Transcriber's note:
Text enclosed by underscores is in italics (_italics_).
Text enclosed by equal signs is in bold face (=bold=).
Throughout, an asterisk (*) before a name denotes extinct
genera/species.
* * * * *
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.