual side, may be
a function of age. Hence, most adults would be characterized by the
reduced posterior plate on the upper premolar. Although age may be the
important factor, intragroup variation cannot be ruled out. It is of
interest to note that in all specimens from the Benson (type series of
_P. minor_) and Curtis Ranch local faunas, the former of late Pliocene
age and the latter of middle Pleistocene age, the enamel plates are
complete on the posterior face of the upper premolar. As mentioned
before, the specimens from Kansas may actually represent the
transitional stages of the early evolution of _Geomys_ in which the
posterior plate of P4 is entirely lost. The enamel pattern of p4 is
like that in other members of the tribe (excepting the genus
_Pliogeomys_). The re-entrant angles of P4 and p4 are widely open
(obtuse) in the examples recovered from late Pliocene and middle
Pleistocene deposits, representing retention of a trait that is
primitive in the Geomyini (see account of phylogeny).
M1 and M2 are elliptical in cross-section and each has an enamel plate
on both the anterior and posterior surface. In the living species (_Z.
trichopus_), the posterior enamel plate fails to reach the labial
margin of the tooth and is restricted to the lingual two-thirds of the
posterior surface; however, the enamel plates are complete in the late
Pliocene species (_Z. minor_) and the middle Pleistocene species (_Z.
persimilis_), being only slightly separated from the anterior plate by
narrow tracts of dentine on the ends of the tooth. M3 is partly
biprismatic in the living species, the two incompletely divided lophs
being separated by a distinct outer sulcus. The posterior loph is
elongated and forms a conspicuous heel paralleling the evolution of
this trait in the genus _Orthogeomys_; therefore, the crown is longer
than wide. The posterior part of the tooth is protected by two lateral
enamel plates; of the two, the lingual plate is especially long and
extends to the end of the heel. M3 has not been recovered in the
Pliocene species, but in the middle Pleistocene species (_Z.
persimilis_) M3 is subtriangular, no longer than wide, and the lateral
inflections are weakly developed. The trend towards elongation of M3
evidently occurred in late Pleistocene evolution of the genus. All
three of the inferior molars are elliptical, and only the posterior
enamel plate is present (as in all other genera of the tribe except
_Pliogeomys_).
The masseteric ridge of the mandible is well developed. In the late
Pliocene species _Z. persimilis_ and _Z. minor_ the mental foramen is
directly beneath the anterior extension of the masseteric ridge, but
in the living species, _Z. trichopus_, the foramen lies well anterior
to the ridge. The basitemporal fossa in the living species is well
developed and deep; in the Pliocene species it is usually distinct but
shallow (late Pliocene specimens of _Z. minor_).
_Referred species._--Three (two extinct and one living; the last has
two subspecies):
*_Zygogeomys minor_ (Gidley), 1922. U. S. Geol. Surv. Prof. Paper,
131:123, December 26. Type from Benson local fauna (late
Pliocene), Cochise County, Arizona; also known from the
Rexroad local fauna, Meade County, Kansas.
*_Zygogeomys persimilis_ Hay, 1927. Carnegie Inst. Washington
Publ., 136. Originally described by Gidley, 1922 (U. S. Geol.
Surv. Prof. Papers, 131:123, December 26) as _Geomys
parvidens_ which was preoccupied by _G. parvidens_ Brown,
1908. Type from Curtis Ranch local fauna (middle
Pleistocene), Cochise County, Arizona.
_Zygogeomys trichopus trichopus_ Merriam, 1895. N. Amer. Fauna,
8:196, January 31. Type from Nahuatzen, Michoacán.
_Zygogeomys trichopus tarascensis_ Goldman, 1938. Proc. Biol. Soc.
Washington, 51:211, December 23. Type from 6 mi. SE
Pátzcuaro, 8,000 ft., Michoacán.
Genus =Geomys= Rafinesque
1817. _Geomys_ Rafinesque, Amer. Monthly Mag., 2(1):45, November.
1817. _Diplostoma_ Rafinesque, Amer. Monthly Mag., 2(1):44-45,
November. Included species: _Diplostoma fusca_ Rafinesque
[= _Mus bursarius_ Shaw] and _Diplostoma alba_ Rafinesque
[= _Mus bursarius_ Shaw] from the Missouri River region.
1820. _Saccophorus_ Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66.
Type: _Mus bursarius_ Shaw, from upper Mississippi Valley.
1823. _Pseudostoma_ Say, Long's Expd. Rocky Mts., I, pp. 406. Type:
_Pseudostoma bursaria_ [= _Mus bursarius_ Shaw], from upper
Mississippi Valley.
1825. _Ascomys_ Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822),
p. 20., fig. 2. Type: _Ascomys canadensis_ Lichtenstein
[= _Mus bursarius_ Say], probably from upper Mississippi Valley.
1944. _Parageomys_ Hibbard, Bull. Geol. Soc. Amer., 55:735, June.
Type: _Parageomys tobinensis_ Hibbard, from Pleistocene, Cudahy
(Tobin) local fauna, Russell Co., Kansas.
_Type._--_Geomys pinetis_ Rafinesque, 1817, restricted to Screven
County, Georgia, in region of the pines.
_Chronologic range._--Late Pliocene faunas of Blancan age (Rexroad,
Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported
from numerous Pleistocene deposits of all stratigraphic levels,
especially from the Great Plains, where common today.
_Description and discussion._--Pocket gophers of this genus are
medium-sized geomyids; none is so small as the average-sized
_Thomomys_. The skull is generalized and lacks the dolichocephalic and
platycephalic specializations seen in the genera _Orthogeomys_ and
_Pappogeomys_, respectively. _Geomys_ closely resembles _Zygogeomys_,
but retains fewer of the primitive characters of the ancestral stock.
At the same time, _Geomys_ has several specializations. Even so, a
considerable amount of parallelism is evident in the phyletic trends
of the two genera.
The upper incisor of _Geomys_ is bisulcate as in _Pliogeomys_ and
_Zygogeomys_; the deeper grove is medial and the shallower grove lies
near the inner border of the tooth. The premolar, above and below, is
bicolumnar; and two columns are joined at their mid-points (deep
re-entrant angles separate the columns at the sides). A permanent
enamel plate protects the anterior face of the anterior loph, and
enamel bands outline each of the re-entrant folds. In p4 a complete
enamel plate covers the posterior surface of the posterior loph. All
of the enamel bands are interrupted by tracts of dentine, except in
the initial stages of wear of the occlusal surface of the newly
erupted tooth. For a short time in living _Geomys_, the enamel bands
are continuous as observed in juveniles of _Geomys bursarius major_
(KU 5628, 8531, and 41540). But, the enamel cap is thin and the
dentine tracts, which are high on the sides of the tooth, are soon
revealed by a minimum of wear on the crown. Therefore, the adult, or
final, pattern characterized by interrupted enamel plates emerges
early in life and remains throughout the life of the individual.
Evidence from fossil _Geomys_, especially from specimens from early
and late Pleistocene deposits, suggests that the final adult pattern
appears later, ontogenetically, than in Recent specimens. Some of the
fossil premolars in initial stages of wear have continuous and
uninterrupted bands of enamel. _Geomys quinni_ of the late Pliocene
and early Pleistocene has the interrupted pattern seen in late
Pleistocene and Recent _Geomys_. Also, in late Pliocene and early
Pleistocene species, the re-entrant folds diverge laterally and form
"open" angles. In later taxa (middle Pleistocene to Recent) the folds
are compressed and parallel-sided, and the "open" folds are found only
in the early stages of wear.
The posterior enamel plate of P4 disappears in the final stages of
wear as the interrupted enamel pattern is formed. In the late
Pleistocene and Recent _Geomys_, the loss of the posterior plate
occurs early in life, usually in the first phases of wear on the
occlusal surface of the newly erupted tooth, but in fossils of
_Geomys_ of corresponding ontogenetic age from the early and middle
Pleistocene, the posterior plate is retained in some individuals until
a later phase of wear, thereby delaying the appearance of the final
pattern. Indeed, in five or fewer per cent of the individuals (see
Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of
enamel is retained throughout life or at least until late in
adulthood. In _Geomys tobinensis_, for example, a thin, but
transversely complete, plate of enamel occurs all the way down to the
base of the loph (Paulson, _loc. cit._) and would persist throughout
life. In _Geomys garbanii_, a vestige on the lingual side of the
posterior surface of a fully adult specimen was noted by White and
Downs (_loc. cit._). Vestiges of the posterior plate occur less
frequently in living geomyids. Paulson (_loc. cit._) found a posterior
plate in one of 75 specimens of _Geomys bursarius dutcheri_. A young
(suture present between exoccipitals and supraoccipital) female of
_Geomys pinetis austrinus_ (KU 23358) has a vestige of the posterior
plate on the lingual side of the tooth as White and Downs (_loc.
cit._) observed in a specimen of _Geomys garbanii_. The enamel, I
suspect, tends to be thicker on the lingual than on the labial side of
the loph and extends farther down the lingual surface in some
individuals; therefore, wear on the occlusal surface erodes it down to
the dentine more rapidly on the labial than on the lingual side. The
tendency of enamel to be retained is a primitive feature.
A lower molar of _Geomys_ is a single elliptical column, and enamel is
restricted to the posterior surface as in _Zygogeomys_, _Orthogeomys_,
and _Pappogeomys_. Paulson (_loc. cit._) found a thin enamel plate on
the anterior surfaces of the lower molars in about five per cent of
the individuals of _Geomys tobinensis_ from the Cudahy local fauna
(middle Pleistocene, deposits of the late Kansan glaciation). An
anterior plate is unknown in other members of the tribe Geomyini,
except in the primitive genus _Pliogeomys_ of the middle Pliocene.
Occurrence of the plate in _Geomys tobinensis_ is an atavistic trait.
Primitive dental patterns occur occasionally in geomyids, as pointed
out above, but the frequency of occurrence in _G. tobinensis_ is
higher than would be expected.
M1 and M2, like the lower molars, are elliptical in cross-section.
Complete enamel plates on the anterior and posterior surfaces are
separated by tracts of dentine on the sides of each tooth. M3 is
usually suborbicular (sometimes subtriangular) in cross-section. The
tooth is not especially elongated posteriorly and usually has no
definite heel; therefore, it is not significantly longer than wide.
Living species of _Geomys_ rarely have a well defined outer re-entrant
fold on M3; less than 10 per cent of the individuals (and usually only
one side in each individual in which it occurs) have it, although a
shallow inconspicuous groove occurs more frequently. The biprismatic
molar characteristic of the ancestral morphotype is less often found
in _Geomys_ than in any other living member of the tribe Geomyini. The
outer re-entrant fold and biprismatic pattern are more often present
in the extinct species _Geomys garbanii_ of the Middle Pleistocene
than in other species. Less than 24 per cent of the third upper molars
in _Geomys garbanii_ lack a tract of the re-entrant fold and more than
38 per cent have a well developed outer fold (see White and Downs,
1961:13, 18). The bicolumnar pattern, although incomplete, would be
clearly evident in those teeth having a well marked re-entrant fold;
the pattern occurs less frequently in those teeth with no fold or only
a slight one. M3 of geomyids is not usually recovered and, therefore,
the occlusal pattern of M3 is unknown in most extinct kinds of
_Geomys_. In Recent _Geomys_ the fold is more common in the eastern
_pinetis_ species-group than in the western _bursarius_ species-group.
The masseteric ridge on the outer side of the mandible is well
developed in all species of the genus. The position of the mental
foramen relative to the anterior part of the ridge varies with
individuals and according to species. The basitemporal fossa is always
present, but is shallower in the late Pliocene and Pleistocene species
than in Recent species. The angular process is short.
_Referred species._--The twelve species, five of which are extinct,
are as follows:
_quinni_ species-group
*_Geomys quinni_ McGrew, 1944. Geol. Ser., Field Mus. Nat. Hist.,
9 (546):49, January 20. Type from Sand Draw local fauna (late
Pliocene), Brown County, Nebraska; also known from
Broadwater-Lisco local faunas (early Pleistocene), Morrill and
Garden counties, Nebraska, Deer Park local fauna (early
Pleistocene), Meade County, Kansas.
*_Geomys paenebursarius_ Strain, 1966. Bull. Texas Memorial Mus.,
10:36. Type from Hudspeth local fauna (early Pleistocene),
Hudspeth County, Texas.
*_Geomys tobinensis_ Hibbard, 1944. Bull. Geol. Soc. Amer.,
55:736. Type from Tobin local fauna (middle Pleistocene),
Russell County, Kansas; also known from Cudahy local fauna
(middle Pleistocene), Meade County, Kansas.
*_Geomys garbanii_ White and Downs, 1961. Contrib. Sci., Los
Angeles Co. Mus., 42:1-34, June 30. Type from Vallecito Creek
local fauna (middle Pleistocene), San Diego County, California.
*_Geomys bisulcatus_ Marsh, 1871. Amer. Jour. Sci., 3:121. Type
from Loup River fossil beds, near Camp Thomas, Nebraska
(probably late Pleistocene).
_bursarius_ species-group
*_Geomys parvidens_ Brown, 1908. Mem. Amer. Mus. Nat. Hist.,
9:194. (An extinct subspecies of _Geomys bursarius_ according to
White and Downs, 1961:6). Type from Conard Fissure local fauna
(late Pleistocene), northern Arkansas.
_Geomys bursarius_ (Shaw, 1800). Trans. Linn. Soc. London, 5:227.
Type from somewhere in Upper Mississippi Valley, North America.
_Geomys arenarius_ Merriam, 1895. N. Amer. Fauna, 8:139, January 31.
Type from El Paso, El Paso County, Texas.
_Geomys personatus_ True, 1889. Proc. U. S. Nat. Mus., 11:159,
January 5. Type from Padre Island, Cameron County, Texas.
_pinetis_ species-group
_Geomys pinetis_ Rafinesque, 1806. Amer. Monthly Mag., 2 (1):45,
November. Type locality restricted to Screven County, Georgia.
_Geomys colonus_ Bangs, 1898. Proc. Boston Soc. Nat. Hist., 28:178,
March. Type from Arnot Plantation, about 4 mi. W St. Marys,
Camden County, Georgia.
_Geomys cumberlandius_ Bangs, 1898. Proc. Boston Soc. Nat. Hist.,
28:180, March. Type from Stafford Place, Cumberland Island,
Camden County, Georgia.
_Geomys fontanelus_ Sherman, 1940. Jour. Mamm., 21:341, August 13.
Type from 7 mi. NW Savannah, Chatham County, Georgia.
Genus =Orthogeomys= Merriam
1895. _Orthogeomys_ Merriam, N. Amer. Fauna 8:172, January 31.
1895. _Heterogeomys_ Merriam, N. Amer. Fauna 8:179, January 31
(type, _Geomys hispidus_ Le Conte, 1862).
1895. _Macrogeomys_ Merriam, N. Amer. Fauna 8:185, January 31 (type,
_Geomys heterodus_ Peters, 1865).
_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca,
México.
_Chronologic range._--Late Pleistocene Wisconsin deposits (San
Josecito Cave local fauna, Nuevo León, México) to Recent.
_Description and discussion._--Species of this genus are of medium to
large size. The skull is strongly dolichocephalic in most species;
the posterior part of the skull is especially narrow. The angular
processes are remarkably short, especially in relation to the length
of the mandible. The nasals and rostrum are relatively broad and
heavy. The pelage is coarse, and often hispid. In some species the
hairs are so sparsely distributed that the body appears almost naked,
and none has so dense a covering of hair as do other genera. The genus
occurs entirely within the tropical life-zones, and most of the
external features seem to be associated with adaptation to tropical
conditions.
The upper incisor is unisulcate; the sulcus is usually near the inner
border of the tooth, but in some species (subgenus _Orthogeomys_) it
is more medial, and in a few individuals with an extremely wide groove
the outer lip of the sulcus may actually reach the middle of the
tooth. The groove is compressed or open. The premolar is a double
column united at the mid-point. The two prisms are of approximately
equal size, and the lateral re-entrant folds are so compressed that
their sides are parallel. Enamel plates cover the anterior surface and
border the re-entrant angles in both upper and lower premolars. As in
other members of the tribe, the lower premolar has a fourth enamel
plate on the posterior surface of the posterior lophid. In the upper
premolar, the enamel plate is reduced to a narrow blade on the lingual
side of the loph as in the living species of the genus _Zygogeomys_.
In the subgenus _Orthogeomys_ the posterior plate is usually absent,
and otherwise is narrow and near the lingual border of the tooth.
Each lower molar, in the final stage of wear, consists of a single
elliptical column having an enamel plate only on the posterior
surface. The first and second upper molars are single elliptical
columns having one enamel plate on the anterior surface and another
on the posterior surface. The plates are separated by a tract of
dentine on each side of the tooth. The third upper molar is partly
bilophodont, and the two lophs are separated by a deep outer
re-entrant fold. In many of the species an inner re-entrant fold also
is retained, but in the adult tooth it is less distinct than the
outer. In all of the species the posterior loph is long and forms a
conspicuous heel; consequently the crown is significantly longer then
wide. Moreover, the posterior loph has an enamel plate on each side.
The labial plate always borders the outer re-entrant fold, and in the
subgenus _Orthogeomys_ is infrequently separated into two small
plates.
The mandible is relatively long. Its masseteric ridge is well
developed and massive. The basitemporal fossa is usually deep and well
defined; it tends to be shallow in the subgenus _Orthogeomys_, and in
young individuals is hardly more than a slight depression.
Key to the Subgenera of _Orthogeomys_
A Frontal wide and greatly inflated; no interorbital
constriction; enamel plate on posterior wall of P4 usually
absent, although sometimes having small plate, restricted
to lingual end of wall. Subgenus _Orthogeomys_ p. 529
A´ Frontal narrow and not greatly inflated; interorbital
region decidedly constricted; enamel plate on posterior wall
of P4 always present but short and restricted to lingual end
of wall.
B Anterior margin of mesopterygoid fossa even with plane of
posterior wall of M3; postorbital bar weakly developed;
anteroposterior occlusal length of M3 equal to, or less
than, combined length of M1 and M2.
Subgenus _Heterogeomys_ p. 530
B´ Anterior margin of mesopterygoid fossa decidedly behind
plane of posterior wall of M3; postorbital bar strongly
developed; anteroposterior occlusal length of M3 more
than combined length of M1 and M2.
Subgenus _Macrogeomys_ p. 531
Subgenus =Orthogeomys= Merriam
1895. _Orthogeomys_ Merriam, N. Amer. Fauna, 8:172, January 31.
_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca,
México.
_Chronologic range._--Known only from the Recent.
_Description._--Skull elongated and narrow (many skulls of nearly
uniform breadth throughout), being extreme in dolichocephalic
specializations; mandibles long and narrow, rami not spreading
laterally, being more nearly parallel-sided than in other subgenera;
angular processes short; breadth across zygomata not significantly
exceeding breadth across mastoid processes (in many skulls
considerably less); interorbital area remarkably broad, lacking deep
constriction; frontals between orbits greatly inflated laterally,
postorbital prominence inconspicuous; mesopterygoid fossa extending to
level of posterior margin of M3; I having sulcus broader than in other
subgenera, mostly on inner half of anterior surface but sometimes
overlapping mid-line; enamel plate lacking from posterior wall of P4,
rarely retaining narrow vestige near lingual border of posterior loph;
M3 having distinct heel, bicolumnar pattern with inner re-entrant fold
usually minute, occlusal length less than in other subgenera, length
less than combined lengths of M1-2; hair generally coarse, sometimes
hispid, sparse, in lowland forms, so sparse as to impart appearance of
nakedness.
_Referred species and subspecies._--Fourteen taxa:
_Orthogeomys grandis alleni_ Nelson and Goldman, 1930. Jour. Mamm.,
11:156, May 9. Type from near Acapulco, 2000 ft., Guerrero.
_Orthogeomys grandis annexus_ Nelson and Goldman, 1933. Proc. Biol.
Soc. Washington, 46:195, October 26. Type from Tuxtla
Gutierrez, 2600 ft., Chiapas.
_Orthogeomys grandis carbo_ Goodwin, 1956. Amer. Mus. Novit.,
1757:5, March 8. Type from Excurano, 2500 ft., Cerro de San
Pedro, 20 km. W Mixtequilla, Oaxaca.
_Orthogeomys grandis felipensis_ Nelson and Goldman, 1930. Jour.
Mamm., 11:157, May 9. Type from Cerro San Felipe, 10 mi. N
Oaxaca, Oaxaca.
_Orthogeomys grandis huixtlae_ Villa, 1944. Anal. Inst. Biol. Univ.
Nac. México, 15:319. Type from Finca Lubeca, 12 km. NE
Huixtla, 850 m., Chiapas.
_Orthogeomys grandis grandis_ (Thomas, 1893). Ann. Mag. Nat. Hist.,
ser. 6, 12:270, October. Type from Dueñas, Guatemala.
_Orthogeomys grandis latifrons_ Merriam, 1895. N. Amer. Fauna,
8:178, January 31. Type from Guatemala, exact locality
unknown.
_Orthogeomys grandis nelsoni_ Merriam, 1895. N. Amer. Fauna, 8:176,
January 31. Type from Mt. Zempoaltepec, 8000 ft., Oaxaca.
_Orthogeomys grandis pluto_ Lawrence, 1933. Proc. New England Zool.
Club, 13:66, May 8. Type from Cerro Cantoral, north of
Tegucigalpa, Honduras.
_Orthogeomys grandis scalops_ (Thomas, 1894). Ann. Mag. Nat. Hist.,
ser. 6, 13:437, May. Type from Tehuantepec, Oaxaca.
_Orthogeomys grandis soconuscensis_ Villa, 1949. Anal. Inst. Biol.
Univ. Nac. México, 19:267, April 8. Type from Finca
Experanza, 710 m., 45 km. (by road) NW Huixtla, Chiapas.
_Orthogeomys grandis guerrerensis_ Nelson and Goldman, 1930. Jour.
Mamm., 11:158, May 9. Type from El Limón, in valley of Río de
las Balsas approximately 20 mi. NW La Unión, Guerrero.
_Orthogeomys cuniculus_ Elliot, 1905. Proc. Biol. Soc. Washington,
18:234, December 9. Type from Zanatepec, Oaxaca.
_Orthogeomys pygacanthus_ Dickey, 1928. Proc. Biol. Soc. Washington,
41:9, February 1. Type from Cacaguatique, 3500 ft., Dept. San
Miguel, El Salvador.
Subgenus =Heterogeomys= Merriam
1895. _Heterogeomys_ Merriam, N. Amer. Fauna, 8:179, January 21.
_Type._--_Geomys hispidus_ Le Conte, 1852, from near Jalapa, Veracruz.
_Chronologic range._--Late Pleistocene, Wisconsin deposits (San
Josecito Cave local fauna, Nuevo León) to the Recent.
_Description._--Skull dolichocephalic (less so than in the other
subgenera); zygomata more widely spreading than in _Orthogeomys_;
ramus and angular process short; interorbital area noticeably
constricted; frontals between orbits neither exceptionally broad or
inflated; mesopterygoid fossa extending to level of posterior margin
of M3; I having sulcus on inner third of anterior surface usually
narrower than in subgenus _Orthogeomys_; enamel plate on posterior
wall of P4 restricted to lingual half of loph; M3 distinctly
biprismatic, posterior loph usually circumscribed by shallow inner
re-entrant fold and outer deep fold well developed in all members of
genus; posterior loph forming conspicuous heel longer than in subgenus
_Orthogeomys_; occlusal length equal to or slightly less than combined
lengths of M1-2; hair coarse and hispid but never so sparse as to
impart appearance of nakedness.
_Referred species and subspecies._--Eleven taxa:
*_Orthogeomys onerosus_ (Russell, 1960). Univ. Kansas Publ., Mus.
Nat. Hist., 9 (21):544, January 14. Type from San Josecito Cave
local fauna, Upper Pleistocene, Nuevo León.
_Orthogeomys hispidus cayoensis_ (Burt, 1937). Occ. Papers Mus.
Zool., Univ. Michigan, 365:1, December 16. Type from Mountain
Pine Ridge, 12 mi. S El Cayo, British Honduras.
_Orthogeomys hispidus chiapensis_ (Nelson and Goldman, 1929). Proc.
Bio. Soc. Washington, 42:151, March 30. Type from Tenejapa, 16
mi. NE San Cristobal, Chiapas.
_Orthogeomys hispidus concavas_ (Nelson and Goldman, 1929). Proc.
Biol. Soc. Washington, 42:148, March 30. Type from Pinal de
Amoles, Querétaro.
_Orthogeomys hispidus hispidus_ (Le Conte, 1852). Proc. Acad. Nat.
Sci. Philadelphia, 6:158. Type from near Jalapa, Veracruz.
_Orthogeomys hispidus latirostris_ (Hall and Alvarez, 1961). Anal.
Escuela Nac. Ciencias Biol., 10:121, December 20. Type from
Hacienda Tamiahua, Cabo Rojo, Veracruz.
_Orthogeomys hispidus negatus_ (Goodwin, 1953). Amer. Mus. Novit.,
1620:1, May 4. Type from Gomez Ferias, 1300 ft., about 45 mi. S
Ciudad Victoria, 10 km. W Pan American Highway, Tamaulipas.
_Orthogeomys hispidus tehuantepecus_ (Goldman, 1939). Jour.
Washington Acad. Sci., 29:174, April 15. Type from mountains 12
mi. NW Santo Domingo and about 60 mi. N Tehuantepec, 1600 ft.,
Oaxaca.
_Orthogeomys hispidus torridas_ (Merriam, 1895). N. Amer. Fauna,
8:183, January 31. Type from Chichicaxtle, Veracruz.
_Orthogeomys hispidus yucatanensis_ (Nelson and Goldman, 1929).
Proc. Biol. Soc. Washington, 42:150, March 30. Type from
Campeche, Campeche.
_Orthogeomys lanius_ (Elliot, 1905). Proc. Biol. Soc. Washington,
18:235, December 9. Type from Xuchil, Veracruz.
Subgenus =Macrogeomys= Merriam
1895. _Macrogeomys_ Merriam, N. Amer. Fauna, 8:185, January 31.
_Type._--_Geomys heterodus_ Peters, 1865, from Costa Rica, exact
locality unknown.
_Chronologic range._--Known only from the Recent.
_Description._--Skull dolichocephalic in varying degree (overlapping
subgenera _Orthogeomys_ and _Heterogeomys_ in this respect); mandibles
elongated, not spreading far laterally; angular processes decidedly
short; breadth across zygomata in no instance significantly exceeding
mastoid breadth; interorbital area strongly constricted; frontals
between orbits slightly inflated laterally (especially in forms having
more strongly dolichocephalic skulls); postorbital prominence
conspicuous; anterior margin of mesopterygoid fossa terminating well
behind M3; I having narrow and deep sulcus entirely on inner third of
anterior surface; enamel plate on posterior wall of P4 restricted to
inner half of loph; M3 bilophodont (outer and inner re-entrant folds
each circumscribing a loph), posterior loph remarkably elongated and
forming pronounced heel, length of crown more than combined lengths of
M1-2; hair wooly in some individuals, harsh in others but seldom
hispid, never so sparse as in subgenus _Orthogeomys_; some species
having white markings, especially on lumbar region and head.
_Referred species and subspecies._--Eleven taxa:
_Orthogeomys heterodus cartagoensis_ (Goodwin, 1943). Amer. Mus.
Novit., 1227:2, April 22. Type from Paso Ancho, Province
Cartago, Costa Rica.
_Orthogeomys heterodus dolichocephalus_ (Merriam, 1895). N. Amer.
Fauna, 8:189, January 31. Type from San José, Costa Rica.
_Orthogeomys heterodus heterodus_ (Peters, 1865). Monatsb. preuss.
Acad. Wiss., Berlin, 1865:177. Type from Costa Rica, exact
locality unknown.
_Orthogeomys cavator nigrescens_ (Goodwin, 1943). Amer. Mus. Novit.,
1227:3, April 22. Type from El Muneco (Río Navarro), 10 mi. S
Cartago, 4000 ft., Province Cartago, Costa Rica.
_Orthogeomys cavator pansa_ (Bangs, 1902). Bull. Mus. Comp. Zool.,
39:44, April. Type from Bogava (= Bugaba), 600 ft., Chiriquí,
Panamá.
_Orthogeomys dariensis_ (Goldman, 1912). Smithsonian Misc. Coll.,
60(2):8, September 20. Type from Cana, 2000 ft., mountains of
eastern Panamá.
_Orthogeomys underwoodi_ (Osgood, 1931). Field Mus. Nat. Hist.,
Publ. 295, Zool. Ser., 185:143, Aug. 3. Type from Alto de
Jabillo Pirris, between San Geronimo and Pozo Azul, western
Costa Rica.
_Orthogeomys cherriei carlosensis_ (Goodwin, 1943). Amer. Mus.
Novit., 1227:3, April 22. Type from Cataratos, San Carlos,
Alajuela, Costa Rica.
_Orthogeomys cherriei cherriei_ (J. A. Allen, 1893). Bull. Amer.
Mus. Nat. Hist., 5:337, December 16. Type from Santa Clara,
Costa Rica.
_Orthogeomys cherriei costaricensis_ (Merriam, 1895). N. Amer.
Fauna, 8:192, January 31. Type from Pacuare, Costa Rica.
_Orthogeomys matagalpae_ (J. A. Allen, 1910). Bull. Amer. Mus. Nat.
Hist., 28:97, April 30. Type from Peña Blanca, Matagalpa,
Nicaragua.
Genus =Pappogeomys= Merriam
1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.
1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.
Type: _Geomys merriami_ Thomas.
1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.
Type: _Geomys gymnurus_ Merriam; Hooper, Jour. Mamm.,
27:397, November 25, 1946.
_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope
Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.
_Chronologic range._--Late Pliocene, from deposits of early Blancan
age (Benson local fauna, Arizona) to the Recent. However in the
Pleistocene, only late Pleistocene records are known, and
_Pappogeomys_ has not been found in early (late Blancan) or middle
(Irvingtonian) Pleistocene local faunas. Presumably the genus was
restricted to México during the Pleistocene until post-Wisconsin time.
_Description and discussion._--The size ranges from as little as in
the smaller kinds of _Thomomys_ to the maximum attained in the
subfamily and matched elsewhere perhaps in only a few of the larger
subspecies of _Orthogeomys grandis_. Depending on the species and
subgenus, the form of the skull varies from generalized to
specialized. The generalized skulls are short and not especially
narrow; the zygomatic arches are spread laterally so far that the
breadth across them exceeds the breadth across the mastoid processes.
The most specialized skulls are platycephalic and the breadth across
the mastoid processes equals or exceeds the breadth across the
zygomatic arches (even so, the zygomatic arches are still relatively
widespread). In correlation with the great breadth of the posterior
part of the cranium, the rami of the mandibles diverge widely
posteriolaterally and the angular processes are remarkably elongated.
The rostrum is moderately broad in most species, but not nearly so
broad and heavy as in _Orthogeomys_.
The single deep, median sulcus on the outer surface of the upper
incisor is slightly displaced to the inner side of the tooth. The
posterior surface of P4 lacks enamel (small vestige found on lingual
end of posterior wall in only two adult individuals--UA 3260 and KU
100442, of the subgenus _Pappogeomys_); the other three plates are
fully developed as usual. The p4 is provided with four fully developed
enamel plates, in the pattern characteristic of the tribe Geomyini. In
the p4 of the late Pliocene species (_P. bensoni_) the re-entrant
angles are open (obtuse), a trait that is evidently primitive in the
Geomyini.
All three lower molars are single, compressed, elliptical columns with
enamel on only the posterior surfaces. M1 and M2 are also elliptical
in cross-section and decidedly anteroposteriorly compressed, like the
lower molars. Nevertheless, the enamel pattern is variable; enamel
plates may be retained completely across both the anterior and
posterior walls of M1 and M2 or only the anterior plate may be
retained without reduction and the posterior plate may be reduced so
that only a vestige is retained on the lingual fourth of the tooth or
the posterior plate may be completely lost.
M3 tends to remain at least incompletely bilophodont by reason of
retaining a permanent labial re-entrant fold in most species (with
exceptions in _Pappogeomys bulleri_ and some old adults of _P.
castanops_). Primitively the occlusal surface of M3 is subtriangular
(subgenus _Pappogeomys_), but in the _castanops_ species-group of the
advanced subgenus _Cratogeomys_, the posterior loph usually is reduced
and the occlusal surface is quadriform or obcordate. Curiously, the
trend towards reduction of the posterior loph is reversed in one
subspecies (_P. merriami fulvescens_) and, the loph has elongated into
a pronounced heel in some specimens, resembling the condition in
_Orthogeomys_. The entire range of variation occurs in _P. m.
fulvescens_. The subtriangular pattern is retained in the most
specialized species of _Cratogeomys_ where that pattern is associated
with extreme platycephaly in the _gymnurus_ species-group. In most
species the posterior loph supports two lateral plates, the outer one
always bordering the labial re-entrant fold. In _Pappogeomys bulleri_
and in the _castanops_ species-group, the outer re-entrant fold of M3
tends to be obsolete, and the tooth becomes quadriform or
suborbiculate in some individuals and loses the bilophodont pattern
that characterizes other species. The lingual enamel plate is
displaced to the posterior surface of the tooth, and one or both
plates may disappear with advancing age. Consequently, only the
anterior enamel plate remains in some adults, and constitutes the
maximum degree of reduction of enamel on M3 in the Geomyinae. In many
adults of _Pappogeomys bulleri_, the enamel investment of the
posterior loph is complete and the two lateral plates are connected,
without interruption around the posterior apex of the tooth, evidently
representing the retention of a primitive character of the ancestral
lineage.
The m3 of _P. bensoni_ from the late Pliocene is distinguished by
minute lateral inflections suggesting the primitive biprismatic
pattern. Also the posterior enamel plates of m1 and m2 are remarkably
long, extending around the ends of the tooth. The associated upper
incisor was unisulcate as in the modern species, and the basitemporal
fossa of the mandible is well developed and deep.
The lower jaw is stout and relatively short. The masseteric ridge is
well developed and has an especially thick crest. The basitemporal
fossa is deep. In most living species, the pelage is soft and dense,
but in one species, _Pappogeomys fumosus_, the hairs are coarse and
hispid somewhat as in _Orthogeomys_.
Key to the Subgenera of _Pappogeomys_
A Enamel plates completely developed across posterior walls of
M1 and M2, except in one species (_P. alcorni_) having enamel
restricted to lingual fourth in M1; sagittal crest lacking
owing to impressions of temporal muscles remaining separated
(even in old adults); zygomata slender, and without platelike
expansion at lateral angle. Subgenus _Pappogeomys_ p. 534
A´ Enamel lacking on posterior walls of M1 and M2; pronounced
sagittal crest developed in adults of both sexes by union
of temporal impressions at middorsal line; zygomata stout
and wide, with lateral angle expanded into broad plate.
Subgenus _Cratogeomys_ p. 535
Subgenus =Pappogeomys= Merriam
1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.
_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope
Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.
_Chronologic range._--Late Pliocene (Benson local fauna, Arizona) to
Recent, but no specimens known from Pleistocene.
_Description._--Small, approximately same size as small subspecies of
_Thomomys umbrinus_ but forefeet larger and claws longer; skull of
generalized shape, broad, relatively short, smoothly rounded, not
especially compressed dorso-ventrally; zygomatic breadth great but not
exceeding mastoid breadth; zygomata relatively slender for geomyid and
lacking platelike expansions at lateral angles; rostrum relatively
narrow; sagittal crest lacking, owing to impressions of temporal
muscles remaining separated; angular process of mandible not
especially elongated; enamel plates extending completely across
posterior wall of M1 and M2, except in one species, _P. alcorni_,
where posterior plate of M1 remains only on lingual fourth of
posterior wall (remainder of plate lacking); with wear, plates
sometimes exceptionally thin completely across posterior face of M2
and especially M1 in a few individuals of _P. bulleri_ much as Paulson
(1961:138-139) describes in extinct _Geomys tobinensis_; one or both
plates rarely disappear in final stages of attrition in old
individuals resulting in same dental pattern found in _Cratogeomys_;
M1 and M2 retaining enamel plate on anterior wall throughout life; M3
usually subtriangular in cross-section but sometimes suborbiculate or
ovoid, crown slightly bilophodont owing to shallowness of labial
re-entrant angle in modern species; posterior loph of M3 not
especially elongated and crown not significantly longer than wide;
both lateral enamel plates of M3 usually well developed and
approximately equal in length, occasionally plates reduced in length
and rarely one or both plates are lost with wear in old individuals;
patch of whitish or buffy hairs surrounding nose of most individuals.
The primitive character of the lower dentition, as described in the
species account above, suggest that _Cratogeomys_ [= _Pappogeomys_]
_bensoni_ Gidley should be referred to the subgenus _Pappogeomys_
rather than _Cratogeomys_. Only the upper dentition would make
positive identification possible; however, reference to the subgenus
_Pappogeomys_ seems to be the best arrangement at this time.
_Referred species._--Three (one extinct):
*_Pappogeomys bensoni_ (Gidley), 1922. U. S. Geol. Surv. Prof.
Papers, 131:123. Type from Benson local fauna (late Pliocene),
Cochise County, Arizona.
_Pappogeomys alcorni_ Russell, 1957. Univ. Kansas Publ. Mus. Nat.
Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.
_Pappogeomys bulleri_ Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6,
vol. 10:196, August. Type from "near Talpa," west slope of
Sierra Madre de Mascota, Jalisco.
Subgenus =Cratogeomys= Merriam
1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.
1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.
Type: _Geomys gymnurus_ Merriam, 1892.
_Type._--_Geomys merriami_ Thomas, 1893, from "Southern México,"
probably in Valley of México.
_Chronologic range._--Late Pleistocene, from Wisconsin deposits (San
Josecito Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave,
New Mexico, local faunas) to the Recent.
_Description._--Size medium to large; skull becoming angular and
rugose with age, and tending towards platycephaly and dorso-ventral
compression; zygomata stout, each bearing platelike expansion at
anterolateral angle into which anterior end of jugal becomes morticed;
breadth across zygomata great relative to length of skull; rostrum
relatively broad; squamosals expanding medially with age eventually
growing over lateral parts of parietals, and sometimes also expanding
laterally displacing postglenoid notch; sagittal crest well developed
in adults of both sexes, but especially high and bladelike in males;
lambdoidal crest prominent in all but young animals, having dorsal
outline broadly convex posteriorly in most species but strongly
sinuous in _gymnurus_-group; enamel plate on posterior wall of P4
absent; enamel plates present only on anterior walls of M1 and M2; M3
variform in occlusal shape (as described in species account), either
subtriangular (_gymnurus_-group), quadriform or obcordate
(_castanops_-group, with exceptions as noted before); lateral plates
of M3 usually present in all species, labial plate approximately as
long as lingual plate in _gymnurus_-group (like that in subgenus
_Pappogeomys_) or distinctly shorter in _castanops_-group (labial
plate scarcely extending beyond border of labial re-entrant fold); one
or both lateral plates tending to disappear with wear in
_castanops_-group, with lingual plate usually disappearing first;
breadth across angular processes clearly more than breadth across
zygomatic processes, especially in _gymnurus_-group.
_Remarks._--In the species of the _castanops_-group the skulls can be
spoken of as generalized and the least platycephalic of the subgenus.
Indeed, the species of the _castanops_-group are hardly more
specialized in this respect than is the subgenus _Pappogeomys_.
In these skulls the breadth across the squamosal processes is less
than that across the zygomatic arches, although the two dimensions
are almost equal in some examples of _P. merriami_ of the
_castanops_-group (where squamosal breadth varies from 85 to 98% of
zygomatic breadth). In the species having marked platycephalic skulls
(_gymnurus_ species-group) the breadth across the squamosal processes
equals or exceeds the breadth across the zygomatic arches (squamosal
breadth rarely 97 to 99% of zygomatic breadth), except in _P. zinseri_
and _P. tylorhinus zodius_.
The variable character of the third upper molar as between species
suggests that this tooth is presently undergoing active evolution. The
structure of this tooth, although differing between taxa, is
remarkably stable in other kinds of Geomyini. The most remarkable
modification of M3 in _Cratogeomys_ is the obcordate pattern
developed in _P. merriami_ of the _castanops_-group. The posterior
loph and entire tooth is shortened somewhat resembling in shape that
of _Thomomys_. Moreover, the posterior loph is twisted labially;
consequently, its posterior surface now forms the labial border of the
weakly defined posterior loph. Owing to the torsion, the lingual
enamel plate has been rotated to the posterior surface of the tooth.
Therefore, the tooth is provided with two transverse enamel plates,
including the plate on the anterior wall of the tooth. The labial
plate is greatly reduced, its total surface being restricted to the
small labial inflection. The highly specialized obcordate M3 is not
found in the most specialized platycephalic skulls characteristic of
the _gymnurus_ species-group. Instead the _gymnurus_-group retains the
primitive subtriangular pattern without significant modification.
_Referred species._--Seven:
_castanops_ species-group
_Pappogeomys castanops_ (Baird, 1852). Report Stanbury's Exp'd. to
Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's
Fort," near present town of Las Animas, Colorado.
_Pappogeomys merriami_ (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6,
12:271, October. Type from "southern Mexico," probably Valley of
México (see Merriam, 1895:152).
_gymnurus_ species-group
_Pappogeomys fumosus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
7:165, September 29. Type from 3 mi. W Colima, Colima.
_Pappogeomys gymnurus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
7:166, September 29. Type from Zapotlan (Ciudad Guzman),
Jalisco.
_Pappogeomys neglectus_ (Merriam, 1902). Proc. Biol. Soc.
Washington, 15:68, March 22. Type from Cerro de la Calentura,
about 8 mi. NW Pinal de Amoles, Querétaro.
_Pappogeomys tylorhinus_ (Merriam, 1895). N. Amer. Fauna, 8:167,
January 31. Type from Tula, Hidalgo.
_Pappogeomys zinseri_ (Goldman, 1939). Jour. Mamm., 20:91, February
15. Type from Lagos, Jalisco.
PHYLOGENY OF THE GEOMYIDAE
The fossil record of the Geomyidae provides a sequence of morphotypes,
each representing a stage in the phyletic development of the family.
Most of the preserved specimens probably represent the stufenreihe
rather than the ahnenreihe, as Simpson (1953:219-220) points out. Even
so, the stufenreihe closely approximates the general trend of
evolution, and the level of structural organization in the different
stages of phyletic development may be ascertained. The actual
ancestral series of most lineages probably will remain unknown, but
hopefully some of the existing gaps will be filled by future
discoveries. From the established record, several clearly defined
lineages can be distinguished; in fact the sequence of origin, pattern
of evolution, and specializations, of the principal lineages are
reasonably well expressed.
Primitive Morphotype
In the earliest known geomyids from the Upper Oligocene and Lower
Miocene, the premolars and molars are biprismatic and bilophodont. In
rodents, this is itself a specialized pattern, and is thought to have
evolved from a more primitive sextituberculate prototype by the union
of individual cusps, and probably also cuspules, forming the two
transverse enamel lophs. The primitive, common ancestor of the
Geomyidae and Heteromyidae with sextituberculate teeth in the early
Tertiary is unknown.
As soon as geomyids attained the early bilophodont stage of evolution,
the basic morphological structure of the family was established. The
family probably first became clearly distinguished from other
Geomyoidea at this stage. In the early bilophodont stages of
evolution, owing to the relatively deep valley between them, the two
columns probably failed to unite in the normal cycle of wear, as they
do in all later geomyids. _Griphomys_ described by Wilson (1940:93)
from the late Eocene of California, has a bilophate pattern in which
the anterior and posterior lophs are separated by a persistent
transverse valley. The occlusal pattern of _Griphomys_ closely
resembles a stage through which the ancestors of the early Miocene
geomyids must have passed in their pre-Miocene evolution, as Wilson
suggests (1949:115-116). Although he (1940:95; 1949:110-118)
tentatively referred _Griphomys_ to the superfamily Geomyoidea and
Simpson (1945:80) went so far as to refer it to the family Geomyidae,
with a notation of _incertae sedis_, its exact relationship to the
pocket gophers is uncertain. However, the structure of the molariform
dentition of _Griphomys_ does not exclude it from the phyletic
ancestry of the Geomyidae. In subsequent stages of evolution the
anterior and posterior columns become united. Thereby part of the
valley floor between the transverse prisms was progressively elevated,
to the stage where attrition on the occlusal surface would unite the
two columns. On the unworn enamel cap of living geomyids the two
transverse enamel folds are separated by a shallow but well defined
valley, briefly reflecting the ancient ancestral pattern.
Union of the lophs may have been either at the mid-points of the two
columns or at the edge of their protomeres. [A protomere is the half
of a tooth containing the protocone or protoconid--lingual side of
upper tooth and labial side of lower tooth. The paramere is the
opposite half of a given tooth--labial side of upper tooth and
lingual side in lower tooth. See Miller and Gidley, 1918:434.] Union
of the columns at the mid-points would have produced the figure-8
occlusal pattern (or H-pattern), which is characteristic of the early
Miocene Geomyinae (_Dikkomys_). Union of the two columns at the
protomeres would have produced the U-shaped pattern of the
Entoptychinae, which also occurred in the early Miocene and were
contemporary with the earliest Geomyinae. Since pre-Miocene geomyids
are unknown, the actual phyletic development of the dentition is a
matter of speculation. Probably the development of the two divergent
lineages, one leading to the Entoptychinae and the other to the
subfamily Geomyinae, occurred in the Oligocene (as depicted in Fig.
3). Of the two lineages, the subfamily Geomyinae, in my view, is the
more primitive and less specialized. Support for this view is
furnished by a reconstruction of the pattern of occlusal wear in
_Dikkomys_ and _Pliosaccomys_, especially on the first and second
molars.
In _Dikkomys_, the anterior and posterior column first unite near
their mid-points in the first stages of wear thus producing a figure-8
shaped (H-shaped) occlusal pattern in the premolar and all three
molars. Evidently in the first two upper molars, the columns unite
closer to their lingual margins than their mid-points, but at any rate
both outer and inner re-entrant folds are evident at this stage of
wear. With continued attrition on m1 and m2 of _Dikkomys_, the
anterior and posterior columns secondarily unite at the edge of their
labial margins thus enclosing a fossette of enamel in the labial half
of the tooth. The lateral coalescence at the ends of the protomeres
occurs because of the shallow vertical depth of the labial re-entrant
fold, and the fossette itself does not reach the base of the crown and
with continued wear it too would disappear, but not until the last
stages of wear, at least in _Dikkomys matthewi_. The lingual
re-entrant fold is deep, and therefore, persistent through all stages
of wear. Although the amount of wear required for its effacement would
be great, the occlusal configuration of the first and second lower
molars in _Dikkomys_ could be eventually ground down to a U-pattern as
in the entoptychids. Only one upper molar of _Dikkomys_, the first,
has been recovered (see Wood, 1936:23, fig. 32B). Although the tooth
is in an early stage of wear, the lingual valley is minute. Less
attrition than required in m1 and m2 would progressively reduce the
lingual fold until it too would essentially form a U-pattern, perhaps
retaining a slight lingual inflection. Hence, the first upper molar
becomes a mirror image of the first lower molar, and the second upper
molar probably had the same pattern as the first (at least it does so
in _Pliosaccomys_). Both of the lateral re-entrant folds of the
premolar are deep vertically, and consequently would not disappear
with occlusal wear. Therefore, the H-pattern of the premolars is
retained throughout life.
The m3 (M3 unknown for _Dikkomys_ or _Pliosaccomys_) also has deep
lateral folds; hence, it too retains the H-pattern in all stages of
attrition, although the isthmus between the two prisms may become
wider in the final phases of wear (as it does in _Pliosaccomys_).
In _Pliosaccomys_, the stages of wear are essentially the same as
those described for _Dikkomys_, except that the anterior and posterior
loph of the first and second molars tend to unite closer to one side
of the tooth, lingual side in upper molars and labial in lower. Only a
slight inflection of the re-entrant fold is evident on the side of
union, and the inflection disappears in the first phases of wear as
the columns unite. Concomitant with the lateral shift in the initial
point of coalescence of the transverse lophs, the occlusal penetration
of the re-entrant fold from the opposite side increases in horizontal
depth, and the fold extends medially more than half way across the
occlusal surface, thus forming a pattern essentially like that of the
entoptychids. The U-pattern in _Pliosaccomys_ appears in the initial
stages of wear without going through an earlier H-pattern as is the
case in its Miocene ancestors of the genus _Dikkomys_, unless the
minute inflection is considered as indicative of that stage. The two
columns of the premolar and m3 are joined near their mid-points as in
_Dikkomys_; therefore, they retain their primitive H-pattern, a
feature unique to the Geomyinae.
The evolutionary trend toward an ontogenetically earlier U-pattern in
the first two molars in the primitive lineage of the Geomyinae
suggests that the U-pattern characteristic of the Entoptychinae was
simply an earlier tendency toward the same specialization that
occurred later in the subfamily Geomyinae. If so, early entoptychines
would have been characterized by an H-pattern in the first stages of
attrition, like _Dikkomys_, and later developed union at the edge of
the protomeres. However, in the entoptychines, all the molariform
dentition, and not merely the first and second molar, became
specialized; consequently the U-pattern was produced on the occlusal
surfaces of each of the cheek teeth. As in _Pliosaccomys_, the
transitional phase, in which the two columns were united at their
mid-points, was eventually eliminated from the pattern of wear and
only the U-pattern, that now appeared in the initial stages of wear,
was retained. In the entoptychines of the early Miocene there is no
suggestion of the H-pattern that characterizes the Geomyinae, except
in the position of the cusps before wear in the lower molars of
_Pleurolicus sulcifrons_, which, according to Wood (1936:6), suggests
the H-pattern. In earlier unknown Oligocene stages of evolution, the
prisms possibly united first at their mid-points, and the columns may
have joined at the side of the tooth only in the terminal stages of
wear. The U-pattern of pre-Miocene entoptychines, therefore, may have
become the dominant occlusal pattern only in the later stages of
phyletic development.
According to the recently expressed views of several paleontologists,
the Entoptychinae constitute the primitive lineage of the family and
the early Geomyinae constitute a specialized offshoot of the
entoptychine ancestral assemblage. The structure of the Entoptychinae,
especially of the less advanced genera, closely approximates that of
the hypothetical primitive morphotype. But, according to my view, the
subfamily Geomyinae constitutes the ancestral assemblage and its
structure is essentially that of the primitive morphotype of the
family. At any rate the structure of the early geomyines more closely
approximates the structure of the ancestral stock than the more
divergent entoptychines. Therefore, the genus _Dikkomys_ of the early
Miocene, the first known geomyine, is considered to be a generalized
geomyid, and, although it is a contemporary of the more specialized
entoptychid assemblage, is considered to be more closely allied to the
ancestral stock.
The entoptychines were the dominant and most highly differentiated
geomyids of the early and middle Miocene. Nevertheless, they became
extinct in the middle Miocene, and the geomyines of that time survived
and later gave rise to the modern pocket gophers. Therefore, the early
history of the family Geomyidae is characterized by an early radiation
and trend toward specialization, followed by survival of the less
specialized Geomyinae and extinction of the more specialized
Entoptychinae.
Entoptychid Radiation
The most abundant geomyids of the early and middle Miocene, the
Entoptychinae, consisted of at least 24 species (see Wood, 1936:4-25)
classified in four genera: _Pleurolicus_, _Gregorymys_, _Grangerimus_,
and _Entoptychus_. The genera were essentially contemporaneous (see
Figure 3). Even so, the subfamily was morphologically varied, pointing
to an earlier origin in the Oligocene (actually a part of the John Day
Fauna, including _Pleurolicus_ may be correlated with late Oligocene
Whitneyian age) followed by a relatively rapid radiation including all
four genera in the early Miocene. Two genera, _Pleurolicus_ and
_Gregorymys_, continued into the Middle Miocene (Hemingfordian). This
divergence, specialization, and subsequent radiation suggest that the
entoptychines evolved into a new major adaptive zone, in the sense
described by Simpson (1945:199-206).
The radiation is correlated geographically and temporally with the
southward retreat of the Neotropical flora of the Tertiary from the
western United States and southward movement of the Arctic flora of
the Tertiary (see Axlerod, 1950; Berry, 1937:31-46; Chaney,
1947:139-148; and Kendeigh, 1961:280-283). In the early Tertiary the
Neotropical-tertiary geoflora occurred northward to at least 49°
latitude in western North America, and the boreal Arctic-tertiary
flora was restricted to a circumpolar zone. The southward and eastward
shift of the Neotropical-tertiary flora, associated with the drying
and chilling of the continent, began in the middle or late Oligocene
and was concurrent with the divergence and radiation of the
Entoptychinae. Beginning in late Oligocene and continuing at least
into middle Miocene, most of the region in which the entoptychines
occurred was occupied by the Arcto-tertiary geoflora of which the
temperate forest division contributed the dominate plant associations.
The maples, chestnuts, dogwoods, beeches, walnuts, oaks, elms,
birches, and sycamores of that flora were the forerunners of today's
eastern deciduous forest. It is my view that the entoptychines became
adapted to the conditions of this paleoecological environment and
radiated rapidly in the Arikareean when the major change occurred in
climax vegetation. The ancestral stock of the Geomyinae was not so
successful in the Arcto-tertiary climax, and most of it probably was
displaced southward along with the tropical flora.
The skeleton in the entoptychines is not so strongly fossorial as in
the modern geomyids (Wilson, 1949:117), and these early geomyids
probably were semi-fossorial with somewhat the same burrowing habits
as those of the living mountain beaver (_Aplodontia_). Inasmuch as the
morphology and taxonomy of the entoptychines were discussed in detail
by Cope (1884) and reviewed later by Wood (_loc. cit._), there is no
need to recount the details here. According to Wood (_op. cit._,
27-28), _Pleurolicus_ occupied a central position in the entoptychid
radiation and perhaps appeared slightly earlier than the other genera.
Wilson (1949) suggested that the lower part of the John Day may
actually be Upper Oligocene rather than Lower Miocene, and this
arrangement is followed here. Also, _Pleurolicus_ is less specialized
than the other genera and occurs in deposits of both the Great Plains
and the Pacific Coast. _Gregorymys_, also little specialized, occurred
only on the Great Plains. The more specialized genera, _Grangerimus_
and _Entoptycus_, evidently appeared somewhat later than _Pleurolicus_
and evolved from it. Except for a record from southern Texas reported
recently by Hibbard and Wilson (1950:621-623) and the new species
described by MacDonald (1963:182) from the Sharps Formation of South
Dakota (early Arikareean), _Grangerimus_ is known only from the
Pacific coast. _Entoptycus_ was restricted to the Pacific Coast (John
Day fauna).
_Entoptycus_ is the most specialized of the known genera; it has
pronounced fossorial adaptations, especially in the skull. Its
molariform teeth are rootless and ever-growing as in the modern
geomyines. Moreover, the continuous enamel bands on only moderately
worn teeth become separated in the final stages of wear into anterior
and posterior enamel plates by tracts of dentine that extend toward
the crown on the sides of each tooth. This extension was made possible
by the union of the two columns at both the lingual and labial margins
of the tooth forming an O-pattern, and the crown is essentially
monoprismatic save for the isolated enamel fossette in the center of
the tooth. The fossette is all that remains of the lateral re-entrant
fold that characterized the preceding U-pattern of the earlier stages
of wear. Late in the sequence of wear, the anterior enamel plate is
lost in the lower molars and the posterior plate in the upper molars.
The U-pattern characterizes the final stages of attrition in the other
genera of the Entoptychinae; none developed the dental specializations
seen in _Entoptycus_. Rootless, ever-growing cheek teeth,
discontinuous enamel patterns, and monoprismatic molars were not
evolved in the subfamily Geomyinae until the late Pliocene.
Phyletic Trends in Subfamily Geomyinae
The subfamily Geomyinae is made up of three groups, recognized
taxonomically for the first time in this account as
tribes--Dikkomyini, Thomomyini, and Geomyini (for full discussion of
classification, see previous account). The phylogeny proposed by me is
illustrated in Figure 3. The tribe Dikkomyini is characterized by
generalized and primitive features that together form the basic
structural foundation of the subfamily. Evolution within the
Dikkomyini resulted in the acquisition and perfection of fossorial
adaptations. The Thomomyini and Geomyini are considerably more
specialized than the ancestral Dikkomyini from which they evolved.
The Geomyini are clearly more specialized than the Thomomyini,
suggesting closer affinity between the Thomomyini and the Dikkomyini
than between the Geomyini and the Dikkomyini. The specializations in
the dentition and the associated changes in the skull of the
Thomomyini and Geomyini permit more efficient mastication of fibrous
vegetation. Along with these specializations, fossorial adaptations
inherited from the Dikkomyini are retained without noteworthy
modification.
_Dikkomys_, the earliest known genus of the tribe Dikkomyini, can be
taken as a starting point of evolution for the subfamily Geomyinae.
The Pliocene genus _Pliosaccomys_ is the only other known geomyine
having primitive features closely resembling those of _Dikkomys_. The
relatively close but previously unrecognized relationship between
_Dikkomys_ and _Pliosaccomys_ can be understood when patterns of wear
on the occlusal surfaces of the cheek teeth are taken into account. It
appears that _Pliosaccomys_ descended from _Dikkomys_-like stock, if
not _Dikkomys_ itself. Although _Dikkomys_ is towards the beginning of
this phyletic sequence and _Pliosaccomys_ towards the end of the
sequence, the primitive features shared by the two provide a
generalized morphotype for the subfamily Geomyinae.
In the molariform dentition, an almost complete series of stages of
wear in _Pliosaccomys_ has been preserved, and those of _Dikkomys_ can
be reconstructed with reasonable accuracy from those that are known
(see Fig. 4):
(1) In the initial stage of wear in _Dikkomys_ the anterior and
posterior columns are separated by an intervening valley (Fig. 4A),
and the occlusal surface of each column bears a loph of dentine
surrounded by a ring of enamel: protoloph on the anterior column and
metaloph on the posterior column of the upper teeth (protolophid and
hypolophid in corresponding positions in the lower teeth). Actually
this stage is not preserved in the known material of _Dikkomys_, but
does occur in both geomyines and entoptychines in all stages of
evolution, and it must have also occurred in _Dikkomys_ in order for
the next two stages, which are preserved, to have developed.
(2) The occlusal surfaces are ground down to a level where the enamel
loops of the two columns join at their mid-points, thus forming an
H-shaped pattern (Fig. 4B), or more exactly a pattern resembling a
figure 8. Probably this was the primitive pattern in the final stage
of wear in the geomyid ancestor of the Oligocene.
[Illustration: FIG. 3. Diagram depicting geologic range and probable
phyletic relationships of the family Geomyidae. Dashed lines represent
parts of lineages that are not represented by fossil records, and
solid lines represent parts of lineages verified by actual specimens.
Question marks indicate uncertainty of suggested ancestry of known
taxa. The relationships within the subfamily Entoptychinae are
modified after Wood (1936), and the temporal range of the Miocene
geomyids have been adjusted to agree with current stratigraphic
correlations. Hence, _Pleurolicus_, _Gregorymys_ and _Dikkomys_ are
illustrated as ranging into the Hemingfordian, rather than being
confined to the Arikareean (see MacDonald, 1963, and Black, 1961).]
(3) In the pre-final stage of wear, the anterior and posterior lophs
of the first and second molars unite secondarily at the edge of their
protomeres (labial side in the lower and lingual in the upper), thus
enclosing an isolated enamel fossette (Fig. 4C). Lateral union occurs
in the lower teeth because the vertical depth of the labial re-entrant
angle is less than the depth of the lingual re-entrant fold. In the
upper teeth the reverse is true. The re-entrant angle on one side of
the premolar is as deep vertically as the angle on the other side of
that tooth, and both reach the base of the crown; therefore, they do
not disappear at any stage of attrition. The same pertains in the
third lower molar.
(4) In the final stage of wear (Fig. 4D), the enamel fossette
disappears as a result of continued attrition on the occlusal surface
in the upper series. The fossette may vary somewhat in vertical depth
in m1 and m2, but the amount of wear required for its effacement would
be greater than in the upper teeth. Therefore, upon wear, the
U-pattern would become characteristic of the final stage in M1 (and
probably also M2), but the modified H-pattern described in Fig. 4C
would prevail in m1 and m2. Perhaps, in extremely worn teeth, the
labial fossette of m1 and m2 would disappear. If this advanced stage
of effacement is obtained, then the two columns would be united across
the entire surface of their protomeres from the center of the crown to
its labial edge, and the occlusal pattern would be in the shape of a
U.
The occlusal pattern, at least in M1 and M2, in the final stages of
wear in _Dikkomys_ resembles that in the subfamily Entoptychinae, but
the U-pattern develops on only the first and probably the second molar
in _Dikkomys_ and not on all of the cheek teeth as it does in the
entoptychines. Judging from the material that has been described, the
U-pattern did not develop in the lower teeth of _Dikkomys_ until the
Hemingfordian (_D. woodi_), upper Rosebud, and specimens of _D.
matthewi_ from the earlier Arikareean, lower Harrison, suggest that
the modified H-pattern, with secondary coalescence at the edge of the
protomeres, persisted throughout life, without developing the
U-pattern in the final stages of wear.
Essentially the same patterns of wear characterize the genus
_Pliosaccomys_, except that the earlier stages were telescoped and the
second stage was omitted while another (final) stage was added. The
stages are reconstructed in sequence in figure 4, and all are based on
preserved dentitions, as follows:
(1) The first phases of wear produced the pattern (Fig. 4E and I)
described for _Dikkomys_ in the previous account (Fig 4A).
(2) A small additional amount of wear produced the 2nd stage (Fig. 4F
and J) characterized by a U-pattern, formed by union of the anterior
and posterior columns at the edge of the protomeres of the first and
second molars, both above and below, without first forming an H-shaped
pattern. Union at the mid-points thus was omitted from the sequence of
wear in these two teeth. In the premolars and third molars the
primitive H-pattern did form, as in _Dikkomys_. The pattern of wear in
the first two molars is the same as in the entoptychines of the early
Miocene. The trend of evolution through which the _Pliosaccomys_
lineage passed must have featured a progressively earlier union at the
edge of the tooth until the lateral coalescence occurred
simultaneously with the median union. At that stage, emphasis was
shifted to the union at the edge of the tooth, and eventually the
teeth failed to unite at their mid-points and the U-pattern developed
directly. Therefore, the horizontally deep re-entrant fold that
separates the two lophs of the U-pattern is equivalent to one fold
plus the apex of the opposite fold.
(3) The horizontal re-entrant fold of the U-pattern was remarkably
shallow vertically and disappeared with little additional wear. Thus
the two parts of M1, and also of M2, are united into a single column
except for a slight inflection on the labial side and this is true
also of m1 and m2 except for a slight inflection on the lingual side
(Fig. 4G and K). The inflection appears to have persisted in the upper
teeth (Fig. 4H), but evidently with slight wear, disappeared in the
lower teeth (Fig. 4L). The final monocolumnar pattern was attained
early ontogenetically, evidently before the permanent premolar had
fully erupted; hence, the earlier stages occurred only in transition,
persisted for only a brief interval in the teeth of juveniles, and the
final stage developed in the young animal and lasted throughout the
rest of its life in _Pliosaccomys_. In _Dikkomys_ the two columns
never united into a single column, and a bilophodont occlusal pattern
persisted throughout life.
The early phyletic development of the subfamily Geomyinae took place
in the tribe Dikkomyini from the early Miocene into the early
Pliocene. Compared with the rapid evolution of the specializations
that distinguish the Entoptychinae, the structural changes in the
early Geomyinae occurred at a remarkably slow rate. In fact the
lineage changed but little from _Dikkomys_ to _Pliosaccomys_, in parts
of the animal that can be compared, as illustrated by the low-crowned
and rooted cheek teeth, the continuous enamel bands, the lack of
grooving of the upper incisor, the retention of the primitive
H-pattern, both above and below, in the premolar and third lower
molar, and the ridges and fossae of the mandible to which the muscles
of mastication attach. The only major changes detected in the known
fragments are in the pattern of wear and the final configuration of
the first and second molars, as described above. The unification of
the two lophs in each of these two teeth into a single column was a
significant step in the evolution of the Geomyinae, and is a stage
between the primitive bilophodont pattern of the early and middle
Miocene geomyines having continuously bicolumnar teeth and the
monolophodont pattern in the modern pocket gophers of both lineages in
which these teeth consist of a single column in all but the initial
stages of wear. The monocolumnar structure of the first and second
molars in the final stages of wear, therefore, is closer to that in
the lineage of _Thomomys_ than it is to that of _Dikkomys_. Other
specializations in the dentition of _Pliosaccomys_, especially in m1
and m2 where the H-pattern has been completely eliminated from the
sequence of wear, are too far advanced for _Pliosaccomys_ to have
given rise to the tribe Geomyini. The teeth in the immediate ancestor
of the Geomyini must have been less specialized in m1 and m2, perhaps
about as in _Dikkomys_. In the m1 and m2 of the tribe Geomyini, the
H-pattern is formed in the initial stages of wear; therefore, in the
early Pliocene ancestor, presently unknown in the fossil record, the
H-pattern probably was present. Even so, the ancestor of the Geomyini
and that of _Pliosaccomys_ probably were closely allied otherwise, and
both probably had attained the highly specialized fossorial
adaptations characterizing all modern pocket gophers, before the
divergence of _Pliosaccomys_ and the Geomyini took place.
The evidence points to a major divergence of the geomyines that lived
in the latest Miocene or the early Pliocene (probably the latter) and
that gave rise to the two modern lineages, Thomomyini and Geomyini
(see Fig. 3). One, the most primitive of the two, gave rise to the
Thomomyini lineage that eventually evolved into _Thomomys_.
_Pliosaccomys_ is closely allied to the ancestry of this lineage,
although it is probably not the actual ancestor, as mentioned
previously. Aside from the aforementioned specializations of the first
and second molars, the features of the Thomomyini are less advanced
than in the other specialized lineage (tribe Geomyini). Primitive
traits retained in the tribe Thomomyini (and also characteristic of
the ancestral tribe Dikkomyini) are: (1) Small size, in general no
larger than the ancestral morphotype; (2) lack of grooving on the
upper incisor (although a slight rudimentary groove is developed
rarely in some living species); (3) retention of anterior and
posterior enamel plates in lower and upper cheek teeth; (4) premolars
having widely open re-entrant folds; (5) smooth and generalized skull
lacking marked angularity, regosity or cresting (neither the sagittal
nor the lambdoidal crest are ordinarily well developed except in
_Thomomys bulbivorus_); (6) forefoot small, less modified for digging
than in the Geomyini.
[Illustration: FIG. 4. Drawings of the molariform dentitions of
_Dikkomys_ and _Pliosaccomys_ (Tribe Dikkomyini) depicting the
patterns of wear on the occlusal surfaces. Ontogenetically, the
stages of wear are arranged from left to right in each row. Stages
not represented by actual specimens have been carefully
reconstructed from information provided by known stages in the
sequence of wear and the dentitions of other geomyines. × 5.
A-D. _Dikkomys woodi_, right lower tooth-row, including p4-m3.
Patterns based on No. P26284 (FMNH) from Upper Rosebud (Middle
Miocene), Shannon Co., South Dakota (B above).
E-H. _Pliosaccomys dubius_, left upper tooth-row, including P4-M2
(M3 unknown). Patterns based on Nos. 1798 and 1799 (LAM) from
Smiths Valley (Middle Pliocene), Lyon Co., Nevada.
I-L. _Pliosaccomys dubius_, right lower tooth-row, including p4-m3.
Patterns based on Nos. 1796 (holotype), 1804, and 1806 (LAM)
from Smiths Valley (Middle Pliocene), Lyon Co., Nevada.
]
The lineage of the Thomomyini is essentially rectilinear and without
the major branching seen in the tribe Geomyini. The one genus,
_Thomomys_, appears first in the Upper Pliocene (early Blancan time),
and the specializations characterizing the lineage had already
developed by that time. Evidently, the early stages of divergence from
the ancestral stock resulted in the development of rootless,
ever-growing, more hypsodont cheek teeth, simplification of M3, and
enlargement of the masseteric ridge on the mandible. The enamel
investment on the sides of the molariform teeth is interrupted owing
to intrusion of tracts of dentine on the sides of each column. Even
so, complete anterior and posterior plates are retained on all of the
cheek teeth (Fig. 5, K and L) and there is no trend toward additional
loss of enamel as in the Geomyini. The enamel on the sides of the
column has little functional value, and its elimination probably
reduces friction during the anteroposterior movements of the lower
jaw, thereby increasing the efficiency of the cutting blades on the
anterior and posterior wall of the tooth. The simplification of M3 was
achieved by union of the two columns of the primitive pattern into a
single column and obliteration of both the labial and lingual
re-entrant folds in the first stages of wear. The adult tooth (see
Fig. 5L) is without trace of the bilophate pattern and is not
elongated; therefore, its structure is essentially the same as that of
the first and second upper molars.
In the Thomomyini, the two lophs of the unworn molars unite entirely
across the width of their surfaces with the first traces of wear (see
Fig. 5, I and J), owing to the shallow and uniform depth of the
transverse valley. In the molars, the final pattern is acquired,
therefore, before the deciduous premolar has been replaced by the
permanent tooth. A relatively shallow re-entrant inflection between
the ends of the parameres sometimes is retained, although it also will
disappear with slight additional wear. Therefore, both lophs tend to
unite completely with the first stages of wear in the Thomomyini, thus
omitting both U and H patterns from the sequence of wear. This is the
highest degree of specialization attained in the Geomyidae in regard
to the patterns of wear, since a sequence of bilophodont patterns
appear in both the Dikkomyini and Geomyini before the monoprismatic
pattern is developed.
[Illustration: FIG. 5. Drawings of molariform dentitions
representative of the tribes Geomyini and Thomomyini depicting
patterns of wear on the occlusal surface. A-D represent, in
ontogenetic sequence from left to right, upper tooth-rows of the
tribe Geomyini. E-H represent, in the same sequence of stages,
lower tooth-rows of the tribe Geomyini. I-L represents both upper
and lower tooth-rows of both pre-final and final stages of wear in
the tribe Thomomyini. All × 5.
A and E. _Geomys bursarius majusculus_, No. 2948 (KU), Douglas Co.,
Kansas. Right upper (A) including DP4-M3; lower left (E)
including dp4-m3.
B and F. _Pappogeomys bulleri burti_, No. 100444 (KU), 10 mi. NNW
Barra de Navidad, Jalisco. Right upper (B) including P4-M3;
right lower (F) including p4-m3 (both P4 and p4 with unworn
enamel caps).
C and G. _Pappogeomys bulleri albinasus_, No. 31044 (KU), 10 mi. S
and 8 mi. W Guadalajara, Jalisco. Right upper (C) including
P4-M3; right lower (G) including p4-m3.
D and H. _Pappogeomys bulleri albinasus_, No. 31002 (KU), W side La
Venta, 13 mi. W and 4 mi. N Guadalajara, Jalisco. Right
upper (D) including P4-M3; right lower (H) including p4-m3.
I and J. _Thomomys talpoides bridgeri_, No. 6865 (KU), 2 mi. up Mink
Creek, Pocatella, Bannock Co., Idaho. Left upper (I),
DP4-M3; left lower (J), dp4-m3.
K and L. _Thomomys talpoides fossor_, No. 13205 (KU), Wasson Ranch,
3 mi. E Creede, Mineral Co., Colorado. Right lower (K),
p4-m3; left upper (L), P4-M3.
]
Relationship of the Geomyini with the ancestral Dikkomyini is most
clearly demonstrated in the sequence of wear on the occlusal surfaces
of the molars. As in all geomyids, the upper part of the crown is
biprismatic in the newly erupted tooth, and the two columns are
separated by an intervening valley. With slight attrition on the
unworn enamel cap, the weakly developed cusps merge and form a
transverse enamel loop on each of the two columns (see third molar in
Fig. 5, A and E), each loop enclosing a core of dentine that had
become exposed. The valley between the two columns is shallow, and
upon further wear of the tooth, the two loops unite. The two columns
become joined at different points in the upper and lower molars
depending on the varying depth of the valley in different teeth.
Therefore, upper and lower molars develop distinctly different
occlusal configurations.
In the lower molars, the pattern characteristic of _Dikkomys_ (Fig.
4C) is preserved without significant modification, as illustrated in
an immature specimen of _Geomys_ (see Fig. 5E). The H-pattern and
modified H-pattern are developed in the same sequence of wear in the
Geomyini. A juvenal female (not illustrated), KU 2931, provides an
example of the intermediate H-pattern. In this specimen, the
protolophid and hypolophid of the left m2 are united only at their
mid-points, indicating that the pattern of wear occurs in the same
sequence in the Geomyini as it did in the Miocene genus _Dikkomys_.
After the two columns have become united at their mid-points, a
secondary union is formed at the edge of their protomeres, thus
enclosing the enamel fossette as illustrated in Figure 5E (this is the
modified H-pattern mentioned above). However, the fossette itself is
shallow and soon disappears with slight wear. At this stage, the
occlusal configuration would be in a U-pattern (m1 in Fig. 5E). The
lingual re-entrant fold is also shallow in vertical depth; therefore,
it is obliterated by wear following the eradication of the labial
fossette. Consequently, the two columns are united into one. In m3
(see Figs. 5E, F, and G), the two columns merge by progressive lateral
expansion of the medial isthmus.
In the first and second upper molars, the two columns unite across the
entire surface of their protomeres from near the lingual edge of the
crown to near its center. A minute inner inflection may be temporarily
retained in some teeth. At this stage (see Fig. 5B), the parameres are
still separated by the labial fissure, and the occlusal pattern is in
the shape of a U, resembling, but not exactly duplicating, the
pre-final pattern of Ml and M2 in the genus _Pliosaccomys_ (see Fig.
4H). The labial fissure is shallow, and, with further wear, the
inflection is worn away and the parameres also unite, thereby forming
a monoprimatic crown in the final stage. In M3, the two lophs first
become united near the edge of their protomeres (see Fig. 5B),
therefore forming a U-pattern similar to that developed in Ml and M2
of _Pliosaccomys_. The connection of the two lophs is not directly at
the end of the protomere; consequently a shallow lingual inflection
remains. The lingual edge of the valley is also shallow, and, with
continued wear a second union of the two lophs takes place near the
ends of their parameres, and the deeper, interior part of the valley
remains as an isolated enamel fossette (see Fig. 5C). The two primary
lophs of the tooth are now joined near both sides, having shallow
lingual and labial re-entrant angles on the sides and the enamel
island in the center. With continued effacement of the occlusal
surface, the fossette will be eradicated, and the pattern of the
occlusal surface will become the partially biprismatic pattern of the
final stages (adult) of wear (see Fig. 5D). M3's of _Dikkomys_ and
_Pliosaccomys_ are not known; however, it seems reasonable to assume
that the pattern of wear in the M3 of Dikkomyini was not essentially
different from that of the Geomyini, except that it is likely that the
U-pattern of the second stage of wear in the Geomyini was probably the
final stage in the genus _Dikkomys_.
Judging from the pre-final stages of wear, the dentition of the
Geomyini provides a curious combination of patterns that resemble in
part the Miocene genus _Dikkomys_ and in part the early and middle
Pliocene genus _Pliosaccomys_. There is no significant variation in
the premolars or third molars (at least in the lower teeth) of the
Geomyinae from the early Miocene to late Pliocene; therefore,
deviations of major significance are in the character of the first and
second molars. In the Geomyini, the patterns of wear of m1 and m2 are
the same as those of _Dikkomys_, and are distinctly different from
those of _Pliosaccomys_ where the two columns first unite at the edge
of their protomeres to form a U-pattern, rather than at their
mid-points to form an H-pattern. Even though the intermediate stages
of ontogeny in m1 and m2 of _Pliosaccomys_ and the Geomyini are
entirely different, the bicolumnar crowns of both eventually unite,
upon wear, into a single column. On the other hand, the patterns of M1
and M2 in the Geomyini most closely resemble those of _Pliosaccomys_,
rather than _Dikkomys_. In this regard it should be pointed out that
the upper molars of _Dikkomys_ are presently represented by only one
tooth, an M1 in an early stage of wear. As described already, the
patterns of M1-2 evidently would be mirror images of m1-2 in
corresponding stages of wear. However, the initial union of the two
columns, in the M1 that is known, is somewhat to the lingual side of
center and the relatively small lingual valley does not reach the base
of the crown, indicating, that eventually with wear, the two columns
of _Dikkomys_ might have become united across the entire surface of
their protomeres as in _Pliosaccomys_. Even so, the two columns of M1
do initially join closer to their mid-points than they do in
_Pliosaccomys_, and, if they did actually unite across their
protomeres, the union would have occurred with subsequent wear. That
is, the first occlusal pattern would be H-shaped (but with the
connection closer to the lingual than the labial side), as in m1 and
m2, and it would become U-shaped only after additional wear. This
sequence of patterns of M1 and M2, as already pointed out, does not
pertain in _Pliosaccomys_ or the Geomyini, since the U-pattern is
formed with the first union of the two columns at the edge of their
protomeres, and the primitive H-pattern is never developed, unless one
counts the slight lingual inflection, that occasionally is formed just
after the two columns unite, as being indicative of the primitive
pattern. As in the lower teeth, the bicolumnar crowns of early
ontogeny in both _Pliosaccomys_ and the Geomyini become eventually
united, with wear, into a single column.
Based upon the foregoing evidence, it would seem likely that the
Geomyini evolved from an early Pliocene (perhaps late Miocene)
Dikkomyini ancestor that had evolved the specializations of M1 and M2
that characterize its relative, _Pliosaccomys_, but had not also
evolved the specializations of m1 and m2 that distinguish
_Pliosaccomys_. Therefore, the ancestor of the Geomyini differed from
the _Pliosaccomys_-Thomomyini lineage in its retention, unmodified, of
the primitive patterns in m1 and m2 that characterized the earliest
known Geomyines (_Dikkomys_). The same patterns are preserved in m1
and m2 of its modern descendents, the living Geomyini. In the
_Pliosaccomys_-Thomomyini lineage the pattern of m1 and m2 are
entirely different, as described above.
The earliest record of the Geomyini is the extinct genus _Pliogeomys_
(see Fig. 6) in the latest Hemphillian (middle Pliocene) and earliest
Blancan (late Pliocene). _Pliogeomys_ is more primitive than any
modern genus of the Geomyini, seems to have been a late survivor of
the primitive stock, but was itself probably a collateral lineage and
not on the direct line of descent. The cheek teeth in _Pliogeomys_ are
rooted and less hypsodont than in the late Pliocene examples of the
modern genera, and the anterior enamel plate of the lower molars shows
no indication of reduction, as would be expected if _Pliogeomys_ were
in the direct line of evolution. Separation of _Pliogeomys_ from the
main stem of the Geomyini probably occurred after several
specializations had already been achieved by the Geomyini. Two
inheritances might have been grooving on the upper incisors and some
reduction in amount of enamel on the sides of the cheek teeth. The
dentine tracts on the sides of the cheek teeth of _Pliogeomys_ are
narrow (see Fig. 7A) and barely separate the enamel blades and there
is no discernible reduction in the anterior enamel blades on its lower
molars. Those blades evidently were lost in the main lineage before
the Pleistocene radiation of the living genera took place.
_Pliogeomys_ is in an intermediate stage in evolution, and was not so
advanced as was the main lineage at the time _Pliogeomys_ died out.
Its structure does provide clues as to phyletic development that took
place in the main lineage.
Specialized trends in the early phylogeny of the Geomyini included:
development of rootless, ever-growing cheek teeth and an increase in
hypsodonty; loss of the bicolumnar structure of the first and second
molars, and, consequently, the formation of a single elliptical column
in the final stage of wear; interruption of the enamel investment of
the molariform teeth and formation of anterior and posterior enamel
plates; and enlargement of the masseteric ridge and fossa. Each of
these trends occurred independently in the Thomomyini, and each is an
example of parallelism in the phyletic evolution of the two lineages.
Three additional specializations lacking in the Thomomyini are the
grooving on upper incisors, loss of anterior enamel plate in lower
molars, and development of a basitemporal fossa on the mandible.
Evidently, two grooves evolved in the ancestral incisors in the same
bisculcate pattern preserved in _Pliogeomys_, _Zygogeomys_ and
_Geomys_. The innermost groove is weakly developed in _Pliogeomys_,
suggesting that this character was in an intermediate stage of
evolution in the ancestral lineage at the time that _Pliogeomys_ split
off. Numerous other specializations in the Geomyini appeared later,
but evolved in the different genera that diverged from the ancestral
lineage and are discussed separately in the next account. Only two of
the major features characterizing the Dikkomyini are retained in the
Geomyini: the H-pattern on the occlusal surface of the m1 and m2
developed during the initial stages of wear, and the bicolumnar
pattern of M3. Adaptive radiation produced the living genera of the
Geomyini in the late Pliocene and early Pleistocene (see Fig. 6) and
subsequent specialization of the ancestral morphology followed.
Parallelism in the molars of later geomyines and the Entoptychinae is
illustrated by the lateral interruption of the enamel investment and
loss of enamel plates and by the omission of the H-pattern stage in
the first and second molars (in _Pliosaccomys_). Resemblance of
dentitions in certain stages of wear in _Pliosaccomys_ and in
entoptychines led some investigators, for instance, Hibbard
(1953:357), to suggest that _Pliosaccomys_ descended from one of the
less specialized entoptychines, possibly _Grangerimus_ but probably
_Gregorymys_. Actually, the highly specialized upper and lower
premolars and third molars of the entoptychines rule them out as
ancestors of the later geomyines. The evolution of entoptychine-like
features in _Pliosaccomys_ is regarded as an example of iteration, a
pattern of parallelism (see Simpson, 1953:248-253) where an
allochronic and independent lineage undergoes the same evolutionary
trend that phyletically characterized an earlier lineage, usually
after the latter has become extinct. In this case, the lineage giving
rise to _Pliosaccomys_ passed through the same phyletic stages in its
evolution in the early Pliocene (and possibly the late Miocene) as did
the entoptychines in the late Oligocene and early Miocene.
Another parallelism by iteration, occurring in the middle and late
Pliocene in both the Thomomyini and Geomyini, is the loss of enamel
from the lateral surfaces of the cheek teeth, and, in the Geomyini
only, the eventual loss of the anterior plate in the lower teeth and
the posterior plate in the upper teeth. Both features were evolved
more than an epoch earlier in the specialized entoptychid genus
_Entoptychus_ of the lower Miocene. In _Entoptychus_, only the
posterior plate of the lower molars and the anterior plate of the
upper molars remained in the final stages of attrition, although a
central enamel fossette, a remnant of the re-entrant fold, remained
throughout life. Iteration is also expressed in the subfamily
Geomyinae by the development of grooving on the upper incisor and the
formation of the basitemporal fossa. A shallow but distinct
basitemporal fossa occurs between the coronoid process and the third
lower molar in the genus _Entoptychus_ and a sulcated upper incisor, a
single shallow groove usually near the median border of the tooth, is
found in the genus _Gregorymys_ of the subfamily Entoptychinae. Both
features are regarded as advanced specializations in the tribe
Geomyini, even though each was evolved in the entoptychines of the
Lower Miocene.
The postcranial skeleton of living genera of pocket gophers, as befits
animals that spend most of their life within underground burrows, are
highly specialized for a fossorial life. Elements of the postcranial
skeleton recovered from Lower Miocene deposits indicate that the
entoptychines were only semi-fossorial (see Cope, 1884:857; Wood,
1936:4-5; Wilson, 1949:117-118). One of the basic trends of the
entoptychines was towards greater fossorial adaptation; the skeleton
of _Entoptychus_ shows a greater degree of fossorial adaptation than
earlier genera of the subfamily. There is no reason to suppose that
the geomyine genus _Dikkomys_, which lived at the same times as the
entoptychines, had acquired any more advanced fossorial adaptations
than had the entoptychines.
The most pronounced fossorial adaptations seem to have evolved only in
the ancestral lineage of the modern geomyines, probably in the latter
part of the Miocene and in the early Pliocene, before the modern
Thomomyini and Geomyini diverged. Extreme fossorial adaptations in
herbivorous rodents, such as those characteristic of the modern pocket
gophers and their immediate ancestors, are thought to have evolved
only in response to pronounced arid conditions. The Entoptychinae and
evidently the early geomyines lived in environments that were either
tropical or temperate, and under conditions more mesic than I would
consider necessary to bring about selection pressure resulting in
fossorial specializations. In late Oligocene and early Miocene,
according to Axelroad (1958:433-509), arid conditions did not exist in
the United States, and the only xerophytic environments in North
America occurred on the Central Plateau of México. Moreover (Axelroad,
_loc. cit._), arid conditions did not develop in the western United
States until the early Pliocene. Geomyids evidently became extinct in
this region at the close of the Middle Miocene, and none appear in
fossil deposits in the western United States until the latest Lower
Pliocene (Clarendonian). The reappearance of geomyids, _Pliosaccomys_,
in the western United States coincides with a trend toward aridity and
the northward movement of the Madro-tertiary geoflora into the Great
Basin and Great Plains from its place of origin on the Central Plateau
of México (Axelroad, _loc. cit._). Later, in the middle and later
Pliocene, the Madro-tertiary geoflora gave rise to the modern
xerophytic plants that now characterize the desert vegetation of North
America.
The Madro-tertiary climax does not appear as a major flora until the
Miocene, but probably originated earlier. According to Axelroad (_loc.
cit._), this xerophytic flora evolved from elements of the
Neotropical-tertiary geoflora that became adapted to arid conditions
that developed in the rain shadow of the high mountains flanking the
Central Plateau of México. Originally, the Madro-tertiary flora
consisted of small trees, shrubs, and grasses. Although some elements
of this flora moved northward in the late Miocene, the major part of
it remained in México until the early Pliocene. In the western United
States, mountain formation increased in intensity in the Pliocene and
continued on into the early Pleistocene. As the mountains became more
elevated, especially the Sierra Nevada and Cascade ranges, they
blocked the prevailing winds from the Pacific Ocean and extensive
aridity developed on their leeward side. As xeric conditions became
widespread, the Madro-tertiary flora successfully occupied the drier
regions of southern California, the Great Basin, and the western parts
of the Great Plains.
While the Entoptychinae probably evolved in response to the
Arcto-tertiary flora, the late Tertiary geomyines probably evolved in
response to the Madro-tertiary geoflora on the Central Plateau of
México. Some of these early geomyines, especially ancestors of the
modern lineages, probably were pushed southward by competition with
the more specialized entoptychines. Most geomyines were pushed out of
the northern area of distribution, except for _Dikkomys_ that survived
in association with the entoptychids throughout the early and middle
Miocene. During this time, and probably continuing on into the late
Miocene, the geomyines occurring to the south in México became adapted
to the arid environments of the Madro-tertiary geoflora.
Of course, information is lacking about climates in several parts of
the late Miocene and early Pliocene. When such information becomes
available it conceivably could modify the hypothesis outlined
immediately above.
The principal trend of evolution in these semi-fossorial rodents was
toward more complete fossorial adaptation, and the pronounced
fossorial features characteristic of the modern pocket gophers were
perfected. This trend continued in response to the intense selection
pressures in this arid environment. The principal structural
characters effected were in the postcranial anatomy, especially in the
skeletal and muscular systems. Consequently, it is not surprising that
in skull and dentition, _Pliosaccomys_ differs but little from
_Dikkomys_. Therefore, most of the basic structural specializations so
far developed for subterranean existence probably had evolved by the
time geomyines moved back north in the early Pliocene. Both modern
lineages, the tribes Thomomyini and Geomyini, have essentially the
same fossorial features, and it seems unlikely that these features
were acquired independently in the relatively short period of time
available to them after their divergence; probably they were inherited
from a common ancestor. These probabilities indicate that the
evolution of the fossorial specialization was in the later phyletic
development of the tribe Dikkomyini.
Plio-Pleistocene radiation of Geomyini
Unlike the lineage of the Thomomyini that remained essentially
rectilinear through out its history, the Geomyini in the late Pliocene
and the early Pleistocene underwent adaptive radiation in a degree
comparable to the earlier radiation of the Entoptychinae, and all of
the later history of the tribe is dominated by the radiation--the
resulting structural diversity. At least four lineages were produced
by the Plio-Pleistocene radiation (see Fig. 6); each originated at
essentially the same time (late Pliocene) presumably from the same
ancestral stock. Each of these lineages within the Geomyini has given
rise to one of the four modern genera: _Zygogeomys_, _Geomys_,
_Orthogeomys_, and _Pappogeomys_.
[Illustration: FIG. 6. Plio-Pleistocene radiation of the Tribe
Geomyini.]
_Morphotype_
The immediate, unknown, ancestor probably lived on the Central Plateau
of México. After the radiation began the ancestors of _Geomys_ and
_Zygogeomys_ extended their ranges northward.
Features of the hypothetical morphotype, that would permit derivation
of the modern genera would include the following: (1) Skull
generalized, neither excessively long and narrow or short and broad;
(2) skull smoothly rounded, without pronounced angularity, rugosity or
cresting (sagittal crest probably lacking, even in old individuals);
(3) zygomata slender, without lateral platelike expansions; (4)
rostrum moderately broad; (5) upper incisors bisulcate, two grooves in
pattern found in _Pliogeomys_, _Zygogeomys_ and _Geomys_; (6) lateral
re-entrant angles of premolars obtuse; (7) p4 having four enamel
plates (one on anterior wall, one on posterior wall, and two lateral
plates) and lower molars having one enamel plate on the posterior wall
of tooth (anterior plate is lacking); (8) P4 having four enamel
plates, in same pattern as described for p4, M1 having two enamel
plates (one anterior and one posterior), M2 same as M1, M3 having
three plates (one anterior, two lateral on sides of posterior loph,
none posterior); (9) M3 subtriangular in cross-section, distinctly
bicolumnar, two columns marked by shallow re-entrant folds and
connected by broad isthmus; (10) masseteric ridge large, forming high
crest bordering masseteric fossa; (11) basitemporal fossa shallow;
(12) angular process of mandible short, its lateral projection barely
exceeding that of zygomatic arch.
_Specializations in Genera_
In relation to the primitive morphotype, increase in size,
simplification of dentition, and changes in shape of skull are
regarded as specializations. Considerable parallelism between the four
lineages is seen. But each lineage is distinguished by a combination
of specialized features, and three by a few unique specializations.
Among trends resulting in simplification of the dentition, reduction
of enamel on the posterior wall of the upper cheek teeth has occurred
in various degrees in all lineages of the Geomyini even to loss of all
enamel on the posterior wall of the premolars and molars in two
genera. Loss of some enamel is more common on P4 than on M1-2, and has
occurred in all genera (see Figs. 7 and 9.)
In evolutionary sequence loss of enamel from M1 and M2 usually occurs
after, but never preceding, the reduction of enamel on P4. Loss of
enamel plates from the posterior face of M1 and M2 is associated with
the evolution of an efficient anterotransverse shearing action of the
teeth.
On the anterior wall of those teeth no reduction of the cutting blade
has been observed; a complete anterior plate is retained in all living
Geomyini.
Presence of both the posterior and anterior plates decreases the
efficiency of transverse shearing, by providing two upper plates
(anterior plate of one tooth and posterior plate of the preceding
tooth) over which the lower cutting blade _simultaneously_ must pass
with each movement. The advantages of shearing over the more common
mechanics of planing are largely lost unless the posterior plates are
eliminated. Also, none of the living Geomyini have retained a
definitive posterior enamel plate on M3, the last upper molar; but two
well-developed lateral plates, that extend almost all of the way back
to the posterior apex of M3, have been retained, and, together
function as a posterior plate. Loss of either or both of the lateral
plates of M3 is rare, and occurs only in old individuals. Their loss
in the final stages of wear may represent the beginning of a new trend
in those species where it occurs (the _castanops_-group of the
subgenus _Cratogeomys_). In any case, reduction of enamel takes place
by transverse shortening of the plate through the complete loss of
enamel on one end, the diminution beginning first on the labial end
and proceeding by progressive atrophy to the lingual end of the plate.
Evidently, when enamel has been eliminated from the labial end of a
plate, the rate of loss decreases markedly, and the last stages of
evolution, terminating in complete loss of an enamel plate, occurs
more slowly. Evolution may be arrested before complete loss has
occurred, and that part of the enamel that remains forms a short,
vestigial plate restricted to the lingual one-fourth or one-third of
the wall. The enamel pattern of the lower dentition is the same in all
of the diverging lineages, with no evidence of additional loss of
enamel from that which had already occurred in their common ancestor
(see Figs. 7 and 9). Reduction and loss of enamel plates began and was
terminated in the lower dentition before reduction began in the upper
dentition.
Other dental specializations have occurred in the shape of the third
upper molar and in the pattern of grooving in the upper incisor.
Unlike M3 of the Thomomyini, that of the Geomyini differs in shape
from M2, and its enamel investment differs from that of M2.
Primitively, M3 was probably subtriangular in cross-section, and the
posterior loph evidently projected posteriorly as a short, rudimentary
heel that formed the apex of the triangle. Other shapes of M3 are
considered to be specializations that have been derived from the
primitive form. In addition to the primitive subtriangular pattern,
the M3 of living Geomyini may be suborbicular, quadriform, elongate,
or obcordate in shape. Usually each lineage is characterized by only
one pattern, but in one genus (_Pappogeomys_) all patterns occur. Of
the different forms, the elongate and obcordate seem to be the most
highly specialized deviations from the triangular-shaped tooth. The
bicolumnar pattern is accentuated in the elongate type (Fig. 7D, F, H)
by deep lateral re-entrant folds, on both the lingual and labial
sides, and by the elongation of the posterior loph into a pronounced
heel. Teeth having this pattern have been illustrated by Merriam
(1895:76-82) in Figures 27 (6 and 7), 28 (c and d), 34 (7 through 15),
and 35 (8).
[Illustration: FIG. 7. Molariform dentitions of the Tribe Geomyini.
Drawings illustrating enamel patterns characteristic of
_Pliogeomys_, _Zygogeomys_, and the subgenera of _Orthogeomys_
(_Orthogeomys_, _Heterogeomys_ and _Macrogeomys_). × 5.
A. _Pliogeomys buisi_, No. 29157 (UMMP), holotype, Buis Ranch
(Upper Middle Pliocene), Beaver Co., Oklahoma. Right lower,
p4-m2 (m3 unknown).
B and C. _Zygogeomys trichopus trichopus_, adult female, No. 51971
(FMNH), Mt. Tancítaro, 10,500 ft., Michoacán. Left upper
(B), P4-M3; right lower (C), p4-m3.
D and E. Subgenus _Orthogeomys_. _Orthogeomys grandis guerrerensis_,
adult female, No. 39807 (KU), 1/2 mi. E La Mira, 300 ft.,
Michoacán. Left upper (D), P4-M3; right lower (E), p4-m3.
F and G. Subgenus _Heterogeomys_. _Orthogeomys hispidus hispidus_,
adult female, No. 23975 (KU), 4 km. W Tlapacoyan, 700 ft.,
Veracruz. Left upper (F), P4-M3; right lower (G), p4-m3.
H and I. Subgenus _Macrogeomys_. _Orthogeomys heterodus
cartagoensis_, adult female, No. 60664 (KU), Rancho
Redando, Volcán Lrozá, Prov. San José, Costa Rica. Left
upper (H), P4-M3; right lower (I), p4-m3.
]
The subcordate form is characterized by pronounced anteroposterior
compression, and retention of a distinct labial re-entrant fold. The
posterior loph apparently has been rotated in such a way that what was
previously its posterior border now lies on the outer margin of the
tooth; therefore, the axis of the posterior loph is strongly oblique
in relation to the anteroposterior bearing of the maxillary tooth-row,
and the median enamel plate also has been rotated and so lies
transversely across the posterior wall of the tooth. Owing to the
rotation of the posterior loph, the apex of the obcordate tooth is at
its lingual side. The subcordate type is illustrated by Merriam (_loc.
cit._) in Figures 27 (3 and 4), 28 (a and b), 34 (3 and 4), and 35 (5,
6, and 7). The suborbicular and quadriform types are less specialized
than the two described above. Both are characterized by reduction,
often obliteration, of the bicolumnar pattern of the subtriangular
ancestral form, especially marked by the decrease in depth of the
lateral re-entrant folds and the decrease in length of the posterior
projection of the posterior loph. With these changes, the tooth
becomes essentially monocolumnar, its occlusal surface oval in outline
in one and squarish in shape in the other. Occlusal views of the
suborbicular form are presented by Merriam (_loc. cit._) in Figure 33
(1, 5, 6, 7, 11, and 12) and the quadriform tooth is depicted in
Figure 29. Grooved upper incisors are characteristic of the living
Geomyini, but variation occurs in the number of grooves, and, if only
one groove is present, its position on the anterior face of the tooth
varies. Except for the previously mentioned (p. 480) abnormal tooth
having three grooves, incisors with no more than two grooves are found
in these pocket gophers, and this number of grooves is taken to be
primitive. Loss of one or the other of the two grooves of the
bisulcate pattern, therefore, is regarded as specialization. However,
complete loss of both grooves never occurs in the Geomyini. Each of
the four major lineages is characterized by one of the three patterns
of grooving, and the particular groove-pattern is remarkably stable in
each group.
Shape of skull varies from dolichocephalic to platycephalic. The
morphology of each has been described in foregoing accounts. The
dolichocephalic skull is highly specialized for planing, a grinding
action of the teeth; whereas, the platycephalic skull is highly
specialized for shearing, a slicing action of the teeth. Of course,
concomitant specializations of the dentition, as described above, are
closely associated with both specialized trends in the skull. Most
kinds of living Geomyini have generalized skulls that show no tendency
toward either of the specialized conditions.
Increase in size of body and skull is seen in most Pleistocene
lineages of the Geomyini. Judging from the smallness of the skull in
late Pliocene species, representing the base of three of these
lineages, the ancestral species of the living assemblage were no
larger than the living species of the subgenus _Pappogeomys_ or the
smaller subspecies of _Geomys bursarius_. The recorded range of
variation in condylobasal length is 36.1 to 45.5 in _Pappogeomys
bulleri_, including both adult males and females. Probably the skulls
of the ancestral species were not significantly larger. Maximum
dimensions of males in living species are 74.5 (subgenus
_Cratogeomys_) and 75.0 (subgenus _Orthogeomys_). These are more than
twice the minima observed in _Pappogeomys bulleri_.
Zygogeomys
This is the least specialized and most primitive of the four lineages,
has a generalized type of skull, two grooves on the anterior face of
each upper incisor, an enamel plate on the posterior wall of P4, open
or divergent lateral re-entrant angles on the premolars, and a
bicolumnar and elongated M3. All of these features are primitive and
essentially as in the ancestral morphotype. No other modern genus
retains so much of the primitive structure. Phyletic trends in
_Zygogeomys_ are not well documented in the fossil record; and only a
few fossils are known and they are fragmentary as discussed before.
The genus is represented in the late Pliocene (_Z. minor_), middle
Pleistocene (_Z. persimilis_), and Recent (_Z. trichopus_). The living
species is a relict population in the mountains of Central México.
Judging from the known material, the phyletic trends in the genus have
been increase in size, reduction of enamel on the posterior face of P4
(occurring only in the living species) where a short enamel plate is
retained on the lingual side of the tooth (see Fig. 7B), loss of the
outer fourth of the enamel blade on the posterior wall of M1 and M2
(also occurring only in the living species), development of a more
pronounced heel on the M3 by progressive elongation of the posterior
loph, reduction in size of the jugal and its displacement ventrally,
which allows the maxillary and squamosal bones to meet along the
dorsal border of the zygomatic arch. The last specialization is seen
in at least one taxon of _Orthogeomys_ (_Orthogeomys cherriei
costaricensis_). In my opinion, too much weight has been given to this
feature in past classifications. Reduction of enamel in the upper
dentition evidently occurred in the late Pleistocene, since the
posterior plates on the upper cheek teeth were complete in specimens
from the middle Pleistocene (_Z. persimilis_).
Geomys
_Geomys_, slightly more specialized than _Zygogeomys_, must also be
regarded as one of the most primitive of the living genera. Primitive
features that have been retained are the generalized type of skull,
the bisulcate pattern of grooves on the upper incisor, and the
retention of enamel plates on both the anterior and posterior walls
of M1 and M2 (see Fig. 9A). All of these primitive features are
shared with _Zygogeomys_. In addition, three other trends, or
specializations, in evolution characterize the phyletic development of
_Geomys_. One major trend is toward loss of the enamel plate from the
posterior wall of P4. No trace of enamel remains on the posterior wall
of this tooth in late Pleistocene or Recent species of _Geomys_, and
at least one of the earlier species (_quinni_) was also characterized
by loss of this enamel plate. Secondly, M3 retains only a vestige of
the primitive bicolumnar pattern after the initial stages of wear. In
most Recent specimens, especially of the species _G. bursarius_, the
lateral re-entrant fold and the heel of M3 are small, and the
re-entrant inflection is hardly evident. The lateral fold is more
frequently well-developed in Irvingtonian species than in living
species (White and Downs, 1961:13), illustrating progressive loss of
the bicolumnar pattern in Pleistocene evolution. A third trend
involves the modification of the lateral folds of the premolars.
Primitively the angles of these folds are broadly open or divergently
V-shaped, and some of the earliest species of _Geomys_, for example
_G. quinni_, have retained this feature throughout life. Nevertheless,
the main trend is toward progressive compression of the folds
resulting in their walls being more nearly perpendicular, and
parallel, to the long axis of the tooth. Obtuse re-entrant angles
persist in premolars of young individuals of Irvingtonian species, but
the adults are characterized by well-compressed folds, as in Recent
species.
Remains of _Geomys_ are abundant, especially from Pleistocene deposits
of the Great Plains, but in most instances specific assignment is
difficult or impossible since only isolated teeth or fragments of
skulls have been preserved. Estimates of phyletic relationships of the
known species of _Geomys_ are depicted in Figure 8; those estimates
are useful in discussing the phyletic development of the genus. One of
the earliest known species, _Geomys quinni_, ranges from Upper
Pliocene to the later stages of the Lower Pleistocene (Aftonian
interglacial deposits). The dentition of _G. quinni_ is essentially
the same as in the living species except that open lateral re-entrant
angles are retained in the premolars. _Geomys paenebursarius_, also of
the early Pleistocene, is a smaller species and seems to be more
directly in the line of evolution of the modern species. As yet
unnamed smaller species of _Geomys_ from the Rexroad fauna (late
Pliocene) and Saunders fauna (latest Aftonian) may also be on the main
line of evolution. Surprisingly, _Geomys tobinensis_ and _Geomys
garbanii_ of later Irvingtonian provincial age are less specialized
than either _Geomys quinni_ or _Geomys paenebursarius_. It is likely
that _G. tobinensis_ and the unnamed species from the Dixon are closer
to the main line of descent than _G. paenebursarius_ suggesting that
the direct ancestral lineage of the living species of _Geomys_ was
more conservative and less specialized than _Geomys paenebursarius_ of
the Lower Pleistocene. _Geomys quinni_ and _G. paenebursarius_ seem to
have acquired specialized dental features in the early Pleistocene.
_Geomys quinni_ was successful on the Great Plains, and persisted into
the late Blancan. The main line may be represented in the early
Pleistocene by _Geomys paenebursarius_ from the Hancock formation of
the Texas Trans-Pecos. The structure of _G. paenebursarius_ indicates
that it is in or close to the main line of descent, and probably
evolved from one of the more primitive late Pliocene species of
_Geomys_ from the Rexroad fauna.
[Illustration: FIG. 8. Tentative arrangement of species of the genus
_Geomys_, depicting phylogenetic trends and probable relationships
within the genus.]
Isolated teeth, to which the name _Geomys bisulcatus_ probably
applies, from Illinoian deposits on the Great Plains, show that the
dentition characteristic of the living _Geomys_ had been developed by
that time. Actually, the Illinoian material is too fragmentary to
show clearly its taxonomic or phyletic affinities with the species of
the later Pleistocene. Even so, the two main stocks of living
_Geomys_, _G. bursarius_ and _G. pinetis_, had certainly been
differentiated by Sangamon time. The other living species evidently
evolved from one or the other of these two stocks in a period of
isolation from the main population, probably in either the Wisconsin
or post-Wisconsin. For example, _Geomys arenarius_ clearly
differentiated from populations of _Geomys bursarius_ that were
isolated by the eastward retreat of the main population from the
southwestern United States as that region became more arid in the
post-Wisconsin.
In review, it seems that the Recent species, represented basically by
_bursarius_ and _pinetis_, evolved from Illinoian species (_Geomys
bisulcatus?_), which descended in turn from the more primitive species
of the early Pleistocene, possibly _Geomys paenebursarius_ or possibly
from descendants of the Saunders species. Actually the Saunders
species may prove to be _Geomys paenebursarius_. At any rate, three
trends that took place during the Pleistocene stage of evolution, in
the direction of the modern species, were an increase in size,
progressive loss of the posterior enamel plate on P4, and a decrease
in the vertical depth of the enamel cap as a result of which the
dentine is reached in the initial phases of attrition on the tooth of
a juvenile. _Geomys garbanii_, occurring at the periphery of the range
of the genus, is regarded as a sterile offshoot of the primitive
_tobinensis_-line of evolution.
Orthogeomys
This is one of the more specialized genera of the Geomyini. Save for
one record in the late Pleistocene (_Orthogeomys onerosus_), there is
no fossil history of the genus upon which to reconstruct its
phylogeny; therefore, its phyletic development must be estimated by
comparing it and the primitive morphotype of the tribe. Results of
that comparison suggest that _Orthogeomys_ has closer affinities with
_Zygogeomys_ than with any of the other genera, and that _Orthogeomys_
may have originated in an early dichotomy of primitive _Zygogeomys_
stock instead of descending from the ancestral stock of the tribe.
Except for the unisulcate incisors and the longer posterior loph on
the third upper molars, the teeth of the two genera do not differ
significantly. As in _Zygogeomys_, the enamel blade on the posterior
wall of P4 has been reduced to a short plate restricted to the lingual
third of the tooth (see Fig. 7F and H). In _Orthogeomys_, the trend in
reduction of enamel is carried to its extreme only in the subgenus
_Orthogeomys_, where this plate has been completely lost in most taxa
(see Fig. 7D). The most significant trends in _Orthogeomys_, and the
principal basis for recognizing the genus, are the dolichocephalic
specializations of the skull, as described elsewhere, and the adaptive
traits that have equipped the genus for living in tropical
environments. The dolichocephalic features are more sharply defined in
the subgenera _Orthogeomys_ and _Macrogeomys_, and are less developed
in the subgenus _Heterogeomys_. Aside from the general dolichocephalic
specializations, trends in _Orthogeomys_ include: Increase in size;
loss of the median one of the two grooves on the anterior face of the
upper incisor in the ancestral stock; increase in the anteroposterior
length of each of the cheek teeth, as well as the aforementioned
elongation of the posterior loph of M3; compression of the lateral
angles of the premolars; and the remarkable increase in the size of
the rostrum.
Pappogeomys
The genus _Pappogeomys_, as it is conceived of in this study, is
comprised of two subgenera; one, _Pappogeomys_, is generalized and
primitive, and the other, _Cratogeomys_, is specialized, and includes
the most highly specialized of the modern pocket gophers. The subgenus
_Pappogeomys_ is regarded as the ancestral lineage, and the subgenus
_Cratogeomys_ is regarded as an early offshoot, probably in the early
Pleistocene, that became progressively more specialized in the course
of its subsequent evolution. In the same period of time, the subgenus
_Pappogeomys_ changed little. It is known only from late Pliocene
fragments and from the living species. The ancestral morphotype is
preserved in _Pappogeomys_. Primitive characters are: (1) Small size;
(2) skull generalized and smoothly rounded; (3) temporal ridges
separate (not uniting into a sagittal crest); (4) enamel plates
retained on both anterior and posterior walls of M1 and M2; (5) M3
bilophate, its posterior loph short. Basic specializations are few and
include loss of the inner groove from the anterior face of the upper
incisor; anteroposterior compression of the lateral re-entrant folds
of the premolars; and loss of enamel from the posterior wall of P4.
All three features have been perpetuated in the advanced subgenus
_Cratogeomys_, suggesting that they were already developed in the
early evolution of the subgenus _Pappogeomys_ before _Cratogeomys_
diverged. Agreement with _Geomys_ is demonstrated by the lack of
enamel on the posterior wall of P4 (see Fig. 9) and by retention of
the posterior enamel plate on M1 and M2. In _Pappogeomys (Pappogeomys)
alcorni_ the enamel from the posterior face of M1 has been lost from
all but the lingual fourth or so of the posterior wall (Fig. 9E).
Reduction of enamel in M1 provides an example of parallelism with the
more advanced subgenus _Cratogeomys_, discussed below.
There is no record as yet of the early evolution of the subgenus
_Cratogeomys_. The features that characterize the subgenus were
already well developed in the first known fossils which are from
Wisconsin deposits of the late Pleistocene. _Cratogeomys_ is not a
homogenous assemblage; instead it is composed of two groups of living
species, the generalized _castanops_ group and the specialized
_gymnurus_ group. The _castanops_ group may be survivors of the
ancestral lineage that diverged in two different stages in the
phyletic development of the main line. Even so, the _castanops_ group
has acquired its peculiar specializations. Indeed, _P. merriami_ of
the _castanops_ group differs from the hypothetical stem more than
does _P. castanops_. Judging from the structure of the living species
of the subgenus _Cratogeomys_ and from the primitive subgenus
_Pappogeomys_, the subgenus _Cratogeomys_ featured five major trends:
(1) Increase in size; (2) formation of sagittal crest by union of the
temporal impressions; (3) increase in rugosity and angularity of the
skull; (4) progressive development of platycephalic specializations,
including the elongation of the angular process of the mandible; (5)
complete loss of enamel plates from the posterior wall of M1 and M2.
Each trend is thought to be adaptive.
[Illustration: FIG. 9. Molariform dentitions of the Tribe Geomyini.
Drawings illustrating enamel patterns characteristic of _Geomys_
and _Pappogeomys_ (including the subgenera _Pappogeomys_ and
_Cratogeomys_). × 5.
A and B. _Geomys bursarius bursarius_, adult female, No. 46275 (KU),
Elk River, Sherborne Co., Minnesota. Left upper (A), P4-M3;
right lower (B), p4-m3.
C and D. Subgenus _Pappogeomys_. _Pappogeomys bulleri albinasus_,
adult female, No. 31002 (KU), W side La Venta, 13 mi. W and
4 mi. N Guadalajara, Jalisco. Left upper (C), P4-M3; right
lower (D), p4-m3.
E and F. Subgenus _Pappogeomys_. _Pappogeomys alcorni_, adult
female, No. 31051 (KU), holotype, 4 mi. W Mazamitla, 6600
ft., Jalisco. Left upper (E), P4-M3; right lower (F),
p4-m3.
G and H. Subgenus _Cratogeomys_. _Pappogeomys gymnurus tellus_,
adult female, No. 31051 (KU), 1 mi. NE Tala, 4400 ft.,
Jalisco. Left upper (G), P4-M3; right lower (H), p4-m3.
]
Loss of enamel is a trend common to all living genera of the tribe
Geomyini, but the greatest loss has occurred in _Cratogeomys_. It has
lost the plates on the posterior walls of M1 and M2 (Fig. 9G). If the
lateral plates of M3 are considered as one functional plate and the
lateral plates on either side of P4 together as two transverse plates,
then, the transverse cutting blades in _Cratogeomys_ number seven in
the upper and seven in the lower cheek teeth compared with 10 in the
upper and seven in the lower in the primitive morphotype. Indeed, in
some species of the subgenus, one or both of the lateral plates on M3
is also lost, usually in old age, resulting in even greater reduction
of enamel. Loss of enamel from the posterior walls of the upper
molars may be associated with changes in the mechanics of mastication
from anteroposterior planing to anterotransverse shearing, as
discussed elsewhere. Merriam (1895:95-96) argues convincingly that the
posterior cutting blades of the upper molars would hinder efficient
shearing action of the teeth; hence, selection would favor their
reduction and eventual loss. Changes in the shape of the skull also
seem to be correlated with the shift from a planing to a shearing type
of mastication. More efficient shearing action, which depends upon
lateral movement of the jaw, can be developed if the functional
muscles insert farther laterally than is possible in the generalized
type of skull. Therefore, platycephalic specializations involved
lateral expansion of the braincase and mandible. Pronounced lateral
expansion has been developed only in the _gymnurus_ group of species,
suggesting that the dental specializations evolved earlier in the
evolution of the subgenus than did the platycephalic specializations
of the skull, and that the _castanops_ group separated from the
_gymnurus_ group before the common ancestor had developed the more
extreme trends in platycephaly. It is interesting to note that the
subtriangular M3 (Fig. 9G) postulated for the ancestral morphotype and
that characterizes the subgenus _Pappogeomys_ is retained also in the
_gymnurus_ group.
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_Transmitted May 29, 1967._
[]
* * * * *
Transcriber's note:
All obvious typographical errors were corrected. Minor changes were
made to standardize the text to match the most prevalent form used.
Typographical Corrections
Page Correction
==== ============
477 cumberlandicus => cumberlandius
535 breath => breadth
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