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The Project Gutenberg eBook of Evolution And Classification Of The Pocket Gophers Of The Subfamily Geomyinae, by Robert J. Russell.
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<div>*** START OF THE PROJECT GUTENBERG EBOOK 40282 ***</div>
<div class="fig_center" style="width: 251px;">
<a name="cover" id="cover"></a>
<img src="images/cover_sm.png" width="251" height="421" alt="" title="" />
</div>
<div class="trans_notes">
Transcriber's note:
<p>Throughout, an asterisk before the name (i.e., *Entoptychinae) represents an extinct family/genus/species.</p>
</div>
<p><span class="pagenum"><a name="Page_473" id="Page_473">[473]</a></span></p>
<div class="fig_center">
<img src="images/bar_double.png" width="100%" height="15" alt="" />
</div>
<p class="center smcap">University of Kansas Publications<br />
Museum of Natural History</p>
<hr style="width: 20%" />
<p class="center">Vol. 16, No. 6, pp. 473-579, 9 figures in text</p>
<p class="center"><img src="images/bar_single.png" width="40%" height="15" alt="" />
August 5, 1968
<img src="images/bar_single.png" width="40%" height="15" alt="" /></p>
<p class="caption1">Evolution and Classification<br />
of the Pocket Gophers of the<br />
Subfamily Geomyinae</p>
<p class="caption3"><b>BY</b></p>
<p class="caption2">ROBERT J. RUSSELL</p>
<p class="center pmt4 pmb2"><span class="smcap">University of Kansas<br />
Lawrence</span><br />
1968</p>
<p><span class="pagenum"><a name="Page_430" id="Page_430">[430]</a></span></p>
<p class="center smcap pmb2">University of Kansas Publications, Museum of Natural History</p>
<p class="center">Editors: E. Raymond Hall, Chairman, Henry S. Fitch,<br />
Frank B. Cross, J. Knox Jones, Jr.</p>
<p class="center pmt4 pmb2">Volume 16, No. 6, pp. 473-579, 9 figs.<br />
Published August 5, 1968</p>
<p class="center pmt4 pmb2"><span class="smcap">University of Kansas</span><br />
Lawrence, Kansas</p>
<p class="center pmb2">PRINTED BY<br />
ROBERT R. (BOB) SANDERS, STATE PRINTER<br />
TOPEKA, KANSAS<br />
<br />
1968<br />
<br />
<img src="images/union_label.png" width="74" height="27" alt="Look for the Union Label" /><br />
31-4628</p>
<p><span class="pagenum"><a name="Page_475" id="Page_475">[475]</a></span></p>
<p class="caption1">Evolution and Classification<br />
of the Pocket Gophers of the<br />
Subfamily Geomyinae</p>
<p class="caption3">BY</p>
<p class="caption2">ROBERT J. RUSSELL</p>
<p class="caption2"><a name="CONTENTS" id="CONTENTS"></a>CONTENTS</p>
<table width="85%" summary="ToC">
<tr>
<td></td>
<td class="tdr">PAGE</td>
</tr>
<tr>
<td class="smcap"><a href="#INTRODUCTION">Introduction</a></td>
<td class="tdr">477</td>
</tr>
<tr>
<td class="smcap"><a href="#MATERIALS_AND_ACKNOWLEDGMENTS">Materials and Acknowledgments</a></td>
<td class="tdr">477</td>
</tr>
<tr>
<td class="smcap"><a href="#TAXONOMIC_CHARACTERS">Taxonomic Characters</a></td>
<td class="tdr">478</td>
</tr>
<tr>
<td class="tdl2"><a href="#Prismatic_Character_of_Molars">Prismatic character of molars</a></td>
<td class="tdr">478</td>
</tr>
<tr>
<td class="tdl2"><a href="#Character_of_Enamel_Patterns">Character of enamel patterns</a></td>
<td class="tdr">479</td>
</tr>
<tr>
<td class="tdl2"><a href="#Grooving_of_Incisors">Grooving of incisors</a></td>
<td class="tdr">480</td>
</tr>
<tr>
<td class="tdl2"><a href="#Masseteric_Ridge_and_Fossa">Masseteric ridge and fossa</a></td>
<td class="tdr">480</td>
</tr>
<tr>
<td class="tdl2"><a href="#Basitemporal_Fossa">Basitemporal fossa</a></td>
<td class="tdr">481</td>
</tr>
<tr>
<td class="tdl2"><a href="#Specializations_of_Skull">Specializations of skull</a></td>
<td class="tdr">481</td>
</tr>
<tr>
<td class="smcap"><a href="#FOSSIL_RECORD">Fossil Record</a></td>
<td class="tdr">484</td>
</tr>
<tr>
<td class="tdl2"><a href="#Miocene">Miocene</a></td>
<td class="tdr">485</td>
</tr>
<tr>
<td class="tdl2"><a href="#Pliocene">Pliocene</a></td>
<td class="tdr">486</td>
</tr>
<tr>
<td class="tdl2"><a href="#Pleistocene">Pleistocene</a></td>
<td class="tdr">490</td>
</tr>
<tr>
<td class="tdl3"><a href="#Thomomys">Thomomys</a></td>
<td class="tdr">492</td>
</tr>
<tr>
<td class="tdl3"><a href="#Zygogeomys">Zygogeomys</a></td>
<td class="tdr">496</td>
</tr>
<tr>
<td class="tdl3"><a href="#Geomys">Geomys</a></td>
<td class="tdr">496</td>
</tr>
<tr>
<td class="tdl3"><a href="#Pappogeomys">Pappogeomys</a></td>
<td class="tdr">503</td>
</tr>
<tr>
<td class="tdl3"><a href="#Orthogeomys">Orthogeomys</a></td>
<td class="tdr">504</td>
</tr>
<tr>
<td class="smcap"><a href="#HISTORY_OF_CLASSIFICATION">History of Classifications</a></td>
<td class="tdr">505</td>
</tr>
<tr>
<td class="smcap"><a href="#CLASSIFICATION">Classification</a></td>
<td class="tdr">512</td>
</tr>
<tr>
<td class="tdl2"><a href="#Family_GEOMYIDAE">Family Geomyidae</a></td>
<td class="tdr">512</td>
</tr>
<tr>
<td class="tdl3"><a href="#Subfamily_Entoptychinae">Subfamily *Entoptychinae</a></td>
<td class="tdr">513</td>
</tr>
<tr>
<td class="tdl5"><a href="#Pleurolicus">Genus *<i>Pleurolicus</i></a></td>
<td class="tdr">514</td>
</tr>
<tr>
<td class="tdl5"><a href="#Gregorymys">Genus *<i>Gregorymys</i></a></td>
<td class="tdr">514</td>
</tr>
<tr>
<td class="tdl5"><a href="#Grangerimus">Genus *<i>Grangerimus</i></a></td>
<td class="tdr">514</td>
</tr>
<tr>
<td class="tdl5"><a href="#Entoptychus">Genus *<i>Entoptychus</i></a></td>
<td class="tdr">514</td>
</tr>
<tr>
<td class="tdl3"><a href="#Subfamily_Geomyinae">Subfamily Geomyinae</a></td>
<td class="tdr">514</td>
</tr>
<tr>
<td class="tdl4"><a href="#Tribe_Dikkomyini">Tribe *Dikkomyini</a></td>
<td class="tdr">515</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Dikkomys">Genus *<i>Dikkomys</i></a></td>
<td class="tdr">516</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Pliosaccomys">Genus *<i>Pliosaccomys</i></a></td>
<td class="tdr">517</td>
</tr>
<tr>
<td class="tdl4"><a href="#Tribe_Thomomyini">Tribe Thomomyini</a><span class="pagenum">
<a name="Page_476" id="Page_476">[476]</a></span></td>
<td class="tdr">518</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Thomomys">Genus <i>Thomomys</i></a></td>
<td class="tdr">518</td>
</tr>
<tr>
<td class="tdl6"><a href="#Subgenus_Pleisothomomys">Subgenus *<i>Pleisothomomys</i></a></td>
<td class="tdr">519</td>
</tr>
<tr>
<td class="tdl6"><a href="#Subgenus_Thomomys">Subgenus <i>Thomomys</i></a></td>
<td class="tdr">520</td>
</tr>
<tr>
<td class="tdl4"><a href="#Tribe_Geomyini">Tribe Geomyini</a></td>
<td class="tdr">521</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Pliogeomys">Genus *<i>Pliogeomys</i></a></td>
<td class="tdr">522</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Zygogeomys">Genus <i>Zygogeomys</i></a></td>
<td class="tdr">523</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Geomys">Genus <i>Geomys</i></a></td>
<td class="tdr">525</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Orthogeomys">Genus <i>Orthogeomys</i></a></td>
<td class="tdr">528</td>
</tr>
<tr>
<td class="tdl6"><a href="#Subgenus_Orthogeomys">Subgenus <i>Orthogeomys</i></a></td>
<td class="tdr">529</td>
</tr>
<tr>
<td class="tdl6"><a href="#Subgenus_Heterogeomys">Subgenus <i>Heterogeomys</i></a></td>
<td class="tdr">530</td>
</tr>
<tr>
<td class="tdl6"><a href="#Subgenus_Macrogeomys">Subgenus <i>Macrogeomys</i></a></td>
<td class="tdr">531</td>
</tr>
<tr>
<td class="tdl5"><a href="#Genus_Pappogeomys">Genus Pappogeomys</a></td>
<td class="tdr">532</td>
</tr>
<tr>
<td class="tdl6"><a href="#Subgenus_Pappogeomys">Subgenus <i>Pappogeomys</i></a></td>
<td class="tdr">534</td>
</tr>
<tr>
<td class="tdl6"><a href="#Subgenus_Cratogeomys">Subgenus <i>Cratogeomys</i></a></td>
<td class="tdr">535</td>
</tr>
<tr>
<td class="smcap"><a href="#PHYLOGENY_OF_THE_GEOMYIDAE">Phylogeny of the Geomyidae</a></td>
<td class="tdr">536</td>
</tr>
<tr>
<td class="tdl2"><a href="#Primitive_Morphotype">Primitive Morphotype</a></td>
<td class="tdr">537</td>
</tr>
<tr>
<td class="tdl2"><a href="#Entoptychid_Radiation">Entoptychid Radiation</a></td>
<td class="tdr">540</td>
</tr>
<tr>
<td class="tdl2"><a href="#Phyletic_Trends">Phyletic Trends in Subfamily Geomyinae</a></td>
<td class="tdr">542</td>
</tr>
<tr>
<td class="tdl2"><a href="#Plio-Pleistocene_radiation">Plio-Pleistocene Radiation of Geomyini</a></td>
<td class="tdr">558</td>
</tr>
<tr>
<td class="tdl3"><a href="#Morphotype">Morphotype</a></td>
<td class="tdr">559</td>
</tr>
<tr>
<td class="tdl3"><a href="#Specializations_in_Genera">Specializations in Genera</a></td>
<td class="tdr">560</td>
</tr>
<tr>
<td class="tdl4"><a href="#Zygogeomys2">Zygogeomys</a></td>
<td class="tdr">564</td>
</tr>
<tr>
<td class="tdl4"><a href="#Geomys2">Geomys</a></td>
<td class="tdr">565</td>
</tr>
<tr>
<td class="tdl4"><a href="#Orthogeomys2">Orthogeomys</a></td>
<td class="tdr">568</td>
</tr>
<tr>
<td class="tdl4"><a href="#Pappogeomys2">Pappogeomys</a></td>
<td class="tdr">569</td>
</tr>
<tr>
<td class="smcap"><a href="#LITERATURE_CITED">Literature Cited</a></td>
<td class="tdr">572</td>
</tr>
</table>
<p><span class="pagenum"><a name="Page_477" id="Page_477">[477]</a></span></p>
<p class="caption2"><a name="INTRODUCTION" id="INTRODUCTION"></a>INTRODUCTION</p>
<p>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.</p>
<p class="caption2"><a name="MATERIALS_AND_ACKNOWLEDGMENTS" id="MATERIALS_AND_ACKNOWLEDGMENTS"></a>MATERIALS AND ACKNOWLEDGMENTS</p>
<div class="smaller">
<p>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: <i>Geomys colonus</i>, <i>G. fontanelus</i>, and
<i>G. cumberlandius</i>, <i>Orthogeomys cuniculus</i> and <i>O. pygacanthus</i> of the subgenus
<i>Orthogeomys</i>, and <i>O. dariensis</i> and <i>O. matagalpae</i> of the subgenus <i>Macrogeomys</i>.
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.</p>
<p>Specimens of the extinct genera <i>Dikkomys</i>, <i>Pliosaccomys</i>, <i>Pliogeomys</i>, <i>Nerterogeomys</i>,
and <i>Parageomys</i> also were studied, as were examples of the extinct
species <i>Geomys quinni</i>, <i>Geomys tobinensis</i>, and <i>Orthogeomys onerosus</i>. Considerable
fossil material of living species, especially of the genera <i>Geomys</i> and
<i>Pappogeomys</i>, was used.</p>
<p>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
<i>Orthogeomys</i> and Pleistocene taxa of <i>Geomys</i> when available specimens are
studied.</p>
<p>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
<span class="pagenum"><a name="Page_478" id="Page_478">[478]</a></span>
the Field Museum of Natural History, and Profs. Emmet T. Hooper and
William H. Burt of the University of Michigan Museum of Zoology.</p>
<p>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.</p>
</div>
<p class="caption2"><a name="TAXONOMIC_CHARACTERS" id="TAXONOMIC_CHARACTERS"></a>TAXONOMIC CHARACTERS</p>
<p>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.</p>
<p>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.</p>
<p class="caption3"><a name="Prismatic_Character_of_Molars" id="Prismatic_Character_of_Molars"></a><i>Prismatic Character of Molars</i></p>
<p>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.</p>
<p>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
<span class="pagenum"><a name="Page_479" id="Page_479">[479]</a></span>
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.</p>
<p>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.</p>
<p>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.</p>
<p class="caption3"><a name="Character_of_Enamel_Patterns" id="Character_of_Enamel_Patterns"></a><i>Character of Enamel Patterns</i></p>
<p>In the primitive genera the enamel pattern is bilophate and the
enamel loop (see <a href="#Fig_4">p. 4B</a>) 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.</p>
<p>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.</p>
<p><span class="pagenum"><a name="Page_480" id="Page_480">[480]</a></span></p>
<p class="caption3"><a name="Grooving_of_Incisors" id="Grooving_of_Incisors"></a><i>Grooving of Incisors</i></p>
<p>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 <i>Geomys</i>
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.</p>
<p>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.</p>
<p>The grooves provide a serrated cutting edge on the occlusal edge
of the upper incisor. In the genus <i>Geomys</i>, 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 <i>Geomys</i> 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.</p>
<p class="caption3"><a name="Masseteric_Ridge_and_Fossa" id="Masseteric_Ridge_and_Fossa"></a><i>Masseteric Ridge and Fossa</i></p>
<p>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
<span class="pagenum"><a name="Page_481" id="Page_481">[481]</a></span>
division of the masseter muscle. The mental foramen lies
immediately anterior, or anteroventral, to the fossa.</p>
<p>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.</p>
<p class="caption3"><a name="Basitemporal_Fossa" id="Basitemporal_Fossa"></a><i>Basitemporal Fossa</i></p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p><span class="pagenum"><a name="Page_482" id="Page_482">[482]</a></span></p>
<div class="fig_center" style="width: 435px">
<a name="Fig_1" id="Fig_1"></a>
<img src="images/fig_1.png" width="435" height="543" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 1.</span>
Types of skulls in the subfamily Geomyinae. × 1.</p>
<p class="key2sp1">A. and B. Generalized type of skull. <i>Geomys bursarius lutescens</i>,
adult, male, No. 77955 KU, 10 mi. N Springview, Keya Paha Co.,
Nebraska.</p>
<p class="key2sp2">A. Dorsal view of skull.<br />
B. Ventral view of lower jaw.</p>
<p class="key2sp1">C. and D. Dolichocephalic type of skull. <i>Orthogeomys</i> (<i>Orthogeomys</i>)
<i>grandis guerrerensis</i>, adult, female, No. 39807 KU, 1/2 mi. E
La Mira, 300 ft., Michoacán, México.</p>
<p class="key2sp2">C. Dorsal view of skull.<br />
D. Ventral view of lower jaw.</p>
<p class="key2sp1">E. and F. Platycephalic type of skull. <i>Pappogeomys</i> (<i>Cratogeomys</i>)
<i>gymnurus tellus</i>, adult, female, No. 33454 KU, 3 mi. W Tala,
4300 ft., Jalisco, México.</p>
<p class="key2sp2">E. Dorsal view of skull.<br />
F. Ventral view of lower jaw.</p>
</div>
<p><span class="pagenum"><a name="Page_483" id="Page_483">[483]</a></span></p>
<p class="caption3"><a name="Specializations_of_Skull" id="Specializations_of_Skull"></a><i>Specializations of Skull</i></p>
<p>The skull in most geomyines is generalized, being neither extremely
long and narrow nor short, broad and flat as in specialized
skulls (see <a href="#Fig_1">p. 1</a>). 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.</p>
<p>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.</p>
<p>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.</p>
<p>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 <i>Orthogeomys</i>
(see <a href="#Fig_1">p. 1, C and D</a>).</p>
<p>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 (<a href="#Fig_1">Fig. 1E, F</a>). 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
<span class="pagenum"><a name="Page_484" id="Page_484">[484]</a></span>
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.</p>
<p>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 <i>Cratogeomys</i> (see <a href="#Fig_1">Fig. 1, E and F</a>)
has a platycephalic skull. The trend toward platycephalic specialization
has been the major feature of evolution in <i>Cratogeomys</i>.</p>
<p class="caption2"><a name="FOSSIL_RECORD" id="FOSSIL_RECORD"></a>FOSSIL RECORD</p>
<p>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
<span class="pagenum"><a name="Page_485" id="Page_485">[485]</a></span>
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.</p>
<p class="caption3"><a name="Miocene" id="Miocene"></a>Miocene</p>
<p><i>Dikkomys</i> is the only genus of the Geomyinae known from the
early and middle Miocene. <i>Dikkomys matthewi</i> 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, <i>Dikkomys woodi</i>, from the Deep River Formation, Meagher
County, Montana. The Deep River Formation is late Hemingfordian
(middle Miocene) in age. No remains of <i>Dikkomys</i> 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.</p>
<p>In the present account, <i>Dikkomys</i> 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.</p>
<div class="smaller">
<p>The numerous records of <i>Thomomys</i> and especially <i>Geomys</i> 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 <i>Geomys</i> date back to the description of <i>Geomys bisculcatus</i> 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 <i>Geomys</i> and
<i>Thomomys</i> 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
<span class="pagenum"><a name="Page_486" id="Page_486">[486]</a></span>
structure of the geomyids known from Miocene and Pliocene beds, but the supposed
early appearance of <i>Geomys</i> and <i>Thomomys</i> led to much confusion concerning
geomyid evolution in the late Tertiary.</p>
</div>
<p>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.</p>
<p class="caption3"><a name="Pliocene" id="Pliocene"></a>Pliocene</p>
<p>The oldest and most primitive Pliocene geomyine is <i>Pliosaccomys
dubius</i> Wilson (1936:20) from the Smith Valley local fauna of
middle Pliocene (Hemphillian) age in Nevada. According to Wilson
(<i>op. cit.</i>:15) the beds probably were deposited near the middle of
Hemphillian time. Shotwell (1956:730) recorded <i>Pliosaccomys
dubius</i> 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 <i>Pliosaccomys</i> (cf.
<i>dubius</i>) 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, <i>Pliosaccomys wilsoni</i>, 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. <i>Pliosaccomys wilsoni</i>
does not differ greatly from <i>P. dubius</i>; however, the few differences
<span class="pagenum"><a name="Page_487" id="Page_487">[487]</a></span>
in dental characters seem to warrant specific recognition. The reduction
of cusps on the metalophid of p4 from three (<i>dubius</i>) to
two (<i>wilsoni</i>) and the lack of accessory cuspules on the protolophid
of p4 in <i>wilsoni</i> are probably specializations, suggesting that <i>P. dubius</i>
even though the more recent in age is the less advanced of the
two. <i>P. wilsoni</i> 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.</p>
<p><i>Pliosaccomys</i> 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 <i>Pliosaccomys</i>.</p>
<p>No other pocket gopher is known from the area in which <i>Pliosaccomys</i>
occurred, and it is unknown after middle Hemphillian age.
<i>Pliosaccomys</i> has closer affinities with <i>Dikkomys</i> 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.</p>
<p><i>Pliogeomys buisi</i> 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 (<i>op. cit.</i>: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.
<span class="pagenum"><a name="Page_488" id="Page_488">[488]</a></span>
Hibbard (1954:356) suggested that pocket gopher remains from
the Saw Rock Canyon local fauna were referable to <i>Pliogeomys
buisi</i>, and, in effect, tentatively assigned them to <i>Pliogeomys</i> (in
his description of the genus Hibbard remarked that the upper
incisor is bisulcate as in <i>Geomys</i>, and the only upper incisor that
he mentions was one of the Saw Rock Canyon fossils and not part
of the Buis Ranch material). <i>Pliogeomys</i> has closer affinities with
modern pocket gophers of the tribe Geomyini than it does with the
middle Pliocene genus <i>Pliosaccomys</i>.</p>
<p>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 <i>Pliogeomys</i>, which occurs in early late Pliocene (see
discussion above), the living genera <i>Zygogeomys</i>, <i>Geomys</i>, <i>Pappogeomys</i>
(in the sense used on <a href="#Page_534">p. 534</a>), and <i>Thomomys</i> first appear
in the late Pliocene. The only other living genus, <i>Orthogeomys</i>,
makes its first appearance in the late Pleistocene.</p>
<p>The earliest record of the genus <i>Thomomys</i> is based on a fragment
of a left mandibular ramus bearing p4 and m1, <i>Thomomys
gidleyi</i> Wilson (1933b:122), from the Hagerman local fauna of
Twin Falls County, Idaho. Wilson (<i>loc. cit.</i>) 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 <i>et al.</i>, 1965:512), on the basis of potassium
argon age determinations, also settled on late Pliocene.</p>
<p>Remains of <i>Nerterogeomys</i> [=<i>Zygogeomys</i>] 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 <i>Geomys minor</i> by Gidley (1922:123), and was later referred by
Gazin (1942:487) to his new genus <i>Nerterogeomys</i>. 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 <i>Nerterogeomys</i>, and tentatively referred them to the
species <i>N. minor</i>. <i>Nerterogeomys</i> cf. <i>minor</i> 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 <i>Geomys</i>, and they may actually be a primitive form of
<i>Geomys</i> that represents an intermediate stage in the development
of the enamel pattern from the uninterrupted loops of the ancestor
<span class="pagenum"><a name="Page_489" id="Page_489">[489]</a></span>
to the discontinuous pattern of modern <i>Geomys</i>. I favor this interpretation;
the evidence, however, is inconclusive, and I have, therefore,
reluctantly allocated them, along with the other specimens
of <i>Nerterogeomys</i>, to the genus <i>Zygogeomys</i>. In an early paper,
Hibbard (1938:244) erroneously referred the same specimens, two
upper premolars of a young individual, to the genus <i>Thomomys</i>,
and the same material was identified with the genus <i>Geomys</i>, also
without specific assignment, in a later paper (Hibbard, 1941b:278).
<i>Thomomys</i> is unknown from the late Pliocene of the Great Plains.
The specimens previously referred to <i>Nerterogeomys</i> are assigned
to the genus <i>Zygogeomys</i> for the first time in this report; for a discussion
of the systematic arrangement see the accounts beyond.
The type and paratype of <i>Nerterogeomys</i> from the Benson local
fauna of Arizona have no indication of enamel reduction.</p>
<p>Specimens of the genus <i>Geomys</i> from the late Pliocene were
referred to the large <i>Geomys quinni</i> McGrew, first by Franzen
(1947:55) and later by Hibbard and Riggs (1949:835) and Hibbard
(1950:171). <i>Geomys quinni</i> has been obtained from the Fox Canyon
locality and Locality 3 of the Rexroad local fauna. At Locality 3,
both <i>Zygogeomys</i> (cf. <i>minor</i>) and <i>Geomys quinni</i> have been found
together, but <i>Geomys quinni</i> 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 <i>Geomys quinni</i>; therefore, it seems unlikely that
the smaller gophers referred to <i>Zygogeomys</i> are actually the young
of <i>Geomys quinni</i>. Hibbard (personal communication, May, 1966)
informed me that specimens of <i>Geomys</i> from the late Pliocene (Fox
Canyon and Rexroad Locality 3) are erroneously referred to <i>G.
quinni</i>. 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 <i>Geomys quinni</i> to other species will restrict <i>quinni</i> to the early
Pleistocene.</p>
<p><i>Cratogeomys bensoni</i> 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 <i>Pappogeomys</i>
since the lower dentitions of all the genera of the tribe
Geomyini have the same enamel pattern, and the subgenera <i>Pappogeomys</i>
<span class="pagenum"><a name="Page_490" id="Page_490">[490]</a></span>
and <i>Cratogeomys</i> 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 <i>Pappogeomys</i>. Discovery of the upper molariform
dentition would make a more precise assignment possible.</p>
<p class="caption3"><a name="Pleistocene" id="Pleistocene"></a>Pleistocene</p>
<p>Numerous specimens of geomyids from many localities and horizons
are available from the Pleistocene of North America. Specimens
of the genera <i>Geomys</i> and <i>Thomomys</i> are especially common.
Few specimens are known of the genera <i>Orthogeomys</i> and <i>Pappogeomys</i>,
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,
<a href="#Fig_2">Figure 2</a>). 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 <i>Geomys</i> and
<i>Thomomys</i> from Pleistocene deposits farther north, and the marked
absence of other genera, may mean that <i>Orthogeomys</i> and <i>Pappogeomys</i>
did not range northward from southern and central México
in most of the Pleistocene. One species of <i>Pappogeomys</i> 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.</p>
<p>The fossil record of <i>Zygogeomys</i>, 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 <i>Zygogeomys</i> have been found. Today
a single species exists as a relic in the mountains of central
México and <i>Zygogeomys</i> may have retreated southward to its present
refugium in the late Pleistocene. Perhaps, <i>Zygogeomys</i> occurred
in northern México and the southwestern United States in
the early and middle Pleistocene (see <a href="#Fig_2">p. 2</a>), occupying the area
between the ranges of <i>Pappogeomys</i> to the south and <i>Geomys</i> to the
<span class="pagenum"><a name="Page_491" id="Page_491">[491]</a></span>
north. Competition with <i>Pappogeomys</i>, and especially <i>Geomys</i>,
during Irvingtonian time may have extirpated <i>Zygogeomys</i> over
most of this area, and by late Pleistocene (Sangamon) much of the
former range of <i>Zygogeomys</i> came to be occupied by one or the
other of its competitors. The occurrence of <i>Geomys garbanii</i> in
southern California (see White and Downs, 1961) and the unidentified
species of <i>Geomys</i> in Aguascalientes (Mooser, 1959; for
<span class="pagenum"><a name="Page_492" id="Page_492">[492]</a></span>
faunal correlation, see Hibbard and Mooser, 1963), both from deposits
of Irvingtonian age, supports this suggestion.</p>
<div class="fig_center" style="width:506px">
<a name="Fig_2" id="Fig_2"></a>
<img src="images/fig_2.png" width="506" height="591" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 2.</span> Probable distribution of the Subfamily Geomyinae in the early Pleistocene
(late Blancan), depicting major areas of differentiation of the modern
genera.</p>
<p style="margin-left: 30%; text-indent:0">
1. <i>Thomomys</i><br />
2. <i>Geomys</i><br />
3. <i>Zygogeomys</i><br />
4. <i>Pappogeomys</i><br />
5. <i>Orthogeomys</i></p>
</div>
<p class="caption4"><a name="Thomomys" id="Thomomys"></a><i>Thomomys</i></p>
<p>The earliest Pleistocene records of <i>Thomomys</i> are mostly isolated
teeth. Although they can be identified as genus <i>Thomomys</i>, most
of the materials are too fragmentary to be identified to species. In
<i>Thomomys</i> two distinct patterns of occlusal surfaces of the molars
can be recognized: the generalized elliptical pattern in the subgenus
<i>Pleisothomomys</i>, not unlike the pattern in other geomyids,
and the pear-shaped pattern in the subgenus <i>Thomomys</i>, 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 <i>Thomomys</i> 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.</p>
<p>Three occurrences of <i>Thomomys</i> 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 <i>Thomomys</i>. 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 <i>Thomomys
bottae</i>. Paulson (1961:137) recorded specimens from the Cudahy
local fauna, Meade County, Kansas. These fragmentary specimens
are referable to the subgenus <i>Thomomys</i>, 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
<i>Thomomys</i> 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
<span class="pagenum"><a name="Page_493" id="Page_493">[493]</a></span>
have been deposited in late Kansan glaciation or in early Yarmouth
interglacial by Shultz and Tanner (<i>op. cit.</i>: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 <i>Thomomys</i> from
Hay Springs local fauna has not been referred to species.</p>
<p>The relative abundance of <i>Geomys</i>, and rarity of <i>Thomomys</i>, 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
<i>Geomys</i>. Perhaps <i>Thomomys</i> evolved earlier to the west, in the
Great Basin and Pacific Coastal regions, and not on the Great Plains.</p>
<p>Upper Pleistocene records of <i>Thomomys</i> 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 <i>Thomomys</i> 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 <i>Thomomys</i>, 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 <i>Thomomys</i>; this plate occurs also in <i>Zygogeomys</i>. 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, <i>op. cit.</i>: 1700).</p>
<p>Local faunas in Maryland and Florida of Rancholabrean age
include <i>Thomomys</i>, in every instance referable to the subgenus
<i>Pleisothomomys</i> on the basis of unconstricted molars. <i>Thomomys
potomacensis</i> (Gidley and Gazin, 1933), from Cumberland Cave
local fauna, Allegany County in western Maryland, is the type of
the genus <i>Pleisothomomys</i> Gidley and Gazin (1933:354). <i>Pleisothomomys</i>
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. <i>T. potomacensis</i>
is significantly larger than <i>T. orientalis</i> Simpson (1928:6),
<span class="pagenum"><a name="Page_494" id="Page_494">[494]</a></span>
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. <i>Thomomys</i> is unknown from Wisconsin deposits
in the eastern United States, and today the genus does not
occur east of the Great Plains.</p>
<p><i>Thomomys</i> of Rancholabrean provincial age from the western
United States and México is known only from Wisconsin beds.</p>
<p>Three extinct species of <i>Thomomys</i>, all referable to the subgenus
<i>Thomomys</i>, have been described. <i>Thomomys microdon</i> 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. <i>T. microdon</i> closely resembles <i>Thomomys monticola</i>
that lives in the area today. <i>Thomomys scudderi</i> Hay (1921:614)
is from the Fossil Lake (or Christmas Lake) local fauna in
central Oregon. Elftman (1931:10-11) referred these specimens to
<i>Thomomys townsendii</i>, and he considered <i>T. scudderi</i> to be a
synonym of <i>T. townsendii</i>. Davis (1937:156-158) disagreed with
Elftman concerning the taxonomic status of <i>T. scudderi</i>, which he
regarded as a valid species. According to Davis, <i>T. scudderi</i> is
more closely allied to <i>Thomomys bottae</i> than to <i>T. townsendii</i>. Cope
(1878:389; 1889:160-165) had referred the same specimens to <i>Thomomys
clusius</i> (now <i>Thomomys talpoides clusius</i>). 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 <i>Thomomys</i> were referred
to the species <i>clusius</i>, <i>talpoides</i>, or <i>bulbivorus</i> (see Elftman,
<i>loc. cit.</i>). 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 <i>Thomomys
vetus</i> Davis (1937:156), also from the Fossil Lake local fauna in
Lake County, Oregon. Davis pointed out that <i>T. vetus</i> differs
from <i>T. scudderi</i> Hay, of the same fauna, in larger size and other
cranial details, and that it is closely allied to the living species
<i>Thomomys townsendii</i>, and not to <i>Thomomys talpoides</i>, which is
the only species of <i>Thomomys</i> living in the area today.</p>
<p><i>Thomomys townsendii</i> was recovered by Gazin (1935:299) from
the American Falls beds (probably Wisconsin deposits) in Idaho.</p>
<p><i>Thomomys talpoides</i> is reported from the Howard Ranch local
<span class="pagenum"><a name="Page_495" id="Page_495">[495]</a></span>
fauna in Hardeman County, western Texas, by Dalquest (1965:69-70),
who referred the isolated teeth to <i>T. talpoides</i> on geographic
grounds, apparently on the erroneous assumption that <i>T. talpoides</i>
was the species of <i>Thomomys</i> nearest geographically to Hardeman
County. Hay (1927:259) reported <i>Thomomys fuscus</i> [= <i>Thomomys
talpoides</i>] from late Pleistocene beds near Wenatchee,
Chelan County, Washington. Hibbard (1951:229) recorded <i>Thomomys
talpoides</i> 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 <i>T. talpoides</i> have
been recovered from numerous areas of Wisconsin glacial drift in
western North America.</p>
<p><i>Thomomys bottae</i> 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, <i>T. b. occipitalis</i>); Papago Springs Cave, Santa
Cruz County, Arizona (Skinner, 1942:150 and 158—probably <i>bottae</i>,
but possibly <i>umbrinus</i> 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 <i>T. leucodon</i>, now a subspecies of <i>T. bottae</i>; also see Hay,
1927:214-215).</p>
<p><i>Thomomys umbrinus</i> has been reported from San Josecito Cave,
Nuevo León, México (Russell, 1960:542); Upper Bercerra, México
(Hibbard, 1955a:51—identified only as <i>Thomomys</i> sp., but undoubtedly
referable to <i>T. umbrinus</i>). Post-Wisconsin remains of
<i>Thomomys umbrinus</i> 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 <i>Thomomys</i>
from Wisconsin deposits in Hawver Cave, Eldorado County, California,
but did not assign them to species. Gilmore (1947:158)
found the remains of <i>Thomomys umbrinus</i> 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.</p>
<p><span class="pagenum"><a name="Page_496" id="Page_496">[496]</a></span></p>
<p class="caption4"><a name="Zygogeomys" id="Zygogeomys"></a><i>Zygogeomys</i></p>
<p>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 <i>et al.</i> (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
<i>Geomys parvidens</i>, which is preoccupied by <i>Geomys parvidens</i>
Brown (1908:194), a name proposed for the pocket gopher from
the Conard Fissure of Arkansas; therefore, Hay (1927:136) proposed
the name <i>Geomys persimilis</i> for the Curtis Ranch species to
replace <i>Geomys parvidens</i> Gidley. <i>Geomys persimilis</i> Hay became
the type species of Gazin's genus <i>Nerterogeomys</i> (1942:507). In
this paper, <i>Nerterogeomys</i> is considered to be a junior synonym of
<i>Zygogeomys</i>.</p>
<p><i>Zygogeomys persimilis</i> 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 <i>Geomys</i> from the Arroyo San Francisco, Cedazo fauna, in Aguascalientes,
México, by Mooser (1959:413) may be referable instead
to <i>Zygogeomys</i>. 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
<i>Zygogeomys</i> have been uncovered in late Pleistocene deposits and
the significance of the absence of <i>Zygogeomys</i> has been discussed
in an earlier paragraph of this section. <i>Geomys</i> has not been found
so far south as Aguascalientes, but <i>Zygogeomys</i> occurs farther south
now and presumably had a more extensive range on the plateau to
the north in the Pleistocene.</p>
<p class="caption4"><a name="Geomys" id="Geomys"></a><i>Geomys</i></p>
<p><i>Geomys</i> is common in Pleistocene deposits, especially on the
Great Plains. Certainly the center of differentiation for <i>Geomys</i>
was in this region, although at times, probably when conditions
were favorable, <i>Geomys</i> 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)
<span class="pagenum"><a name="Page_497" id="Page_497">[497]</a></span>
described <i>Geomys quinni</i> 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,
<i>Geomys quinni</i> occurs also in the late Pliocene deposits of southwestern
Kansas. Also, <i>Geomys quinni</i> 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 <i>et al.</i>, 1951: table 1). The Broadwater-Lisco is currently
regarded as Aftonian deposits (Schultz and Stout, <i>loc. cit.</i>; Hibbard,
1958:11). Hibbard (1956:174) identified <i>Geomys quinni</i> from the
Deer Park local fauna, probably deposited during the early Aftonian
interglacial, of Meade County, Kansas. Strain (1966:36)
described <i>Geomys paenebursarius</i> 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 (<i>op. cit.</i>: 164)
recovered isolated teeth of <i>Geomys</i> from the Dixon local fauna,
regarded by him (<i>op. cit.</i>: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 <i>Geomys</i> from the Dixon are known only from isolated
teeth. The teeth are small, and suggest that a smaller species
of <i>Geomys</i> may have occurred along with the more common and
larger <i>G. quinni</i> during the early Pleistocene (see discussion beyond
of the Saunders <i>Geomys</i>). <i>Geomys quinni</i> was widespread and
common throughout the central Great Plains from the late Pliocene
(Rexroad fauna) through the early Pleistocene (Nebraskan and
Aftonian deposits).</p>
<p>Hibbard (1956:179) referred the pocket gopher remains taken
from the Saunders local fauna in Meade County, Kansas, to <i>Geomys
tobinensis</i>, 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 <i>Geomys</i> is distinct from <i>Geomys
tobinensis</i>; hence, the small pocket gopher from the Saunders local
fauna is probably an unnamed species, perhaps more closely allied
to <i>paenebursarius</i> than to <i>quinni</i>. The small <i>Geomys</i> reported from
the Aftonian Broadwater-Lisco local fauna of Nebraska (Schultz
and Stout, 1948:563) may also be the same as the Saunders pocket
<span class="pagenum"><a name="Page_498" id="Page_498">[498]</a></span>
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.</p>
<p>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 <i>et al.</i>, 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
<i>Geomys</i> is referable to any living species. Two Irvingtonian species
have been described. Hibbard (1944:735) named <i>Parageomys
tobinensis</i> [= <i>Geomys tobinensis</i>] 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 <i>Geomys tobinensis</i>
the pocket gopher recovered from the Saunders local fauna, a late
Aftonian deposit of Meade County, Kansas, and reduced the technical
name <i>Parageomys</i> from generic to subgeneric rank. Paulson
(<i>op. cit.</i>:138) pointed out that the Saunders specimens differ from
<i>G. tobinensis</i>, and he, therefore, restricted the name to the small
<i>Geomys</i> of the Cudahy and Tobin local faunas of Irvingtonian
provincial age. <i>G. tobinensis</i> is markedly smaller than the Blancan
<i>G. quinni</i>. 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.</p>
<p>Recently, White and Downs (1961:8) described a new Irvingtonian
species, <i>Geomys garbanii</i>, from the middle Pleistocene Vallecito
Creek local fauna of San Diego County, California. Many
well preserved fossils of the new species were recovered. <i>Geomys
garbanii</i> is of medium size (approximately the size of one of the
larger subspecies of <i>G. bursarius</i>), and significantly larger than the
Irvingtonian <i>Geomys tobinensis</i> of the Great Plains. The Vallecito
Creek occurrence of <i>Geomys</i> is the first authenticated record from
the Pacific Coast region. Matthew (1902:320) erroneously referred
remains of <i>Thomomys</i> to the genus <i>Geomys</i> in his revised list of
Cope's earlier report on the Fossil Lake (or Silver Lake) fauna (see
discussion of <i>Thomomys</i> above).</p>
<p><span class="pagenum"><a name="Page_499" id="Page_499">[499]</a></span></p>
<p>A number of Irvingtonian fossil remains of <i>Geomys</i> have not been
identified with particular species. Hibbard (1941a:206) found
<i>Geomys</i> in the Borchers local fauna (deposited in the time of the
Yarmouthian interglacial) of Meade County, Kansas. Also, <i>Geomys</i>
has been reported from several sites in Nebraska. Schultz and Tanner
(1957:67) reported <i>Geomys</i> 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 <i>Geomys</i> 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 <i>Microtus pennsylvanicus</i>, 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.</p>
<p>Mooser (1959:413) identified as <i>Geomys</i> 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 <i>Zygogeomys</i> rather than
<i>Geomys</i>. 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 <i>Geomys</i> in México. Two of these from San Josecito Cave
in Nuevo León have since been identified with the genera <i>Orthogeomys</i>
and <i>Pappogeomys</i> (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
<span class="pagenum"><a name="Page_500" id="Page_500">[500]</a></span>
from the "Hochtals von Mexiko" listed as <i>Geomys</i> by Freudenberg
(1921:139). His generic identification is doubtful and the specimens
should be compared with Mexican genera of the Geomyinae.</p>
<p>Upper Pleistocene records of <i>Geomys</i> 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 <i>et al.</i>, 1965:512-515) and post-Irvingtonian.
The presence of remains of <i>Bison</i> and/or <i>Microtus pennsylvanicus</i>
are currently considered mammalian index fossils of
Rancholabrean faunas. In the Illinoian, <i>Geomys</i> 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 <i>pinetis</i>, species-group seems to have remained
somewhat more generalized, and the western, or <i>bursarius</i>,
species-group has become more specialized. The Rancholabrean
<i>Geomys</i> from deposits in the southeastern United States are referable
(see Ray, 1963:325) to <i>Geomys pinetis</i>.</p>
<p>Marsh (1871:121) described <i>Geomys bisulcatus</i> 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 <a href="#Page_485">p. 485</a>), 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 (<i>loc. cit.</i>) as mostly
Yarmouthian deposits. <i>Geomys bisulcatus</i>, judging from the original
description and Hibbard's discussion of the cotypes (1954:357),
does not differ significantly from <i>Geomys bursarius</i>. However,
<i>Geomys bisulcatus</i> is tentatively retained as a valid species. Based
on the evidence cited above it seems unlikely that <i>Geomys bisulcatus</i>
occurred in pre-Irvingtonian times as often suggested in the literature.</p>
<p>The genus <i>Geomys</i> has been identified in several faunas of
<span class="pagenum"><a name="Page_501" id="Page_501">[501]</a></span>
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 <i>Geomys</i> cf. <i>bursarius</i>. Hibbard and
Taylor (1960:167) reported a baculum tentatively identified as that
of <i>Geomys</i> from the early Illinoian Butler Springs local fauna (including
the Adams fauna) of Meade County, Kansas. Hibbard
(1963:206) recorded the genus <i>Geomys</i> from the Mt. Scott local
fauna (late Illinoian deposits) of Meade County, Kansas; the specimens
probably are referable to the living species <i>bursarius</i>. From
McPherson County, Kansas, Hibbard (1952:7) reported the genus
<i>Geomys</i> 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 <i>G.
bursarius</i>; however the taxonomic status of <i>G. bisulcatus</i> 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
<i>Geomys</i> from the Great Plains is more closely allied to the living
species of <i>Geomys</i> than it is to the earlier Irvingtonian species.</p>
<p><i>Geomys bursarius</i> has been collected from a number of Sangamon
fossil sites on the Great Plains. Although specific identification of
specimens of <i>Geomys</i> from Illinoian faunas is uncertain, the Great
Plains <i>Geomys</i> 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 <i>Geomys</i> recovered
from the Cragin Quarry local fauna (early Sangamon) of Meade
County, Kansas, and the living species <i>Geomys bursarius</i>. 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
<i>Geomys</i> (referable to <i>G. bursarius</i>) from the Rezabek local fauna
of Lincoln County, Kansas. According to Schultz <i>et al.</i> (1951:6 and
table 1) the genus <i>Geomys</i> 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
<span class="pagenum"><a name="Page_502" id="Page_502">[502]</a></span>
specimens mentioned. Dalquest reported <i>Geomys bursarius</i> 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).</p>
<p><i>Geomys bursarius</i> 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 <i>et al.</i>,
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, <i>op. cit.</i>:343); Wisconsin drift near Galena, Illinois, and mouth
of Platte River in eastern Nebraska (Leidy, 1869:406).</p>
<p>Brown (1908:194) described <i>Geomys parvidens</i> 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 <i>et al.</i> (1965:510-511) regarded the fauna as
a composite including both Irvingtonian and Rancholabrean elements.
White and Downs (1961:21) considered <i>G. parvidens</i> to be
a subspecies of <i>Geomys bursarius</i>.</p>
<p>The first Pleistocene occurrence of <i>Geomys</i> in the southeastern
United States is from the Reddick I deposits reported by Gut and
Ray (1963:325), who found the remains of <i>Geomys pinetis</i> 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 <i>Geomys floridanus</i> [= <i>pinetis</i>] from
Saber-tooth Cave deposits of Citrus County, Florida. The Saber-tooth
Cave (or Lecanto Cave) local fauna is considered by Kurten
(<i>op. cit.</i>:219) also to be a Sangamon deposit. <i>Geomys floridanus</i>
<span class="pagenum"><a name="Page_503" id="Page_503">[503]</a></span>
[= <i>pinetis</i>] was reported from the Seminole Field deposits by
Simpson (1929:563); both Simpson and Kurten (<i>op. cit.</i>: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 <i>Geomys pinetis</i> from the Melbourne
Bone Bed of Brevard County, Florida. The Melbourne local fauna
is considered to be from Wisconsin deposits by Kurten (<i>op. cit.</i>:220).
The eastern species of <i>Geomys</i> 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 <i>Thomomys</i> occurred in this
region at approximately the same time; both genera occur in Saber-tooth
Cave deposits.</p>
<p class="caption4"><a name="Pappogeomys" id="Pappogeomys"></a><i>Pappogeomys</i></p>
<p>The genus <i>Pappogeomys</i> 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 <i>Pappogeomys</i> had been differentiated by late
Pliocene time. The absence of <i>Pappogeomys</i>, 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 <i>Cratogeomys</i> (long in use as a generic name but in the
present paper reduced to subgeneric rank in the genus <i>Pappogeomys</i>).
Schultz and Howard (1935:280) found <i>Cratogeomys</i>
[= <i>Pappogeomys</i>] <i>castanops</i> 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 (<i>loc. cit.</i>) also referred the mandible of a small
pocket gopher to the genus <i>Pappogeomys</i> [= subgenus <i>Pappogeomys</i>].
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 <i>Pappogeomys</i> has occurred beyond its present
range in the late Pleistocene; therefore the small mandible is most
likely that of a young individual of <i>Pappogeomys castanops</i>. Russell
<span class="pagenum"><a name="Page_504" id="Page_504">[504]</a></span>
(1960:543) referred specimens collected at San Josecito Cave in
Nuevo León, México, to the group of small subspecies <i>Cratogeomys</i>
[= <i>Pappogeomys</i>] <i>castanops</i>. Also, Russell (<i>loc. cit.</i>) identified
a rostral fragment as of the genus <i>Cratogeomys</i> [= subgenus <i>Cratogeomys</i>]
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.</p>
<p>Hibbard (1955a:52-53) identified <i>Cratogeomys</i> [= <i>Pappogeomys</i>]
<i>tylorhinus</i> 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 <i>gymnurus</i>-group of species.</p>
<p>Several specimens of the subgenus <i>Cratogeomys</i> have been
reported from beds of latest Wisconsin (certainly after the glacial
maximum) or post-Wisconsin age. Gilmore (1947:158) found fossil
remains of <i>Cratogeomys</i> [= <i>Pappogeomys</i>] <i>castanops</i> 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 <i>Cratogeomys</i> [= <i>Pappogeomys</i>]
<i>tylorhinus</i> from sub-Recent deposits of Capa III in the Cueva La
Nopalera in southwestern Hidalgo, México. <i>Pappogeomys merriami</i>
lives in the area today. Mayer-Oakes (1959:373) reported remains
of <i>Cratogeomys</i> [= <i>Pappogeomys</i>] <i>merriami</i> 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.</p>
<p class="caption4"><a name="Orthogeomys" id="Orthogeomys"></a><i>Orthogeomys</i></p>
<p>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 <i>Orthogeomys</i> 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 <i>Orthogeomys</i> from San Josecito Cave differs from living species,
and has been named <i>Heterogeomys</i> [= <i>Orthogeomys</i>] <i>onerosus</i>
Russell (<i>loc. cit.</i>), and is evidently referable to the subgenus
<i>Heterogeomys</i>. As mentioned before, the San Josecito Cave local
fauna represents deposits of Wisconsin glaciation.</p>
<p><span class="pagenum"><a name="Page_505" id="Page_505">[505]</a></span></p>
<p class="caption2"><a name="HISTORY_OF_CLASSIFICATION" id="HISTORY_OF_CLASSIFICATION"></a>HISTORY OF CLASSIFICATION</p>
<p>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 <i>Sorex mexicana</i> on
the basis of Hernandez's account without having seen any specimens.
Lichtenstein in 1827 applied the technical name <i>Ascomys
mexicana</i> to three specimens collected by Deppe from unknown
localities on the tableland of México. Merriam (<i>loc. cit.</i>) pointed
out that the name <i>mexicanus</i> of Lichtenstein in 1827 is a <i>nomen
nudum</i>, and that it is preoccupied by <i>mexicanus</i> used by Kerr in
1792. The latter can not be technically identified with any particular
species of geomyid.</p>
<p>Bartram in 1791 wrote of the pocket gopher of Florida, without
formally describing it. The first available technical name is <i>Mus
bursarius</i> of Shaw in 1800. Rafinesque in 1817 proposed the first
generic names for the geomyids when he described <i>Geomys</i> and
<i>Diplostoma</i>. In 1839, Waterhouse referred the genus <i>Geomys</i> to
his family Arvicolidae, considered by him to be a subgroup of
muroids. In 1841, he suggested that <i>Geomys</i> was related to <i>Bathyergus</i>
and <i>Spalax</i>. 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 <i>Heteromys</i>, <i>Saccomys</i>, <i>Perognathus</i>,
and <i>Dipodomys</i>. 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 <i>Geomys</i> and <i>Thomomys</i> and the
same genera (at least in part) of heteromyids that Waterhouse
classified in the "family" Saccomyina.</p>
<p>In 1839 the name <i>Thomomys</i> 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
<i>Geomys</i>.</p>
<p>A third family name, Sciurospalacoides, was proposed by Brandt
(1855:188) who referred <i>Geomys</i> and <i>Thomomys</i> 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 <i>Spalax</i>, <i>Sipheus</i>, and <i>Ellobius</i>). Brandt took
exception to the classification of Waterhouse (1848), who united
the geomyids and heteromyids in one family. Brandt placed the
<span class="pagenum"><a name="Page_506" id="Page_506">[506]</a></span>
heteromyid genera in other groups: <i>Perognathus</i> in the Muridae,
and <i>Macrocolus</i> [= <i>Dipodomys</i>] in the Macrolini, a subfamily of
the family Dipodoides.</p>
<p>Modern classification of the pocket gophers begins with Baird in
1858. The important classifications are summarized in <a href="#Table_1">Table 1</a>;
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.</p>
<p>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.</p>
<p>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 <a href="#Table_1">Table 1</a>). 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 <i>Saccomys</i>
Frédéric Cuvier, 1823, was a synonym of <i>Heteromys</i> 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 <i>Saccomys</i>, 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 <i>Saccomys</i> with <i>Heteromys</i> and insisted
on retaining the technical terms Saccomyidae and Saccomyini.</p>
<p>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, <i>Geomys</i> and <i>Thomomys</i>. Merriam described
<span class="pagenum"><a name="Page_507" id="Page_507">[507]</a></span>
much new material, especially from México and Central America,
and proposed seven new genera (see <a href="#Table_1">Table 1</a>). 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 <i>Platygeomys</i> as a synonym of <i>Cratogeomys</i>.
However, Merriam's genera have been recognized in all subsequent
classifications except for the current review (see <a href="#Table_1">Table 1</a>).</p>
<p>Cope described the first known fossil geomyids in 1878, and
published an excellent review of the two genera, <i>Pleurolicus</i> and
<i>Entoptycus</i>, 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 <i>Pleurolicus</i> and
<i>Entoptycus</i> to the Saccomyidae along with the living genera <i>Thomomys</i>
and <i>Geomys</i>. 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, <i>Dikkomys</i>, of the Geomyinae. He followed the
supergeneric classification of Miller and Gidley (1918).</p>
<p>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 <a href="#Table_1">Table 1</a>, the list of genera is principally
from Simpson (1945) but generic names used by Wood (1955) are
included. This is the currently accepted classification.</p>
<p>The new classification proposed in this paper (see <a href="#Table_1">Table 1</a>)
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.</p>
<p><i>Pliogeomys</i> is placed in the same tribe (Geomyini) as <i>Zygogeomys</i>,
<i>Geomys</i>, <i>Orthogeomys</i>, and <i>Pappogeomys</i>. That tribe includes
the most specialized Geomyinae. <i>Zygogeomys</i>, <i>Geomys</i>, <i>Orthogeomys</i>,
and <i>Pappogeomys</i> 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 <i>Pliogeomys</i> may not be the
actual ancestor, it closely resembles the primitive morphotype.</p>
<p><a id="Table_1"></a><span class="pagenum"><a name="Page_508" id="Page_508">[508]</a></span></p>
<p class="tbl_caption"><span class="smcap">Table 1.</span>—History of the classification of the Superfamily Geomyoidea</p>
<table width="100%" summary="Geomyoides Superfamily Classifications">
<tr>
<td class="brdt2 brdb">Baird 1858</td>
<td class="brdt2 brdb brdl">Gill 1872<br />Coues 1877</td>
<td class="brdt2 brdb brdl">Winge 1887 and 1924</td>
<td class="brdt2 brdb brdl">Merriam 1895<br />Ellerman 1940</td>
<td class="brdt2 brdb brdl">Wood 1935<br />Wood 1936</td>
<td class="brdt2 brdb brdl">Simpson 1945<br />Wood 1955</td>
<td class="brdt2 brdb brdl" colspan="2">Names used in present paper</td>
</tr>
<tr>
<td class="brdb">Family<br /><span class="pdl2">Saccomyidae</span></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Geomyidae</span></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Saccomyidae</span></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Geomyidae</span></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Geomyidae</span></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Geomyidae</span></td>
<td class="brdb brdl" colspan="2">Family<br /><span class="pdl2">Geomyidae</span></td>
</tr>
<tr>
<td class="brdbd">Subfamily<br /><span class="pdl2">Geomyinae</span></td>
<td class="brdbd brdl"></td>
<td class="brdbd brdl">"Group"<br /><span class="pdl2">Geomyini</span></td>
<td class="brdbd brdl"></td>
<td class="brdbd brdl">Subfamily<br /><span class="pdl2">Geomyinae</span></td>
<td class="brdbd brdl">Subfamily<br /><span class="pdl2">Geomyinae</span></td>
<td class="brdbd brdl" colspan="2">Subfamily<br /><span class="pdl2">Geomyinae</span></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2">Tribe<br /><span class="pdl2">Dikkomyini</span></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Dikkomys</i></td>
<td class="brdl">*<i>Dikkomys</i></td>
<td class="brdl" colspan="2">*<i>Dikkomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Pliosaccomys</i></td>
<td class="brdl" colspan="2">*<i>Pliosaccomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2">Tribe<br /><span class="pdl2">Thomomyini</span></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Pleisothomomys</i></td>
<td class="brdl">*<i>Pleisothomomys</i></td>
<td class="brdl" rowspan="2" style="width: 15px;">
<img src="images/brace_2r.png" width="15" height="38" alt="}" title="}" />
</td>
<td class="vmid tdl0" rowspan="2"><i>Thomomys</i></td>
</tr>
<tr>
<td><i>Thomomys</i></td>
<td class="brdl"><i>Thomomys</i></td>
<td class="brdl"><i>Thomomys</i></td>
<td class="brdl"><i>Thomomys</i></td>
<td class="brdl"><i>Thomomys</i></td>
<td class="brdl"><i>Thomomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2">Tribe<br /><span class="pdl2">Geomyini</span></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Pliogeomys</i></td>
<td class="brdl" colspan="2">*<i>Pliogeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"><i>Zygogeomys</i></td>
<td class="brdl"><i>Zygogeomys</i></td>
<td class="brdl"><i>Zygogeomys</i></td>
<td class="brdl vmid" rowspan="2">
<img src="images/brace_2r.png" width="15" height="38" alt="}" title="}" />
</td>
<td class="vmid tdl0" rowspan="2"><i>Zygogeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Nerterogeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td><i>Geomys</i></td>
<td class="brdl"><i>Geomys</i></td>
<td class="brdl"><i>Geomys</i></td>
<td class="brdl"><i>Geomys</i></td>
<td class="brdl"><i>Geomys</i></td>
<td class="brdl"><i>Geomys</i></td>
<td class="brdl vmid" rowspan="2">
<img src="images/brace_2r.png" width="15" height="38" alt="}" title="}" />
</td>
<td class="vmid tdl0" rowspan="2"><i>Geomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Parageomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"><span class="pagenum"><a name="Page_509" id="Page_509">[509]</a></span></td>
<td class="brdl"></td>
<td class="brdl"><i>Orthogeomys</i></td>
<td class="brdl"><i>Orthogeomys</i></td>
<td class="brdl"><i>Orthogeomys</i></td>
<td class="brdl vmid" rowspan="3">
<img src="images/brace_3r.png" width="13" height="86" alt="}" title="}" />
</td>
<td class="vmid tdl0" rowspan="3"><i>Orthogeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"><i>Heterogeomys</i></td>
<td class="brdl"><i>Heterogeomys</i></td>
<td class="brdl"><i>Heterogeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"><i>Macrogeomys</i></td>
<td class="brdl"><i>Macrogeomys</i></td>
<td class="brdl"><i>Macrogeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"><i>Pappogeomys</i></td>
<td class="brdl"><i>Pappogeomys</i></td>
<td class="brdl"><i>Pappogeomys</i></td>
<td class="brdbd brdl vmid" rowspan="3">
<img src="images/brace_3r.png" width="13" height="86" alt="}" title="}" />
</td>
<td class="vmid tdl0" rowspan="3"><i>Pappogeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"><i>Cratogeomys</i></td>
<td class="brdl"><i>Cratogeomys</i></td>
<td class="brdl"><i>Cratogeomys</i></td>
</tr>
<tr>
<td class="brdbd"></td>
<td class="brdbd brdl"></td>
<td class="brdbd brdl"></td>
<td class="brdbd brdl"><i>Platygeomys</i></td>
<td class="brdbd brdl"><i>Platygeomys</i></td>
<td class="brdbd brdl"><i>Platygeomys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">Subfamily<br /><span class="pdl2">Entoptychinae</span></td>
<td class="brdl">Subfamily<br /><span class="pdl2">Entoptychinae</span></td>
<td class="brdl" colspan="2">Subfamily<br /><span class="pdl2">Entoptychinae</span></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl">*<i>Pleurolicus</i></td>
<td class="brdl"></td>
<td class="brdl">*<i>Pleurolicus</i></td>
<td class="brdl">*<i>Pleurolicus</i></td>
<td class="brdl" colspan="2">*<i>Pleurolicus</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Gregorymys</i></td>
<td class="brdl">*<i>Gregorymys</i></td>
<td class="brdl" colspan="2">*<i>Gregorymys</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">*<i>Grangerimus</i></td>
<td class="brdl">*<i>Grangerimus</i></td>
<td class="brdl" colspan="2">*<i>Grangerimus</i></td>
</tr>
<tr>
<td class="brdb"></td>
<td class="brdb brdl"></td>
<td class="brdb brdl">*<i>Entoptychus</i></td>
<td class="brdb brdl"></td>
<td class="brdb brdl">*<i>Entoptychus</i></td>
<td class="brdb brdl">*<i>Entoptychus</i></td>
<td class="brdb brdl" colspan="2">*<i>Entoptychus</i></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl">Geomyidae<br /><span class="pdl2"><i>incertae sedis</i></span></td>
<td class="brdl" colspan="2">Geomyidae<br /><span class="pdl2"><i>incertae sedis</i></span></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl"></td>
<td class="brdl" colspan="2"></td>
</tr>
<tr>
<td></td>
<td class="brdl"></td>
<td class="brdl">"Group"<br />Gymnoptychine**</td>
<td class="brdl"></td>
<td class="brdl">*<i>Gidleumys</i></td>
<td class="brdl">*<i>Diplolophus</i></td>
<td class="brdl" colspan="2">*<i>Diplolophus</i></td>
</tr>
<tr>
<td class="brdb"></td>
<td class="brdb brdl"></td>
<td class="brdb brdl"><i>Gymnoptychus</i></td>
<td class="brdb brdl"></td>
<td class="brdb brdl"></td>
<td class="brdb brdl">*<i>Griphomys</i></td>
<td class="brdb brdl" colspan="2">*<i>Griphomys</i></td>
</tr>
<tr>
<td class="brdb">Subfamily<br /><span class="pdl2">Saccomyinae</span></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Saccomyidae</span></td>
<td class="brdb brdl">"Group"<br /><span class="pdl2">Saccomyini</span></td>
<td class="brdb brdl"></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Heteromyidae</span></td>
<td class="brdb brdl">Family<br /><span class="pdl2">Heteromyidae</span></td>
<td class="brdb brdl" colspan="2">Family<br /><span class="pdl2">Heteromyidae</span></td>
</tr>
</table>
<p class="spec_acc">* Denotes extinct genera.</p>
<p class="spec_acc">** Winge included in his family Saccomyidae the "group" Gymnoptychine and the contained genus <i>Gymnoptychus</i> Cope, 1873, which genus currently is
placed in the family Eomyidae. The type of <i>Gymnoptychus</i> Cope, 1873, is synonymous with <i>Ischyromys</i> Leidy, 1856, and the valid name for the genus is
<i>Adjidaumo</i> Hay, 1899.</p>
<p><span class="pagenum"><a name="Page_510" id="Page_510">[510]</a></span></p>
<p><i>Pliosaccomys</i>, on the other hand, represents the terminal stages
of a long trend that began with the <i>Dikkomys</i>-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 (<i>Dikkomys</i>) and middle Pliocene (<i>Pliosaccomys</i>)
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.</p>
<p>The Pliocene ancestor of <i>Thomomys</i> is unknown but probably
resembled <i>Pliosaccomys</i>, with which it may have been a contemporary.
<i>Thomomys</i> is the least specialized of the modern Geomyinae,
and, consequently, shows the most resemblance to the ancestral
tribe. The specializations of <i>Thomomys</i>, 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,
<i>Thomomys</i> is set apart in classification from the other living pocket
gophers.</p>
<p>Merriam's genera <i>Orthogeomys</i>, <i>Heterogeomys</i>, and <i>Macrogeomys</i>
are closely related. Each of these taxa is retained as a subgenus
of a single genus, <i>Orthogeomys</i>. Some species of <i>Macrogeomys</i>
seem to be more closely allied to the subgenus <i>Orthogeomys</i> and
others to the subgenus <i>Heterogeomys</i>. 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
<i>Heterogeomys</i> 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.</p>
<p><i>Nerterogeomys</i> Gazin here is arranged as a junior synonym of
<i>Zygogeomys</i>. Both are less specialized than any of the other Geomyini,
except <i>Pliogeomys</i>. The single living species (<i>Zygogeomys
tricopus</i>) is obviously a relic. Its range is small. The two subspecies
<span class="pagenum"><a name="Page_511" id="Page_511">[511]</a></span>
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, <i>Zygogeomys</i> and <i>Nerterogeomys</i> are closely related and
the latter is best placed as a synonym of the former. Both are
admittedly closely related to <i>Geomys</i>. <i>Zygogeomys</i> and <i>Geomys</i>
share several characters, particularly primitive ones; there is considerable
parallelism, especially marked in Irvingtonian species of
<i>Geomys</i>. Nevertheless, <i>Geomys</i> is more specialized, particularly
in the dentition, and it has developed some <i>Pappogeomys</i>-like
specializations. <i>Zygogeomys</i> has retained more of the primitive
characters of the tribe. A strong case could be made for recognizing
only one genus, <i>Geomys</i>, containing <i>Zygogeomys</i> as one of two subgenera.
Nevertheless, the characters separating <i>Zygogeomys</i> and
<i>Geomys</i> are of considerable importance and I consider the two
kinds to be distinct genera.</p>
<p>The species of <i>Geomys</i>, both living and extinct, form a distinct
and well-marked group. The genus is less primitive in most respects
than <i>Zygogeomys</i> and <i>Orthogeomys</i> and it is less specialized than
<i>Pappogeomys</i>, excluding the ancestral stock (subgenus <i>Pappogeomys</i>).
Some specimens of species of Irvingtonian age (<i>Geomys
tobinensis</i> and <i>Geomys garbanii</i>, especially the former) retain primitive
enamel plates as does <i>Zygogeomys</i>; 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 <i>Geomys</i> than in either Recent or late Pliocene and early
Pleistocene representatives (<i>Geomys paenebursarius</i>, <i>Geomys
quinni</i>) 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 <i>Geomys</i> is uncertain. At present I favor the latter
explanation, and view <i>G. paenebursarius</i> and <i>G. quinni</i> as early progressive
species that evolved dental specializations that were maintained
in the main line of phylogeny.</p>
<p>Hibbard proposed the generic name <i>Parageomys</i> (1944:55), but
later regarded it as a subgenus of <i>Geomys</i> (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 <i>Geomys</i> is better understood, <i>Parageomys</i> may
serve as a subgeneric taxon in which the primitive species of <i>Geomys</i>
can be grouped, but as of now <i>Parageomys</i> is arranged as a synonym
of <i>Geomys</i>.</p>
<p><span class="pagenum"><a name="Page_512" id="Page_512">[512]</a></span></p>
<p><i>Pappogeomys</i> and <i>Cratogeomys</i> 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 <i>Cratogeomys</i> diverged from
primitive <i>Pappogeomys</i>-stock in the earlier Pleistocene, at least
before the end of the Irvingtonian. <i>Cratogeomys</i> probably originated
on the Mexican Plateau and probably underwent its subsequent
evolution there. The living species of the subgenus <i>Pappogeomys</i>
are evidently relics of the ancestral stock of the genus.
Hooper (1946:397), I think correctly, considered <i>Platygeomys</i> as
congeneric with <i>Cratogeomys</i>, although the highest degree of
specialization of the genus is attained in those species formerly
classed in the genus <i>Platygeomys</i>. Even so, in my opinion, the differences
are insufficient to warrant even subgeneric recognition.</p>
<p class="caption2"><a name="CLASSIFICATION" id="CLASSIFICATION"></a>CLASSIFICATION</p>
<p class="caption3"><a name="Family_GEOMYIDAE" id="Family_GEOMYIDAE"></a>Family GEOMYIDAE Gill, 1872</p>
<div class="smaller">
<p>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 <i>Orthogeomys</i> scant, harsh or scattered bristles); color varying
from pale tints of buffy (almost white) to metallic black.</p>
<p>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 <i>Macrogeomys</i>;
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
<span class="pagenum"><a name="Page_513" id="Page_513">[513]</a></span>
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.</p>
<p>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.</p>
<p>Key to the Subfamilies of Geomyidae</p>
<div class="key2sp1">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</div>
<div class="key2sp9"><a href="#Subfamily_Entoptychinae">p. 513</a></div>
<div class="key2sp1">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</div>
<div class="key2sp9"><a href="#Subfamily_Geomyinae">p. 514</a></div>
</div>
<p class="caption4"><a name="Subfamily_Entoptychinae" id="Subfamily_Entoptychinae"></a>
Subfamily <span class="smcap">Entoptychinae</span> Miller and Gidley, 1918</p>
<div class="smaller">
<p>Anterior face of upper incisor usually smooth, sometimes bearing faint groove
in center or near medial margin of tooth, at least in <i>Gregorymys</i>; cheek
teeth hypsodont, medium to high crowned, and rooted in all but <i>Entoptychus</i>
(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 <a href="#Page_537">p. 537</a>) 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 <i>Entoptychus</i> 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 <i>Entoptychus</i>, where enamel disappears first from sides of crowns (following
<span class="pagenum"><a name="Page_514" id="Page_514">[514]</a></span>
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.</p>
<p>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.</p>
<p class="wrd_sp">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:</p>
<p class="spec_acc"><a name="Pleurolicus" id="Pleurolicus">*</a><i>Pleurolicus</i> Cope, 1878. Proc. Amer. Phil. Soc., 18:66.</p>
<p class="spec_acc"><a name="Gregorymys" id="Gregorymys"></a>*<i>Gregorymys</i> Wood, 1936. Amer. Mus. Novit., 866:9.</p>
<p class="spec_acc"><a name="Grangerimus" id="Grangerimus"></a>*<i>Grangerimus</i> Wood, 1936. Amer. Mus. Novit., 866:13.</p>
<p class="spec_acc"><a name="Entoptychus" id="Entoptychus"></a>*<i>Entoptychus</i> Cope, 1878. Proc. Amer. Phil. Soc., 18:64.</p>
<p class="wrd_sp">Five new species have been described since Wood's (1936) revision. They
are: <i>Pleurolicus clasoni</i> MacDonald (1963:180); <i>Gregorymys kayi</i> Wood
(1950:335); <i>Gregorymys montanensis</i> Hibbard and Keenmon (1950:198);
<i>Grangerimus dakotensis</i> MacDonald (1963:182); <i>Grangerimus sellardsi</i> Hibbard
and Wilson (1950:623).</p>
</div>
<p class="caption4"><a name="Subfamily_Geomyinae" id="Subfamily_Geomyinae"></a>Subfamily <span class="smcap">Geomyinae</span> Baird, 1858</p>
<div class="smaller">
<p>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.</p>
<p>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
<span class="pagenum"><a name="Page_515" id="Page_515">[515]</a></span>
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.</p>
<p>Key to the Tribes of the Geomyinae</p>
<div class="key2sp1">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</div>
<div class="key2sp9"><a href="#Tribe_Dikkomyini">p. 515</a></div>
<div class="key2sp1">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
<i>Pliogeomys</i>), crowns of maximum height; third lower molar
monoprismatic, without trace of inner and outer re-entrant folds
as in first and second lower molars.</div>
<div class="key2sp2">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, <i>T. townsendii</i>);
forefoot small and narrow with claws not elongated for digging.
Tribe Thomomyini</div>
<div class="key2sp9"><a href="#Tribe_Thomomyini">p. 518</a></div>
<div class="key2sp2">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 <i>Zygogeomys</i>);
forefoot large and broad, with elongated claws for digging.
Tribe Geomyini</div>
<div class="key2sp9"><a href="#Tribe_Geomyini">p. 521</a></div>
</div>
<p class="caption4"><a name="Tribe_Dikkomyini" id="Tribe_Dikkomyini"></a>
Tribe <span class="smcap"><b>Dikkomyini</b></span>, new tribe</p>
<div class="smaller">
<p class="wrd_sp"><i>Genotype.</i>—<i>Dikkomys</i> Wood, 1936.</p>
<p class="wrd_sp"><i>Chronologic and geographic range.</i>—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 <i>Dikkomys</i> and <i>Pliosaccomys</i> beyond.</p>
<p class="wrd_sp"><i>Diagnosis.</i>—Small Geomyinae; lacking specializations of more advanced
tribes; upper incisors smooth, at least in <i>Pliosaccomys</i>; 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 <i>Pliosaccomys</i>); 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);
<span class="pagenum"><a name="Page_516" id="Page_516">[516]</a></span>
mandible stout, its angle mostly above mandibular tooth-row; masseteric
ridge low; basitemporal fossa barely discernable in some fragments of
<i>Pliosaccomys</i>; postcranial skeleton unknown.</p>
<p>Key to the Genera of the Tribe Dikkomyini</p>
<div class="key2sp1">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 <i>Dikkomys</i></div>
<div class="key2sp9"><a href="#Genus_Dikkomys">p. 516</a></div>
<div class="key2sp1">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 <i>Pliosaccomys</i></div>
<div class="key2sp9"><a href="#Genus_Pliosaccomys">p. 517</a></div>
</div>
<p class="caption4"><a name="Genus_Dikkomys" id="Genus_Dikkomys"></a>Genus <b>Dikkomys</b> Wood</p>
<div class="smaller">
<p class="spec_acc">1936. <i>Dikkomys</i> Wood, Amer. Mus. Novit., 866:26, July 2.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Dikkomys matthewi</i> Wood, 1936, from Lower Harrison deposits
near Agate, Sioux County, Nebraska.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—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.</p>
<p><i>Description.</i>—Size small, about as in small kinds of <i>Thomomys</i>; 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.</p>
<p class="wrd_sp">Evidently, <i>Dikkomys matthewi</i> is more primitive than <i>Dikkomys woodi</i>.
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 <i>Dikkomys woodi</i>, 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.</p>
<p><span class="pagenum"><a name="Page_517" id="Page_517">[517]</a></span></p>
<p>Specimen (No. P 26284 FMNH) reported as <i>Dikkomys matthewi</i> by Galbreath
(1948:316) is referable to the recently described species <i>Dikkomys
woodi</i> Black, 1961.</p>
<p><i>Specimens examined.</i>—One, no. P 26284, Field Mus. Nat. Hist., from upper
Rosebud, Shannon Co., South Dakota.</p>
<p><i>Referred species.</i>—two:</p>
<p class="spec_acc"><i>Dikkomys matthewi</i> Wood, 1936. Amer. Mus. Novit., 866:26, July.
Type from early Arikareean Lower Harrison deposits (early Miocene)
near Agate, Sioux County, Nebraska.</p>
<p class="spec_acc"><i>Dikkomys woodi</i> 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.</p>
</div>
<p class="caption4"><a name="Genus_Pliosaccomys" id="Genus_Pliosaccomys"></a>Genus <b>Pliosaccomys</b> Wilson</p>
<div class="smaller">
<p class="spec_acc">1936. <i>Pliosaccomys</i> Wilson, Carnegie Inst. Washington Publ.,
473:20, May 21.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Pliosaccomys dubius</i> Wilson, 1936, from Smiths Valley local fauna in
Lyon County, Nevada.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—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).</p>
<p><i>Description.</i>—Size small (alveolar length of mandibular tooth-row measuring
6.0 in holotype), about as in <i>Thomomys monticola</i>; 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, <a href="#Page_546">p. 546</a>);
m3 (M3 unknown, but probably with same form as in Geomyini, see
<a href="#Page_552">p. 552</a>) 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.</p>
<p><i>Specimens examined.</i>—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.</p>
<p><i>Referred species.</i>—two:</p>
<p class="spec_acc">*<i>Pliosaccomys dubius</i> 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.</p>
<p><span class="pagenum"><a name="Page_518" id="Page_518">[518]</a></span></p>
<p class="spec_acc">*<i>Pliosaccomys wilsoni</i> 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.</p>
</div>
<p class="caption4"><a name="Tribe_Thomomyini" id="Tribe_Thomomyini"></a>
Tribe <span class="smcap"><b>Thomomyini</b></span>, new tribe</p>
<div class="smaller">
<p class="wrd_sp"><i>Type.</i>—<i>Thomomys</i> Wied-Neuwied, 1839.</p>
<p class="wrd_sp"><i>Chronologic and geographic range.</i>—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.</p>
<p><i>Diagnosis.</i>—Size small to medium (basilar length exclusive of <i>T. bulbivorus</i>,
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 <i>T. monticola</i> 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 <i>Pliosaccomys</i>, 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 (<i>T. bulbivorus</i>); 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 <i>T. townsendii</i>; 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.</p>
<p>The tribe Thomomyini is monotypic, including only the genus <i>Thomomys</i>.</p>
</div>
<p class="caption4"><a name="Genus_Thomomys" id="Genus_Thomomys"></a>Genus <b>Thomomys</b> Wied-Neuwied</p>
<div class="smaller">
<p class="spec_acc">1839. <i>Thomomys</i> Wied-Neuwied, Nova Acta Phys. Med. Acad. Caesar.
Leop.-Carol., 19(1):377.</p>
<p class="spec_acc">1836. <i>Oryctomys</i> Eydoux and Gervais (in part), Mag. de Zool., 6:20, pl. 21.
Type: <i>Oryctomys</i> (<i>Saccophorus</i>) <i>bottae</i>, from coast of California,
probably near Monterey.</p>
<p class="spec_acc">1903. <i>Megascapheus</i> Elliot, Field Columb. Mus., Publ. 76, Zool. Ser.,
3(11):190, July 25. Type: <i>Diplostoma bulbivorum</i> Richardson, from
Columbia River, probably near Portland, Ore.</p>
<p class="spec_acc">1933. <i>Pleisothomomys</i> Gidley and Gazin, Jour. Mamm. 14:354. Type:
<i>Pleisothomomys potomacensis</i> Gidley and Gazin, from Pleistocene,
Cumberland Cave local fauna, Allegany County, Maryland.</p>
<p><i>Chronologic range.</i>—Known from late Pliocene to Recent.</p>
<p><i>Description.</i>—Same as that given for the tribe Thomomyini above.</p>
<p><i>Discussion.</i>—Features characterizing <i>Thomomys</i> and the tribe Thomomyini
are more advanced than those characterizing the tribe Dikkomyini. Also, the
<span class="pagenum"><a name="Page_519" id="Page_519">[519]</a></span>
Thomomyini retain more of the primitive features of the Geomyinae than do
the more specialized tribe Geomyini.</p>
<p>Specializations are few, but include the third molar being a single column
both above and below, enamel plates, and a masseteric ridge.</p>
<p>Key to the Subgenera of <i>Thomomys</i></p>
<div class="key2sp1">A Molars sub-crescent or ovate in cross-section, not becoming
abruptly narrower at one end of tooth.
Subgenus <i>Pleisothomomys</i></div>
<div class="key2sp9"><a href="#Subgenus_Pleisothomomys">p. 519</a></div>
<div class="key2sp1">A´ Molars pear-shaped, not sub-crescent or ovate, in cross-section,
crown becoming abruptly narrow at one end of tooth.
Subgenus <i>Thomomys</i></div>
<div class="key2sp9"><a href="#Subgenus_Thomomys">p. 520</a></div>
</div>
<p class="caption4"><a name="Subgenus_Pleisothomomys" id="Subgenus_Pleisothomomys"></a>Subgenus <b>Pleisothomomys</b> Gidley and Gazin</p>
<div class="smaller">
<p class="spec_acc">1933. <i>Pleisothomomys</i> Gidley and Gazin, Jour. Mamm., 14:354,
November 13.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Pleisothomomys potomacensis</i> Gidley and Gazin, 1933.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—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 <i>Pleisothomomys</i> was restricted to that region while
the subgenus <i>Thomomys</i> occupied the western United States and parts of
Canada and México as it does today.</p>
<p><i>Description and Comparison.</i>—Separated from subgenus <i>Thomomys</i> 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 <i>Thomomys vetus</i> of the late
Pleistocene (Wisconsin deposits), referred to the subgenus <i>Thomomys</i> on the
basis of its alleged relationship to <i>Thomomys townsendii</i> (see Davis, 1937:156-158),
are less distinctly pear-shaped, and are more sub-crescentic, than
in any other known species of the subgenus <i>Thomomys</i>. <i>Pleisothomomys</i> Gidley
and Gazin (<i>loc. cit.</i>) 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 <i>Thomomys</i> occurring in Pleistocene times in the eastern
United States.</p>
<p><i>Referred species.</i>—Three (all extinct):</p>
<p class="spec_acc">*<i>Thomomys gidleyi</i> Wilson, 1933. Carnegie Inst. Washington Publ.
440:122, December. Type from Hagerman beds, late Pliocene,
Idaho.</p>
<p class="spec_acc">*<i>Thomomys potomacensis</i> Gidley and Gazin, 1933. Jour. Mamm.,
14:354, November 13. Type from Cumberland Cave, middle and late
Pleistocene, Maryland.</p>
<p class="spec_acc">*<i>Thomomys orientalis</i> Simpson, 1928. Amer. Mus. Novit., 328:6,
October 26. Type from Saber-tooth Cave, late Pleistocene,
Florida.</p>
</div>
<p><span class="pagenum"><a name="Page_520" id="Page_520">[520]</a></span></p>
<p class="caption4"><a name="Subgenus_Thomomys" id="Subgenus_Thomomys"></a>
Subgenus <b>Thomomys</b> Wied-Neuwied</p>
<div class="smaller">
<p class="spec_acc">1839. <i>Thomomys</i> Wied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar.
Leop. Carol., 19(1):377.</p>
<p class="spec_acc">1903. <i>Megascapheus</i> Elliot, Field Columb. Mus., Publ. 76, Zool.
Ser., 3 (11):190, July 25. Type: <i>Diplostoma bulbivorum</i>
Richardson, from Columbia River, probably near Portland, Oregon.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Thomomys rufescens</i> Wied-Neuwied, 1839.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—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).</p>
<p><i>Description.</i>—Molars pear-shaped in cross-section, becoming abruptly narrow
at one end of the tooth. The teeth of the late Pleistocene species <i>Thomomys
vetus</i> are less distinctly pear-shaped than other referred species (see remarks
in the description of the subgenus <i>Pleisothomomys</i>).</p>
<p>Essentially on the basis of its significantly larger size and details of the
skull, Elliott (1903:190) proposed subgeneric recognition of <i>Thomomys bulbivorus</i>
and described the subgenus <i>Megascapheus</i> to include it. Also the molars
of <i>Thomomys bulbivorus</i> 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 <i>Thomomys
bulbivorus</i> 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 <i>bulbivorus</i> species-group.</p>
<p><i>Referred species.</i>—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).</p>
<p><i>bulbivorus</i> species-group</p>
<p class="spec_acc"><i>Thomomys bulbivorus</i> (Richardson, 1829). Fauna Boreali-Americana,
1:206. Type from Columbia River, probably near Portland, Oregon.</p>
<p><i>umbrinus</i> species-group</p>
<p class="spec_acc">*<i>Thomomys scudderi</i> Hay, 1921. Proc. U. S. Nat. Mus., 49:614.
Type from Fossil Lake beds, late Pleistocene, Oregon.</p>
<p class="spec_acc"><i>Thomomys umbrinus</i> (Richardson, 1829). Fauna Boreali-Americana,
1:202. Type from southern México, probably near Boca de
Monte, Veracruz.</p>
<p class="spec_acc"><i>Thomomys bottae</i> (Eydoux and Gervais, 1836). Mag. de Zool., Paris,
6:23. Type from coast of California, probably near Monterey.</p>
<p class="spec_acc">*<i>Thomomys vetus</i> Davis, 1937. Jour. Mamm., 18:156, May 12. Type
from Fossil Lake beds, late Pleistocene, Oregon.</p>
<p class="spec_acc"><i>Thomomys townsendii</i> (Bachman, 1839). Jour. Acad. Nat. Sci.
Philadelphia, 8:105. Type probably from near Nampa, Canyon
Co., Idaho (erroneously given as "Columbia River").</p>
<p><i>talpoides</i> species-group</p>
<p class="spec_acc">*<i>Thomomys microdon</i> Sinclair, 1905. Bull. Dept. Geol. Univ.
California, 4:145-161. Type from Potter Creek Cave, late
Pleistocene, California.</p>
<p class="spec_acc"><i>Thomomys monticola</i> J. A. Allen, 1893. Bull. Amer. Mus. Nat. Hist.,
5:48, April 28. Type from Mt. Tallac, 7500 ft., El Dorado
Co., California.</p>
<p><span class="pagenum"><a name="Page_521" id="Page_521">[521]</a></span></p>
<p class="spec_acc"><i>Thomomys talpoides</i> (Richardson, 1828). Zool. Jour., 3:518. Type
locality fixed at near Fort Carlton (Carlton House),
Saskatchewan River, Saskatchewan, Canada.</p>
<p class="spec_acc"><i>Thomomys mazama</i> Merriam, 1897. Proc. Biol. Soc. Washington,
11:214, July 15. Type from Anna Creek, 6000 ft., near Crater
Lake, Mt. Mazama, Klamath Co., Washington.</p>
</div>
<p class="caption4"><a name="Tribe_Geomyini" id="Tribe_Geomyini"></a>
Tribe <span class="smcap"><b>Geomyini</b></span>, new tribe</p>
<div class="smaller">
<p class="wrd_sp"><i>Genotype.</i>—<i>Geomys</i> Rafinesque, 1817.</p>
<p class="wrd_sp"><i>Chronologic and geographic range.</i>—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.</p>
<p><i>Diagnosis.</i>—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 (<i>Pliogeomys</i>); 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 <i>Dikkomys</i>, and upper molars as in
<i>Pliosaccomys</i> (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 <i>Geomys</i> and <i>Pappogeomys</i>
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.</p>
<p>Enamel of cheek teeth reduced to interrupted plates, with exception of p4
in <i>Pliogeomys</i>; 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 <i>Pappogeomys</i>) 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
<i>Pliogeomys</i> (only Geomyini having both anterior and posterior enamel plates
on lower molars).</p>
<p>Skull primitively generalized, but becoming specialized towards either
dolichocephaly (<i>Orthogeomys</i>) or platycephaly (<i>Pappogeomys</i>) 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 (<i>Pliogeomys</i> and early members of <i>Zygogeomys</i>); pelage usually
soft, but harsh and hispid in some genera; forefeet broad and massive, claws
long and stout for digging; body form remarkably fossorial.</p>
<p>The tribe Geomyini includes the most highly specialized members of the
subfamily Geomyinae.</p>
<p><span class="pagenum"><a name="Page_522" id="Page_522">[522]</a></span></p>
<p>Key to the Genera of the Tribe Geomyini</p>
<div class="key2sp1">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 <i>Pliogeomys</i></div>
<div class="key2sp9"><a href="#Genus_Pliogeomys">p. 522</a></div>
<div class="key2sp1">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 <i>Geomys tobinensis</i> of middle Pleistocene);
masseteric crest strongly developed and massive.</div>
<div class="key2sp2">B Enamel plate on posterior wall of P4, but usually restricted
to lingual end of tooth (usually absent in subgenus <i>Orthogeomys</i>
of genus <i>Orthogeomys</i>); M3 conspicuously bicolumnar,
longer than wide owing to elongation of posterior loph.</div>
<div class="key2sp3">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 <i>Zygogeomys</i></div>
<div class="key2sp9"><a href="#Genus_Zygogeomys">p. 523</a></div>
<div class="key2sp3">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 <i>Orthogeomys</i></div>
<div class="key2sp9"><a href="#Genus_Orthogeomys">p. 528</a></div>
<div class="key2sp2">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.</div>
<div class="key2sp4">D Upper incisor bisulcate; skull generalized; both anterior
and posterior walls of M1 and M2 having complete enamel
plates. Genus <i>Geomys</i></div>
<div class="key2sp9"><a href="#Genus_Geomys">p. 525</a></div>
<div class="key2sp4">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, <i>Pappogeomys bulleri</i>); enamel
plate on posterior wall of M2 also absent in advanced
species (subgenus <i>Cratogeomys</i>).
Genus <i>Pappogeomys</i></div>
<div class="key2sp9"><a href="#Genus_Pappogeomys">p. 532</a></div>
</div>
<p class="caption4"><a name="Genus_Pliogeomys" id="Genus_Pliogeomys"></a>Genus <b>Pliogeomys</b> Hibbard</p>
<div class="smaller">
<p class="spec_acc">1954. <i>Pliogeomys</i> Hibbard, Michigan Acad. Sci., Arts and
Letters, 39:353.</p>
<p class="wrd_sp"><i>Genotype.</i>—<i>Pliogeomys buisi</i> Hibbard, 1954, from Buis Ranch local fauna
(middle Pliocene), Beaver County, Oklahoma.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—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, <i>op. cit.</i>:342).</p>
<p><i>Description and discussion.</i>—The size of members of this small genus of the
Geomyinae is about the same as in smaller adults of <i>Geomys bursarius</i>. According
to Hibbard (<i>op. cit.</i>:353), the holotype is smaller than specimens
from the Rexroad local fauna referred to <i>Geomys quinni</i> and larger than specimens
referred to <i>Zygogeomys</i> cf. <i>minor</i>. 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.</p>
<p><span class="pagenum"><a name="Page_523" id="Page_523">[523]</a></span></p>
<p>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 <i>Pliosaccomys</i>)
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 <i>Nerterogeomys</i> [= <i>Zygogeomys</i>] cf. <i>minor</i> (see Hibbard, 1954:fig.
5, A and B) and <i>Pliosaccomys dubius</i> (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.</p>
<p>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.</p>
<p><i>Specimens examined.</i>—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.</p>
<p><i>Referred species.</i>—One.</p>
<p class="spec_acc">*<i>Pliogeomys buisi</i> Hibbard, 1954. Papers Michigan Acad. Sci., Arts, and
Letters, 39:353. Type from Buis local fauna, latest middle Pliocene,
Beaver County, Oklahoma.</p>
</div>
<p class="caption4"><a name="Genus_Zygogeomys" id="Genus_Zygogeomys"></a>Genus <b>Zygogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Zygogeomys</i> Merriam, N. Amer. Fauna, 8:195, January 31.</p>
<p class="spec_acc">1942. <i>Nerterogeomys</i> Gazin, Proc. U. S. Nat. Mus., 92:507
(type, <i>Geomys persimilis</i> Hay, 1927).</p>
<p class="wrd_sp"><i>Type.</i>—<i>Zygogeomys trichopus</i> Merriam, 1895, from Nahuatzen, Michoacán.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—Late Pliocene (Benson and Curtis Ranch local faunas,
Arizona, and ?Rexroad Formation, Kansas) to Recent.</p>
<p><i>Description and discussion.</i>—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 <i>Orthogeomys cherriei
costaricensis</i>).</p>
<p><span class="pagenum"><a name="Page_524" id="Page_524">[524]</a></span></p>
<p>The upper incisor, recovered in material from the late Pliocene and middle
Pleistocene, is bisulcate as in the genus <i>Geomys</i> and the primitive genus <i>Pliogeomys</i>.
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 <i>Nerterogeomys</i> cf. <i>minor</i> 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 <i>P. minor</i>) 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 <i>Geomys</i> 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 <i>Pliogeomys</i>). 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).</p>
<p>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 (<i>Z. trichopus</i>),
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 (<i>Z. minor</i>) and the
middle Pleistocene species (<i>Z. persimilis</i>), 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 <i>Orthogeomys</i>;
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 (<i>Z. persimilis</i>) 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 <i>Pliogeomys</i>).</p>
<p>The masseteric ridge of the mandible is well developed. In the late Pliocene
species <i>Z. persimilis</i> and <i>Z. minor</i> the mental foramen is directly beneath the
anterior extension of the masseteric ridge, but in the living species, <i>Z. trichopus</i>,
the foramen lies well anterior to the ridge. The basitemporal fossa in the living
<span class="pagenum"><a name="Page_525" id="Page_525">[525]</a></span>
species is well developed and deep; in the Pliocene species it is usually distinct
but shallow (late Pliocene specimens of <i>Z. minor</i>).</p>
<p><i>Referred species.</i>—Three (two extinct and one living; the last has two subspecies):</p>
<p class="spec_acc">*<i>Zygogeomys minor</i> (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.</p>
<p class="spec_acc">*<i>Zygogeomys persimilis</i> Hay, 1927. Carnegie Inst. Washington
Publ., 136. Originally described by Gidley, 1922 (U. S. Geol.
Surv. Prof. Papers, 131:123, December 26) as <i>Geomys
parvidens</i> which was preoccupied by <i>G. parvidens</i> Brown,
1908. Type from Curtis Ranch local fauna (middle
Pleistocene), Cochise County, Arizona.</p>
<p class="spec_acc"><i>Zygogeomys trichopus trichopus</i> Merriam, 1895. N. Amer. Fauna,
8:196, January 31. Type from Nahuatzen, Michoacán.</p>
<p class="spec_acc"><i>Zygogeomys trichopus tarascensis</i> Goldman, 1938. Proc. Biol. Soc.
Washington, 51:211, December 23. Type from 6 mi. SE
Pátzcuaro, 8,000 ft., Michoacán.</p>
</div>
<p class="caption4"><a name="Genus_Geomys" id="Genus_Geomys"></a>Genus <b>Geomys</b> Rafinesque</p>
<div class="smaller">
<p class="spec_acc">1817. <i>Geomys</i> Rafinesque, Amer. Monthly Mag., 2(1):45, November.</p>
<p class="spec_acc">1817. <i>Diplostoma</i> Rafinesque, Amer. Monthly Mag., 2(1):44-45,
November. Included species: <i>Diplostoma fusca</i> Rafinesque
[= <i>Mus bursarius</i> Shaw] and <i>Diplostoma alba</i> Rafinesque
[= <i>Mus bursarius</i> Shaw] from the Missouri River region.</p>
<p class="spec_acc">1820. <i>Saccophorus</i> Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66.
Type: <i>Mus bursarius</i> Shaw, from upper Mississippi Valley.</p>
<p class="spec_acc">1823. <i>Pseudostoma</i> Say, Long's Expd. Rocky Mts., I, pp. 406.
Type: <i>Pseudostoma bursaria</i> [= <i>Mus bursarius</i> Shaw],
from upper Mississippi Valley.</p>
<p class="spec_acc">1825. <i>Ascomys</i> Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822),
p. 20., fig. 2. Type: <i>Ascomys canadensis</i> Lichtenstein
[= <i>Mus bursarius</i> Say], probably from upper Mississippi Valley.</p>
<p class="spec_acc">1944. <i>Parageomys</i> Hibbard, Bull. Geol. Soc. Amer., 55:735, June.
Type: <i>Parageomys tobinensis</i> Hibbard, from Pleistocene, Cudahy
(Tobin) local fauna, Russell Co., Kansas.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys pinetis</i> Rafinesque, 1817, restricted to Screven County,
Georgia, in region of the pines.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—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.</p>
<p><i>Description and discussion.</i>—Pocket gophers of this genus are medium-sized
geomyids; none is so small as the average-sized <i>Thomomys</i>. The skull is generalized
and lacks the dolichocephalic and platycephalic specializations seen in
the genera <i>Orthogeomys</i> and <i>Pappogeomys</i>, respectively. <i>Geomys</i> closely resembles
<i>Zygogeomys</i>, but retains fewer of the primitive characters of the
ancestral stock. At the same time, <i>Geomys</i> has several specializations. Even so,
a considerable amount of parallelism is evident in the phyletic trends of the two
genera.</p>
<p>The upper incisor of <i>Geomys</i> is bisulcate as in <i>Pliogeomys</i> and <i>Zygogeomys</i>;
the deeper grove is medial and the shallower grove lies near the inner border
<span class="pagenum"><a name="Page_526" id="Page_526">[526]</a></span>
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 <i>Geomys</i>, the enamel bands are continuous as observed in juveniles of
<i>Geomys bursarius major</i> (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 <i>Geomys</i>,
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. <i>Geomys quinni</i> of the late Pliocene and early
Pleistocene has the interrupted pattern seen in late Pleistocene and Recent
<i>Geomys</i>. 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.</p>
<p>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
<i>Geomys</i>, 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 <i>Geomys</i> 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 <i>Geomys tobinensis</i>, for example, a thin, but transversely
complete, plate of enamel occurs all the way down to the base of the loph
(Paulson, <i>loc. cit.</i>) and would persist throughout life. In <i>Geomys garbanii</i>, a
vestige on the lingual side of the posterior surface of a fully adult specimen was
noted by White and Downs (<i>loc. cit.</i>). Vestiges of the posterior plate occur less
frequently in living geomyids. Paulson (<i>loc. cit.</i>) found a posterior plate in one
of 75 specimens of <i>Geomys bursarius dutcheri</i>. A young (suture present between
exoccipitals and supraoccipital) female of <i>Geomys pinetis austrinus</i>
(KU 23358) has a vestige of the posterior plate on the lingual side of the tooth
as White and Downs (<i>loc. cit.</i>) observed in a specimen of <i>Geomys garbanii</i>.
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.</p>
<p>A lower molar of <i>Geomys</i> is a single elliptical column, and enamel is restricted
to the posterior surface as in <i>Zygogeomys</i>, <i>Orthogeomys</i>, and <i>Pappogeomys</i>.
Paulson (<i>loc. cit.</i>) found a thin enamel plate on the anterior surfaces
of the lower molars in about five per cent of the individuals of <i>Geomys tobinensis</i>
<span class="pagenum"><a name="Page_527" id="Page_527">[527]</a></span>
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 <i>Pliogeomys</i> of the middle Pliocene.
Occurrence of the plate in <i>Geomys tobinensis</i> is an atavistic trait. Primitive
dental patterns occur occasionally in geomyids, as pointed out above, but the
frequency of occurrence in <i>G. tobinensis</i> is higher than would be expected.</p>
<p>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 <i>Geomys</i> 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 <i>Geomys</i> 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 <i>Geomys garbanii</i> of the Middle
Pleistocene than in other species. Less than 24 per cent of the third upper
molars in <i>Geomys garbanii</i> 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 <i>Geomys</i>. In Recent <i>Geomys</i> the fold is more common in
the eastern <i>pinetis</i> species-group than in the western <i>bursarius</i> species-group.</p>
<p>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.</p>
<p><i>Referred species.</i>—The twelve species, five of which are extinct, are as
follows:</p>
<p><i>quinni</i> species-group</p>
<p class="spec_acc">*<i>Geomys quinni</i> 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.</p>
<p class="spec_acc">*<i>Geomys paenebursarius</i> Strain, 1966. Bull. Texas Memorial Mus.,
10:36. Type from Hudspeth local fauna (early Pleistocene),
Hudspeth County, Texas.</p>
<p class="spec_acc">*<i>Geomys tobinensis</i> 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.</p>
<p class="spec_acc">*<i>Geomys garbanii</i> 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.</p>
<p class="spec_acc">*<i>Geomys bisulcatus</i> Marsh, 1871. Amer. Jour. Sci., 3:121. Type
from Loup River fossil beds, near Camp Thomas, Nebraska
(probably late Pleistocene).</p>
<p><span class="pagenum"><a name="Page_528" id="Page_528">[528]</a></span></p>
<p><i>bursarius</i> species-group</p>
<p class="spec_acc">*<i>Geomys parvidens</i> Brown, 1908. Mem. Amer. Mus. Nat. Hist.,
9:194. (An extinct subspecies of <i>Geomys bursarius</i> according
to White and Downs, 1961:6). Type from Conard Fissure local
fauna (late Pleistocene), northern Arkansas.</p>
<p class="spec_acc"><i>Geomys bursarius</i> (Shaw, 1800). Trans. Linn. Soc. London, 5:227.
Type from somewhere in Upper Mississippi Valley, North
America.</p>
<p class="spec_acc"><i>Geomys arenarius</i> Merriam, 1895. N. Amer. Fauna, 8:139, January 31.
Type from El Paso, El Paso County, Texas.</p>
<p class="spec_acc"><i>Geomys personatus</i> True, 1889. Proc. U. S. Nat. Mus., 11:159,
January 5. Type from Padre Island, Cameron County, Texas.</p>
<p><i>pinetis</i> species-group</p>
<p class="spec_acc"><i>Geomys pinetis</i> Rafinesque, 1806. Amer. Monthly Mag., 2 (1):45,
November. Type locality restricted to Screven County,
Georgia.</p>
<p class="spec_acc"><i>Geomys colonus</i> Bangs, 1898. Proc. Boston Soc. Nat. Hist., 28:178,
March. Type from Arnot Plantation, about 4 mi. W St. Marys,
Camden County, Georgia.</p>
<p class="spec_acc"><i>Geomys cumberlandius</i> Bangs, 1898. Proc. Boston Soc. Nat. Hist.,
28:180, March. Type from Stafford Place, Cumberland Island,
Camden County, Georgia.</p>
<p class="spec_acc"><i>Geomys fontanelus</i> Sherman, 1940. Jour. Mamm., 21:341, August 13.
Type from 7 mi. NW Savannah, Chatham County, Georgia.</p>
</div>
<p class="caption4"><a name="Genus_Orthogeomys" id="Genus_Orthogeomys"></a>Genus <b>Orthogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Orthogeomys</i> Merriam, N. Amer. Fauna 8:172, January 31.</p>
<p class="spec_acc">1895. <i>Heterogeomys</i> Merriam, N. Amer. Fauna 8:179, January 31
(type, <i>Geomys hispidus</i> Le Conte, 1862).</p>
<p class="spec_acc">1895. <i>Macrogeomys</i> Merriam, N. Amer. Fauna 8:185, January 31
(type, <i>Geomys heterodus</i> Peters, 1865).</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys scalops</i> Thomas, 1894, from Tehuantepec, Oaxaca, México.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—Late Pleistocene Wisconsin deposits (San Josecito Cave
local fauna, Nuevo León, México) to Recent.</p>
<p><i>Description and discussion.</i>—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.</p>
<p>The upper incisor is unisulcate; the sulcus is usually near the inner border
of the tooth, but in some species (subgenus <i>Orthogeomys</i>) 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
<span class="pagenum"><a name="Page_529" id="Page_529">[529]</a></span>
as in the living species of the genus <i>Zygogeomys</i>. In the subgenus <i>Orthogeomys</i>
the posterior plate is usually absent, and otherwise is narrow and near
the lingual border of the tooth.</p>
<p>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 <i>Orthogeomys</i> is infrequently separated into
two small plates.</p>
<p>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 <i>Orthogeomys</i>, and in young individuals is hardly
more than a slight depression.</p>
<p>Key to the Subgenera of <i>Orthogeomys</i></p>
<div class="key2sp1">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 <i>Orthogeomys</i></div>
<div class="key2sp9"><a href="#Subgenus_Orthogeomys">p. 529</a></div>
<div class="key2sp1">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.</div>
<div class="key2sp2">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 <i>Heterogeomys</i></div>
<div class="key2sp9"><a href="#Subgenus_Heterogeomys">p. 530</a></div>
<div class="key2sp2">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 <i>Macrogeomys</i></div>
<div class="key2sp9"><a href="#Subgenus_Macrogeomys">p. 531</a></div>
</div>
<p class="caption4"><a name="Subgenus_Orthogeomys" id="Subgenus_Orthogeomys"></a>
Subgenus <b>Orthogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Orthogeomys</i> Merriam, N. Amer. Fauna, 8:172, January 31.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys scalops</i> Thomas, 1894, from Tehuantepec, Oaxaca, México.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—Known only from the Recent.</p>
<p><i>Description.</i>—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
<span class="pagenum"><a name="Page_530" id="Page_530">[530]</a></span>
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.</p>
<p><i>Referred species and subspecies.</i>—Fourteen taxa:</p>
<p class="spec_acc"><i>Orthogeomys grandis alleni</i> Nelson and Goldman, 1930. Jour. Mamm.,
11:156, May 9. Type from near Acapulco, 2000 ft., Guerrero.</p>
<p class="spec_acc"><i>Orthogeomys grandis annexus</i> Nelson and Goldman, 1933. Proc. Biol.
Soc. Washington, 46:195, October 26. Type from Tuxtla
Gutierrez, 2600 ft., Chiapas.</p>
<p class="spec_acc"><i>Orthogeomys grandis carbo</i> Goodwin, 1956. Amer. Mus. Novit.,
1757:5, March 8. Type from Excurano, 2500 ft., Cerro de San
Pedro, 20 km. W Mixtequilla, Oaxaca.</p>
<p class="spec_acc"><i>Orthogeomys grandis felipensis</i> Nelson and Goldman, 1930. Jour.
Mamm., 11:157, May 9. Type from Cerro San Felipe, 10 mi. N
Oaxaca, Oaxaca.</p>
<p class="spec_acc"><i>Orthogeomys grandis huixtlae</i> Villa, 1944. Anal. Inst. Biol. Univ.
Nac. México, 15:319. Type from Finca Lubeca, 12 km. NE
Huixtla, 850 m., Chiapas.</p>
<p class="spec_acc"><i>Orthogeomys grandis grandis</i> (Thomas, 1893). Ann. Mag. Nat. Hist.,
ser. 6, 12:270, October. Type from Dueñas, Guatemala.</p>
<p class="spec_acc"><i>Orthogeomys grandis latifrons</i> Merriam, 1895. N. Amer. Fauna,
8:178, January 31. Type from Guatemala, exact locality
unknown.</p>
<p class="spec_acc"><i>Orthogeomys grandis nelsoni</i> Merriam, 1895. N. Amer. Fauna, 8:176,
January 31. Type from Mt. Zempoaltepec, 8000 ft., Oaxaca.</p>
<p class="spec_acc"><i>Orthogeomys grandis pluto</i> Lawrence, 1933. Proc. New England Zool.
Club, 13:66, May 8. Type from Cerro Cantoral, north of
Tegucigalpa, Honduras.</p>
<p class="spec_acc"><i>Orthogeomys grandis scalops</i> (Thomas, 1894). Ann. Mag. Nat. Hist.,
ser. 6, 13:437, May. Type from Tehuantepec, Oaxaca.</p>
<p class="spec_acc"><i>Orthogeomys grandis soconuscensis</i> 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.</p>
<p class="spec_acc"><i>Orthogeomys grandis guerrerensis</i> 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.</p>
<p class="spec_acc"><i>Orthogeomys cuniculus</i> Elliot, 1905. Proc. Biol. Soc. Washington,
18:234, December 9. Type from Zanatepec, Oaxaca.</p>
<p class="spec_acc"><i>Orthogeomys pygacanthus</i> Dickey, 1928. Proc. Biol. Soc. Washington,
41:9, February 1. Type from Cacaguatique, 3500 ft., Dept. San
Miguel, El Salvador.</p>
</div>
<p class="caption4"><a name="Subgenus_Heterogeomys" id="Subgenus_Heterogeomys"></a>Subgenus <b>Heterogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Heterogeomys</i> Merriam, N. Amer. Fauna, 8:179, January 21.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys hispidus</i> Le Conte, 1852, from near Jalapa, Veracruz.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—Late Pleistocene, Wisconsin deposits (San Josecito
Cave local fauna, Nuevo León) to the Recent.</p>
<p><i>Description.</i>—Skull dolichocephalic (less so than in the other subgenera);
zygomata more widely spreading than in <i>Orthogeomys</i>; 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 <i>Orthogeomys</i>; 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
<span class="pagenum"><a name="Page_531" id="Page_531">[531]</a></span>
deep fold well developed in all members of genus; posterior loph forming conspicuous
heel longer than in subgenus <i>Orthogeomys</i>; 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.</p>
<p><i>Referred species and subspecies.</i>—Eleven taxa:</p>
<p class="spec_acc">*<i>Orthogeomys onerosus</i> (Russell, 1960). Univ. Kansas Publ., Mus.
Nat. Hist., 9 (21):544, January 14. Type from San Josecito
Cave local fauna, Upper Pleistocene, Nuevo León.</p>
<p class="spec_acc"><i>Orthogeomys hispidus cayoensis</i> (Burt, 1937). Occ. Papers Mus.
Zool., Univ. Michigan, 365:1, December 16. Type from Mountain
Pine Ridge, 12 mi. S El Cayo, British Honduras.</p>
<p class="spec_acc"><i>Orthogeomys hispidus chiapensis</i> (Nelson and Goldman, 1929). Proc.
Bio. Soc. Washington, 42:151, March 30. Type from Tenejapa,
16 mi. NE San Cristobal, Chiapas.</p>
<p class="spec_acc"><i>Orthogeomys hispidus concavas</i> (Nelson and Goldman, 1929). Proc.
Biol. Soc. Washington, 42:148, March 30. Type from Pinal de
Amoles, Querétaro.</p>
<p class="spec_acc"><i>Orthogeomys hispidus hispidus</i> (Le Conte, 1852). Proc. Acad. Nat.
Sci. Philadelphia, 6:158. Type from near Jalapa, Veracruz.</p>
<p class="spec_acc"><i>Orthogeomys hispidus latirostris</i> (Hall and Alvarez, 1961). Anal.
Escuela Nac. Ciencias Biol., 10:121, December 20. Type from
Hacienda Tamiahua, Cabo Rojo, Veracruz.</p>
<p class="spec_acc"><i>Orthogeomys hispidus negatus</i> (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.</p>
<p class="spec_acc"><i>Orthogeomys hispidus tehuantepecus</i> (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.</p>
<p class="spec_acc"><i>Orthogeomys hispidus torridas</i> (Merriam, 1895). N. Amer. Fauna,
8:183, January 31. Type from Chichicaxtle, Veracruz.</p>
<p class="spec_acc"><i>Orthogeomys hispidus yucatanensis</i> (Nelson and Goldman, 1929).
Proc. Biol. Soc. Washington, 42:150, March 30. Type from
Campeche, Campeche.</p>
<p class="spec_acc"><i>Orthogeomys lanius</i> (Elliot, 1905). Proc. Biol. Soc. Washington,
18:235, December 9. Type from Xuchil, Veracruz.</p>
</div>
<p class="caption4"><a name="Subgenus_Macrogeomys" id="Subgenus_Macrogeomys"></a>Subgenus <b>Macrogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Macrogeomys</i> Merriam, N. Amer. Fauna, 8:185, January 31.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys heterodus</i> Peters, 1865, from Costa Rica, exact locality unknown.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—Known only from the Recent.</p>
<p><i>Description.</i>—Skull dolichocephalic in varying degree (overlapping subgenera
<i>Orthogeomys</i> and <i>Heterogeomys</i> 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 <i>Orthogeomys</i>;
some species having white markings, especially on lumbar region and head.</p>
<p><span class="pagenum"><a name="Page_532" id="Page_532">[532]</a></span></p>
<p><i>Referred species and subspecies.</i>—Eleven taxa:</p>
<p class="spec_acc"><i>Orthogeomys heterodus cartagoensis</i> (Goodwin, 1943). Amer. Mus.
Novit., 1227:2, April 22. Type from Paso Ancho, Province
Cartago, Costa Rica.</p>
<p class="spec_acc"><i>Orthogeomys heterodus dolichocephalus</i> (Merriam, 1895). N. Amer.
Fauna, 8:189, January 31. Type from San José, Costa Rica.</p>
<p class="spec_acc"><i>Orthogeomys heterodus heterodus</i> (Peters, 1865). Monatsb. preuss.
Acad. Wiss., Berlin, 1865:177. Type from Costa Rica, exact
locality unknown.</p>
<p class="spec_acc"><i>Orthogeomys cavator nigrescens</i> (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.</p>
<p class="spec_acc"><i>Orthogeomys cavator pansa</i> (Bangs, 1902). Bull. Mus. Comp. Zool.,
39:44, April. Type from Bogava (= Bugaba), 600 ft., Chiriquí,
Panamá.</p>
<p class="spec_acc"><i>Orthogeomys dariensis</i> (Goldman, 1912). Smithsonian Misc. Coll.,
60(2):8, September 20. Type from Cana, 2000 ft., mountains of
eastern Panamá.</p>
<p class="spec_acc"><i>Orthogeomys underwoodi</i> (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.</p>
<p class="spec_acc"><i>Orthogeomys cherriei carlosensis</i> (Goodwin, 1943). Amer. Mus.
Novit., 1227:3, April 22. Type from Cataratos, San Carlos,
Alajuela, Costa Rica.</p>
<p class="spec_acc"><i>Orthogeomys cherriei cherriei</i> (J. A. Allen, 1893). Bull. Amer.
Mus. Nat. Hist., 5:337, December 16. Type from Santa Clara,
Costa Rica.</p>
<p class="spec_acc"><i>Orthogeomys cherriei costaricensis</i> (Merriam, 1895). N. Amer.
Fauna, 8:192, January 31. Type from Pacuare, Costa Rica.</p>
<p class="spec_acc"><i>Orthogeomys matagalpae</i> (J. A. Allen, 1910). Bull. Amer. Mus. Nat.
Hist., 28:97, April 30. Type from Peña Blanca, Matagalpa,
Nicaragua.</p>
</div>
<p class="caption4"><a name="Genus_Pappogeomys" id="Genus_Pappogeomys"></a>Genus <b>Pappogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Pappogeomys</i> Merriam, N. Amer. Fauna, 8:145, January 31.</p>
<p class="spec_acc">1895. <i>Cratogeomys</i> Merriam, N. Amer. Fauna, 8:150, January 31.
Type: <i>Geomys merriami</i> Thomas.</p>
<p class="spec_acc">1895. <i>Platygeomys</i> Merriam, N. Amer. Fauna, 8:162, January 31.
Type: <i>Geomys gymnurus</i> Merriam; Hooper, Jour. Mamm.,
27:397, November 25, 1946.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys bulleri</i> Thomas, 1892, from near Talpa, west slope Sierra
de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—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 <i>Pappogeomys</i> 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.</p>
<p><i>Description and discussion.</i>—The size ranges from as little as in the smaller
kinds of <i>Thomomys</i> to the maximum attained in the subfamily and matched
elsewhere perhaps in only a few of the larger subspecies of <i>Orthogeomys
grandis</i>. 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
<span class="pagenum"><a name="Page_533" id="Page_533">[533]</a></span>
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 <i>Orthogeomys</i>.</p>
<p>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 <i>Pappogeomys</i>);
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 (<i>P. bensoni</i>)
the re-entrant angles are open (obtuse), a trait that is evidently primitive in
the Geomyini.</p>
<p>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.</p>
<p>M3 tends to remain at least incompletely bilophodont by reason of retaining
a permanent labial re-entrant fold in most species (with exceptions in
<i>Pappogeomys bulleri</i> and some old adults of <i>P. castanops</i>). Primitively the
occlusal surface of M3 is subtriangular (subgenus <i>Pappogeomys</i>), but in the
<i>castanops</i> species-group of the advanced subgenus <i>Cratogeomys</i>, 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 (<i>P. merriami fulvescens</i>) and, the loph has elongated into a
pronounced heel in some specimens, resembling the condition in <i>Orthogeomys</i>.
The entire range of variation occurs in <i>P. m. fulvescens</i>. The subtriangular
pattern is retained in the most specialized species of <i>Cratogeomys</i> where that
pattern is associated with extreme platycephaly in the <i>gymnurus</i> species-group.
In most species the posterior loph supports two lateral plates, the outer
one always bordering the labial re-entrant fold. In <i>Pappogeomys bulleri</i> and
in the <i>castanops</i> 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 <i>Pappogeomys bulleri</i>, 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.</p>
<p>The m3 of <i>P. bensoni</i> 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
<span class="pagenum"><a name="Page_534" id="Page_534">[534]</a></span>
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.</p>
<p>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, <i>Pappogeomys
fumosus</i>, the hairs are coarse and hispid somewhat as in <i>Orthogeomys</i>.</p>
<p>Key to the Subgenera of <i>Pappogeomys</i></p>
<div class="key2sp1">A Enamel plates completely developed across posterior walls of M1
and M2, except in one species (<i>P. alcorni</i>) 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 <i>Pappogeomys</i></div>
<div class="key2sp9"><a href="#Subgenus_Pappogeomys">p. 534</a></div>
<div class="key2sp1">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 <i>Cratogeomys</i></div>
<div class="key2sp9"><a href="#Subgenus_Cratogeomys">p. 535</a></div>
</div>
<p class="caption4"><a name="Subgenus_Pappogeomys" id="Subgenus_Pappogeomys"></a>Subgenus <b>Pappogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Pappogeomys</i> Merriam, N. Amer. Fauna, 8:145, January 31.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys bulleri</i> Thomas, 1892, from near Talpa, west slope Sierra de
Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—Late Pliocene (Benson local fauna, Arizona) to Recent,
but no specimens known from Pleistocene.</p>
<p><i>Description.</i>—Small, approximately same size as small subspecies of <i>Thomomys
umbrinus</i> 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, <i>P. alcorni</i>, 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 <i>P. bulleri</i> much as Paulson
(1961:138-139) describes in extinct <i>Geomys tobinensis</i>; one or both plates
rarely disappear in final stages of attrition in old individuals resulting in same
dental pattern found in <i>Cratogeomys</i>; 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.</p>
</div>
<p>The primitive character of the lower dentition, as described in the species
account above, suggest that <i>Cratogeomys</i> [= <i>Pappogeomys</i>] <i>bensoni</i> Gidley
should be referred to the subgenus <i>Pappogeomys</i> rather than <i>Cratogeomys</i>.
Only the upper dentition would make positive identification possible; however,
reference to the subgenus <i>Pappogeomys</i> seems to be the best arrangement at
this time.</p>
<p><span class="pagenum"><a name="Page_535" id="Page_535">[535]</a></span></p>
<div class="smaller">
<p><i>Referred species.</i>—Three (one extinct):</p>
<p class="spec_acc">*<i>Pappogeomys bensoni</i> (Gidley), 1922. U. S. Geol. Surv. Prof.
Papers, 131:123. Type from Benson local fauna (late
Pliocene), Cochise County, Arizona.</p>
<p class="spec_acc"><i>Pappogeomys alcorni</i> Russell, 1957. Univ. Kansas Publ. Mus. Nat.
Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.</p>
<p class="spec_acc"><i>Pappogeomys bulleri</i> Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6,
vol. 10:196, August. Type from "near Talpa," west slope of
Sierra Madre de Mascota, Jalisco.</p>
</div>
<p class="caption4"><a name="Subgenus_Cratogeomys" id="Subgenus_Cratogeomys"></a>
Subgenus <b>Cratogeomys</b> Merriam</p>
<div class="smaller">
<p class="spec_acc">1895. <i>Cratogeomys</i> Merriam, N. Amer. Fauna, 8:150, January 31.</p>
<p class="spec_acc">1895. <i>Platygeomys</i> Merriam, N. Amer. Fauna, 8:162, January 31.
Type: <i>Geomys gymnurus</i> Merriam, 1892.</p>
<p class="wrd_sp"><i>Type.</i>—<i>Geomys merriami</i> Thomas, 1893, from "Southern México," probably
in Valley of México.</p>
<p class="wrd_sp"><i>Chronologic range.</i>—Late Pleistocene, from Wisconsin deposits (San Josecito
Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave, New
Mexico, local faunas) to the Recent.</p>
<p><i>Description.</i>—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
<i>gymnurus</i>-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 (<i>gymnurus</i>-group),
quadriform or obcordate (<i>castanops</i>-group, with exceptions as noted before);
lateral plates of M3 usually present in all species, labial plate approximately
as long as lingual plate in <i>gymnurus</i>-group (like that in subgenus <i>Pappogeomys</i>)
or distinctly shorter in <i>castanops</i>-group (labial plate scarcely extending beyond
border of labial re-entrant fold); one or both lateral plates tending to disappear
with wear in <i>castanops</i>-group, with lingual plate usually disappearing
first; breadth across angular processes clearly more than breadth across
zygomatic processes, especially in <i>gymnurus</i>-group.</p>
<p><i>Remarks.</i>—In the species of the <i>castanops</i>-group the skulls can be spoken
of as generalized and the least platycephalic of the subgenus. Indeed, the
species of the <i>castanops</i>-group are hardly more specialized in this respect than
is the subgenus <i>Pappogeomys</i>. 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 <i>P. merriami</i> of the <i>castanops</i>-group
(where squamosal breadth varies from 85 to 98% of zygomatic breadth).
In the species having marked platycephalic skulls (<i>gymnurus</i> 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 <i>P. zinseri</i> and <i>P. tylorhinus zodius</i>.</p>
<p>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 <i>Cratogeomys</i>
<span class="pagenum"><a name="Page_536" id="Page_536">[536]</a></span>
is the obcordate pattern developed in <i>P. merriami</i> of the <i>castanops</i>-group.
The posterior loph and entire tooth is shortened somewhat resembling in
shape that of <i>Thomomys</i>. 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 <i>gymnurus</i> species-group. Instead the <i>gymnurus</i>-group retains
the primitive subtriangular pattern without significant modification.</p>
<p><i>Referred species.</i>—Seven:</p>
<p><i>castanops</i> species-group</p>
<p class="spec_acc"><i>Pappogeomys castanops</i> (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.</p>
<p class="spec_acc"><i>Pappogeomys merriami</i> (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6,
12:271, October. Type from "southern Mexico," probably Valley
of México (see Merriam, 1895:152).</p>
<p><i>gymnurus</i> species-group</p>
<p class="spec_acc"><i>Pappogeomys fumosus</i> (Merriam, 1892). Proc. Biol. Soc. Washington,
7:165, September 29. Type from 3 mi. W Colima, Colima.</p>
<p class="spec_acc"><i>Pappogeomys gymnurus</i> (Merriam, 1892). Proc. Biol. Soc. Washington,
7:166, September 29. Type from Zapotlan (Ciudad Guzman),
Jalisco.</p>
<p class="spec_acc"><i>Pappogeomys neglectus</i> (Merriam, 1902). Proc. Biol. Soc.
Washington, 15:68, March 22. Type from Cerro de la Calentura,
about 8 mi. NW Pinal de Amoles, Querétaro.</p>
<p class="spec_acc"><i>Pappogeomys tylorhinus</i> (Merriam, 1895). N. Amer. Fauna, 8:167,
January 31. Type from Tula, Hidalgo.</p>
<p class="spec_acc"><i>Pappogeomys zinseri</i> (Goldman, 1939). Jour. Mamm., 20:91, February
15. Type from Lagos, Jalisco.</p>
</div>
<p class="caption2"><a name="PHYLOGENY_OF_THE_GEOMYIDAE" id="PHYLOGENY_OF_THE_GEOMYIDAE"></a>
PHYLOGENY OF THE GEOMYIDAE</p>
<p>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.</p>
<p><span class="pagenum"><a name="Page_537" id="Page_537">[537]</a></span></p>
<p class="caption3"><a name="Primitive_Morphotype" id="Primitive_Morphotype"></a>
Primitive Morphotype</p>
<p>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.</p>
<p>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. <i>Griphomys</i> 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 <i>Griphomys</i>
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 <i>Griphomys</i> to the superfamily Geomyoidea
and Simpson (1945:80) went so far as to refer it to the
family Geomyidae, with a notation of <i>incertae sedis</i>, its exact relationship
to the pocket gophers is uncertain. However, the structure
of the molariform dentition of <i>Griphomys</i> 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.</p>
<p>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
<span class="pagenum"><a name="Page_538" id="Page_538">[538]</a></span>
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 (<i>Dikkomys</i>). 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 <a href="#Fig_3">Fig. 3</a>). 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 <i>Dikkomys</i> and <i>Pliosaccomys</i>, especially
on the first and second molars.</p>
<p>In <i>Dikkomys</i>, 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 <i>Dikkomys</i>, 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 <i>Dikkomys matthewi</i>.
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 <i>Dikkomys</i> could be eventually ground
down to a U-pattern as in the entoptychids. Only one upper molar
of <i>Dikkomys</i>, 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
<span class="pagenum"><a name="Page_539" id="Page_539">[539]</a></span>
pattern as the first (at least it does so in <i>Pliosaccomys</i>). 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.</p>
<p>The m3 (M3 unknown for <i>Dikkomys</i> or <i>Pliosaccomys</i>) 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 <i>Pliosaccomys</i>).</p>
<p>In <i>Pliosaccomys</i>, the stages of wear are essentially the same as
those described for <i>Dikkomys</i>, 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 <i>Pliosaccomys</i> 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 <i>Dikkomys</i>, 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 <i>Dikkomys</i>; therefore, they retain their primitive H-pattern, a
feature unique to the Geomyinae.</p>
<p>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 <i>Dikkomys</i>, 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
<i>Pliosaccomys</i>, 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
<span class="pagenum"><a name="Page_540" id="Page_540">[540]</a></span>
the Geomyinae, except in the position of the cusps before
wear in the lower molars of <i>Pleurolicus sulcifrons</i>, 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.</p>
<p>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
<i>Dikkomys</i> 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.</p>
<p>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.</p>
<p class="caption3"><a name="Entoptychid_Radiation" id="Entoptychid_Radiation"></a>
Entoptychid Radiation</p>
<p>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: <i>Pleurolicus</i>, <i>Gregorymys</i>,
<i>Grangerimus</i>, and <i>Entoptychus</i>. The genera were essentially contemporaneous
(see <a href="#Fig_3">Figure 3</a>). Even so, the subfamily was morphologically
varied, pointing to an earlier origin in the Oligocene
(actually a part of the John Day Fauna, including <i>Pleurolicus</i> may
be correlated with late Oligocene Whitneyian age) followed by a
relatively rapid radiation including all four genera in the early
<span class="pagenum"><a name="Page_541" id="Page_541">[541]</a></span>
Miocene. Two genera, <i>Pleurolicus</i> and <i>Gregorymys</i>, 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).</p>
<p>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.</p>
<p>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 (<i>Aplodontia</i>).
Inasmuch as the morphology and taxonomy of the
entoptychines were discussed in detail by Cope (1884) and reviewed
later by Wood (<i>loc. cit.</i>), there is no need to recount the
details here. According to Wood (<i>op. cit.</i>, 27-28), <i>Pleurolicus</i> 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, <i>Pleurolicus</i> is less specialized than the
<span class="pagenum"><a name="Page_542" id="Page_542">[542]</a></span>
other genera and occurs in deposits of both the Great Plains and
the Pacific Coast. <i>Gregorymys</i>, also little specialized, occurred
only on the Great Plains. The more specialized genera, <i>Grangerimus</i>
and <i>Entoptycus</i>, evidently appeared somewhat later than
<i>Pleurolicus</i> 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), <i>Grangerimus</i>
is known only from the Pacific coast. <i>Entoptycus</i> was restricted
to the Pacific Coast (John Day fauna).</p>
<p><i>Entoptycus</i> 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 <i>Entoptycus</i>. Rootless, ever-growing cheek teeth, discontinuous
enamel patterns, and monoprismatic molars were not evolved in the
subfamily Geomyinae until the late Pliocene.</p>
<p class="caption3"><a name="Phyletic_Trends" id="Phyletic_Trends"></a>
Phyletic Trends in Subfamily Geomyinae</p>
<p>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 <a href="#Fig_3">Figure 3</a>. 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.
<span class="pagenum"><a name="Page_543" id="Page_543">[543]</a></span>
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.</p>
<p><i>Dikkomys</i>, the earliest known genus of the tribe Dikkomyini, can
be taken as a starting point of evolution for the subfamily Geomyinae.
The Pliocene genus <i>Pliosaccomys</i> is the only other known
geomyine having primitive features closely resembling those of
<i>Dikkomys</i>. The relatively close but previously unrecognized relationship
between <i>Dikkomys</i> and <i>Pliosaccomys</i> can be understood
when patterns of wear on the occlusal surfaces of the cheek teeth
are taken into account. It appears that <i>Pliosaccomys</i> descended
from <i>Dikkomys</i>-like stock, if not <i>Dikkomys</i> itself. Although <i>Dikkomys</i>
is towards the beginning of this phyletic sequence and <i>Pliosaccomys</i>
towards the end of the sequence, the primitive features
shared by the two provide a generalized morphotype for the
subfamily Geomyinae.</p>
<p>In the molariform dentition, an almost complete series of stages
of wear in <i>Pliosaccomys</i> has been preserved, and those of <i>Dikkomys</i>
can be reconstructed with reasonable accuracy from those that are
known (see <a href="#Fig_4">Fig. 4</a>):</p>
<p>(1) In the initial stage of wear in <i>Dikkomys</i> the anterior and
posterior columns are separated by an intervening valley
(<a href="#Fig_4">Fig. 4A</a>),
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 <i>Dikkomys</i>, but does occur in both geomyines and entoptychines
in all stages of evolution, and it must have also occurred in <i>Dikkomys</i>
in order for the next two stages, which are preserved, to
have developed.</p>
<p>(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 (<a href="#Fig_4">Fig. 4B</a>), 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.</p>
<p><span class="pagenum"><a name="Page_544" id="Page_544">[544]</a></span></p>
<div class="fig_center" style="width:500px">
<a name="Fig_3" id="Fig_3"></a>
<img src="images/fig_3.png" width="406" height="563" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 3.</span> 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,
<i>Pleurolicus</i>, <i>Gregorymys</i> and <i>Dikkomys</i> are illustrated as ranging into the Hemingfordian,
rather than being confined to the Arikareean (see MacDonald, 1963,
and Black, 1961).</p>
</div>
<p><span class="pagenum"><a name="Page_545" id="Page_545">[545]</a></span></p>
<p>(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 (<a href="#Fig_4">Fig. 4C</a>). 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.</p>
<p>(4) In the final stage of wear (<a href="#Fig_4">Fig. 4D</a>), 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 <a href="#Fig_4">Fig. 4C</a> 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.</p>
<p>The occlusal pattern, at least in M1 and M2, in the final stages
of wear in <i>Dikkomys</i> resembles that in the subfamily Entoptychinae,
but the U-pattern develops on only the first and probably the
second molar in <i>Dikkomys</i> 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 <i>Dikkomys</i> until the Hemingfordian (<i>D. woodi</i>), upper Rosebud,
and specimens of <i>D. matthewi</i> 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.</p>
<p>Essentially the same patterns of wear characterize the genus
<i>Pliosaccomys</i>, 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:</p>
<p>(1) The first phases of wear produced the pattern (<a href="#Fig_4">p. 4E</a> and
I) described for <i>Dikkomys</i> in the previous account (Fig 4A).</p>
<p><span class="pagenum"><a name="Page_546" id="Page_546">[546]</a></span></p>
<p>(2) A small additional amount of wear produced the 2nd stage
(<a href="#Fig_4">Fig. 4F and J</a>) 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 <i>Dikkomys</i>. 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 <i>Pliosaccomys</i> 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.</p>
<p>(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 (<a href="#Fig_4">Fig. 4G and K</a>). The inflection appears to have persisted in
the upper teeth (<a href="#Fig_4">Fig. 4H</a>), but evidently with slight wear, disappeared
in the lower teeth (<a href="#Fig_4">Fig. 4L</a>). 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 <i>Pliosaccomys</i>.
In <i>Dikkomys</i> the two columns never united into a single column,
and a bilophodont occlusal pattern persisted throughout life.</p>
<p>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 <i>Dikkomys</i> to
<i>Pliosaccomys</i>, 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
<span class="pagenum"><a name="Page_547" id="Page_547">[547]</a></span>
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
<i>Thomomys</i> than it is to that of <i>Dikkomys</i>. Other specializations in
the dentition of <i>Pliosaccomys</i>, especially in m1 and m2 where the
H-pattern has been completely eliminated from the sequence of
wear, are too far advanced for <i>Pliosaccomys</i> 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 <i>Dikkomys</i>. 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 <i>Pliosaccomys</i> probably were closely allied
otherwise, and both probably had attained the highly specialized
fossorial adaptations characterizing all modern pocket gophers, before
the divergence of <i>Pliosaccomys</i> and the Geomyini took place.</p>
<p>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 <a href="#Fig_3">Fig. 3</a>). One, the most primitive of the two,
gave rise to the Thomomyini lineage that eventually evolved into
<i>Thomomys</i>. <i>Pliosaccomys</i> 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
<span class="pagenum"><a name="Page_548" id="Page_548">[548]</a></span>
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 <i>Thomomys bulbivorus</i>);
(6) forefoot small, less modified for digging than in the Geomyini.</p>
<div class="fig_center" style="width:500px">
<a name="Fig_4" id="Fig_4"></a>
<img src="images/fig_4.png" width="349" height="537" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 4.</span> Drawings of the molariform dentitions of <i>Dikkomys</i> and <i>Pliosaccomys</i>
(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.</p>
<div class="smaller">
<p class="key2sp1">A-D. <i>Dikkomys woodi</i>, right lower tooth-row, including p4-m3. Patterns
based on No. P26284 (FMNH) from Upper Rosebud (Middle Miocene),
Shannon Co., South Dakota (B above).</p>
<p class="key2sp1">E-H. <i>Pliosaccomys dubius</i>, 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.</p>
<p class="key2sp1">I-L. <i>Pliosaccomys dubius</i>, 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.</p>
</div>
</div>
<p><span class="pagenum"><a name="Page_549" id="Page_549">[549]</a></span></p>
<p>The lineage of the Thomomyini is essentially rectilinear and without
the major branching seen in the tribe Geomyini. The one genus,
<i>Thomomys</i>, 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 (<a href="#Fig_5">Fig. 5, K and L</a>) 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 <a href="#Fig_5">Fig. 5L</a>) 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.</p>
<p>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 <a href="#Fig_5">Fig. 5, I and J</a>), 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
<span class="pagenum"><a name="Page_550" id="Page_550">[550]</a></span>
bilophodont patterns appear in both the Dikkomyini and Geomyini
before the monoprismatic pattern is developed.</p>
<div class="fig_center" style="width:500px">
<a name="Fig_5" id="Fig_5"></a>
<img src="images/fig_5.png" width="419" height="690" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 5.</span> 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.</p>
<div class="smaller">
<p class="key2sp1">A and E. <i>Geomys bursarius majusculus</i>, No. 2948 (KU), Douglas Co., Kansas.
Right upper (A) including DP4-M3; lower left (E) including
dp4-m3.</p>
<p class="key2sp1">B and F. <i>Pappogeomys bulleri burti</i>, 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).</p>
<p class="key2sp1">C and G. <i>Pappogeomys bulleri albinasus</i>, No. 31044 (KU), 10 mi. S and 8
mi. W Guadalajara, Jalisco. Right upper (C) including P4-M3;
right lower (G) including p4-m3.</p>
<p class="key2sp1">D and H. <i>Pappogeomys bulleri albinasus</i>, 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.</p>
<p class="key2sp1">I and J. <i>Thomomys talpoides bridgeri</i>, No. 6865 (KU), 2 mi. up Mink
Creek, Pocatella, Bannock Co., Idaho. Left upper (I), DP4-M3;
left lower (J), dp4-m3.</p>
<p class="key2sp1">K and L. <i>Thomomys talpoides fossor</i>, No. 13205 (KU), Wasson Ranch, 3 mi.
E Creede, Mineral Co., Colorado. Right lower (K), p4-m3; left
upper (L), P4-M3.</p>
</div>
</div>
<p>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
<span class="pagenum"><a name="Page_551" id="Page_551">[551]</a></span>
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 <a href="#Fig_5">Fig. 5, A and E</a>), 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.</p>
<p>In the lower molars, the pattern characteristic of <i>Dikkomys</i>
(<a href="#Fig_4">Fig. 4C</a>) is preserved without significant modification, as illustrated
in an immature specimen of <i>Geomys</i> (see <a href="#Fig_5">Fig. 5E</a>). 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 <i>Dikkomys</i>. After the two columns have become united
at their mid-points, a secondary union is formed at the edge of their
<span class="pagenum"><a name="Page_552" id="Page_552">[552]</a></span>
protomeres, thus enclosing the enamel fossette as illustrated in
<a href="#Fig_5">Figure 5E</a> (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 <a href="#Fig_5">Fig. 5E</a>). 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.</p>
<p>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 <a href="#Fig_5">Fig. 5B</a>),
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
<i>Pliosaccomys</i> (see <a href="#Fig_4">Fig. 4H</a>). 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 <a href="#Fig_5">Fig. 5B</a>), therefore forming a U-pattern
similar to that developed in Ml and M2 of <i>Pliosaccomys</i>. 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 <a href="#Fig_5">Fig. 5C</a>). 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 <a href="#Fig_5">Fig. 5D</a>). M3's of <i>Dikkomys</i> and <i>Pliosaccomys</i> 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 <i>Dikkomys</i>.</p>
<p>Judging from the pre-final stages of wear, the dentition of the
<span class="pagenum"><a name="Page_553" id="Page_553">[553]</a></span>
Geomyini provides a curious combination of patterns that resemble
in part the Miocene genus <i>Dikkomys</i> and in part the early and
middle Pliocene genus <i>Pliosaccomys</i>. 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 <i>Dikkomys</i>, and are distinctly different
from those of <i>Pliosaccomys</i> 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 <i>Pliosaccomys</i> 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 <i>Pliosaccomys</i>, rather than <i>Dikkomys</i>. In this
regard it should be pointed out that the upper molars of <i>Dikkomys</i>
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 <i>Dikkomys</i> might have
become united across the entire surface of their protomeres as in
<i>Pliosaccomys</i>. Even so, the two columns of M1 do initially join
closer to their mid-points than they do in <i>Pliosaccomys</i>, 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 <i>Pliosaccomys</i>
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 <i>Pliosaccomys</i>
and the Geomyini become eventually united, with wear, into a
single column.</p>
<p><span class="pagenum"><a name="Page_554" id="Page_554">[554]</a></span></p>
<p>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, <i>Pliosaccomys</i>, but had not
also evolved the specializations of m1 and m2 that distinguish
<i>Pliosaccomys</i>. Therefore, the ancestor of the Geomyini differed
from the <i>Pliosaccomys</i>-Thomomyini lineage in its retention, unmodified,
of the primitive patterns in m1 and m2 that characterized the
earliest known Geomyines (<i>Dikkomys</i>). The same patterns are
preserved in m1 and m2 of its modern descendents, the living
Geomyini. In the <i>Pliosaccomys</i>-Thomomyini lineage the pattern of
m1 and m2 are entirely different, as described above.</p>
<p>The earliest record of the Geomyini is the extinct genus <i>Pliogeomys</i>
(see <a href="#Fig_6">Fig. 6</a>) in the latest Hemphillian (middle Pliocene)
and earliest Blancan (late Pliocene). <i>Pliogeomys</i> 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 <i>Pliogeomys</i> 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 <i>Pliogeomys</i> were in the direct line of evolution.
Separation of <i>Pliogeomys</i> 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 <i>Pliogeomys</i> are narrow (see <a href="#Fig_7">Fig. 7A</a>)
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. <i>Pliogeomys</i> is in an intermediate
stage in evolution, and was not so advanced as was the
main lineage at the time <i>Pliogeomys</i> died out. Its structure does
provide clues as to phyletic development that took place in the
main lineage.</p>
<p>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
<span class="pagenum"><a name="Page_555" id="Page_555">[555]</a></span>
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 <i>Pliogeomys</i>, <i>Zygogeomys</i> and <i>Geomys</i>. The innermost
groove is weakly developed in <i>Pliogeomys</i>, suggesting that this character
was in an intermediate stage of evolution in the ancestral
lineage at the time that <i>Pliogeomys</i> 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.</p>
<p>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 <i>Pliosaccomys</i>).
Resemblance of dentitions in certain stages of wear in <i>Pliosaccomys</i>
and in entoptychines led some investigators, for instance, Hibbard
(1953:357), to suggest that <i>Pliosaccomys</i> descended from one of the
less specialized entoptychines, possibly <i>Grangerimus</i> but probably
<i>Gregorymys</i>. 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 <i>Pliosaccomys</i> 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 <i>Pliosaccomys</i> 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.</p>
<p><span class="pagenum"><a name="Page_556" id="Page_556">[556]</a></span></p>
<p>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 <i>Entoptychus</i> of the lower Miocene. In <i>Entoptychus</i>,
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 <i>Entoptychus</i> and a
sulcated upper incisor, a single shallow groove usually near the
median border of the tooth, is found in the genus <i>Gregorymys</i> 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.</p>
<p>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 <i>Entoptychus</i> shows a greater degree of
fossorial adaptation than earlier genera of the subfamily. There is
no reason to suppose that the geomyine genus <i>Dikkomys</i>, which
lived at the same times as the entoptychines, had acquired any
more advanced fossorial adaptations than had the entoptychines.</p>
<p>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
<span class="pagenum"><a name="Page_557" id="Page_557">[557]</a></span>
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, <i>loc. cit.</i>), 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, <i>Pliosaccomys</i>, 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, <i>loc. cit.</i>). 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.</p>
<p>The Madro-tertiary climax does not appear as a major flora until
the Miocene, but probably originated earlier. According to Axelroad
(<i>loc. cit.</i>), 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.</p>
<p>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 <i>Dikkomys</i>
that survived in association with the entoptychids throughout the
<span class="pagenum"><a name="Page_558" id="Page_558">[558]</a></span>
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.</p>
<p>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.</p>
<p>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, <i>Pliosaccomys</i> differs but little from <i>Dikkomys</i>.
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.</p>
<p class="caption3"><a name="Plio-Pleistocene_radiation" id="Plio-Pleistocene_radiation"></a>
Plio-Pleistocene radiation of Geomyini</p>
<p>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 <a href="#Fig_6">Fig. 6</a>); 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: <i>Zygogeomys</i>, <i>Geomys</i>,
<i>Orthogeomys</i>, and <i>Pappogeomys</i>.</p>
<p><span class="pagenum"><a name="Page_559" id="Page_559">[559]</a></span></p>
<div class="fig_center" style="width:472px">
<a name="Fig_6" id="Fig_6"></a>
<img src="images/fig_6.png" width="472" height="645" alt="" title="" />
<p class="fig_caption center"><span class="smcap">Fig. 6.</span>
Plio-Pleistocene radiation of the Tribe Geomyini.</p>
</div>
<p class="caption4"><a name="Morphotype" id="Morphotype"></a><i>Morphotype</i></p>
<p>The immediate, unknown, ancestor probably lived on the Central
Plateau of México. After the radiation began the ancestors of
<i>Geomys</i> and <i>Zygogeomys</i> extended their ranges northward.</p>
<p><span class="pagenum"><a name="Page_560" id="Page_560">[560]</a></span></p>
<p>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 <i>Pliogeomys</i>, <i>Zygogeomys</i> and
<i>Geomys</i>; (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.</p>
<p class="caption4"><a name="Specializations_in_Genera" id="Specializations_in_Genera"></a>
<i>Specializations in Genera</i></p>
<p>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.</p>
<p>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.)</p>
<p>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.</p>
<p><span class="pagenum"><a name="Page_561" id="Page_561">[561]</a></span></p>
<p>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.</p>
<p>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 <i>simultaneously</i> 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
<i>castanops</i>-group of the subgenus <i>Cratogeomys</i>). 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.</p>
<p>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
<span class="pagenum"><a name="Page_562" id="Page_562">[562]</a></span>
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
<span class="pagenum"><a name="Page_563" id="Page_563">[563]</a></span>
characterized by only one pattern, but in one genus (<i>Pappogeomys</i>)
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 (<a href="#Fig_7">Fig. 7D, F, H</a>) 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).</p>
<div class="fig_center" style="width: 500px">
<a name="Fig_7" id="Fig_7"></a>
<img src="images/fig_7.png" width="450" height="692" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 7.</span> Molariform dentitions of the Tribe Geomyini. Drawings illustrating
enamel patterns characteristic of <i>Pliogeomys</i>, <i>Zygogeomys</i>, and the subgenera
of <i>Orthogeomys</i> (<i>Orthogeomys</i>, <i>Heterogeomys</i> and <i>Macrogeomys</i>). × 5.</p>
<div class="smaller">
<p class="key2sp1">A. <i>Pliogeomys buisi</i>, No. 29157 (UMMP), holotype, Buis Ranch
(Upper Middle Pliocene), Beaver Co., Oklahoma. Right lower,
p4-m2 (m3 unknown).</p>
<p class="key2sp1">B and C. <i>Zygogeomys trichopus trichopus</i>, adult female, No. 51971 (FMNH),
Mt. Tancítaro, 10,500 ft., Michoacán. Left upper (B), P4-M3;
right lower (C), p4-m3.</p>
<p class="key2sp1">D and E. Subgenus <i>Orthogeomys</i>. <i>Orthogeomys grandis guerrerensis</i>, adult
female, No. 39807 (KU), 1/2 mi. E La Mira, 300 ft., Michoacán.
Left upper (D), P4-M3; right lower (E), p4-m3.</p>
<p class="key2sp1">F and G. Subgenus <i>Heterogeomys</i>. <i>Orthogeomys hispidus hispidus</i>, adult
female, No. 23975 (KU), 4 km. W Tlapacoyan, 700 ft., Veracruz.
Left upper (F), P4-M3; right lower (G), p4-m3.</p>
<p class="key2sp1">H and I. Subgenus <i>Macrogeomys</i>. <i>Orthogeomys heterodus cartagoensis</i>, adult
female, No. 60664 (KU), Rancho Redando, Volcán Lrozá, Prov.
San José, Costa Rica. Left upper (H), P4-M3; right lower (I),
p4-m3.</p>
</div>
</div>
<p>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 (<i>loc. cit.</i>) 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
<span class="pagenum"><a name="Page_564" id="Page_564">[564]</a></span>
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
(<i>loc. cit.</i>) 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
(<a href="#Page_480">p. 480</a>) 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.</p>
<p>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.</p>
<p>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 <i>Pappogeomys</i> or the
smaller subspecies of <i>Geomys bursarius</i>. The recorded range of
variation in condylobasal length is 36.1 to 45.5 in <i>Pappogeomys
bulleri</i>, 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 <i>Cratogeomys</i>)
and 75.0 (subgenus <i>Orthogeomys</i>). These are more than
twice the minima observed in <i>Pappogeomys bulleri</i>.</p>
<p class="caption4"><a name="Zygogeomys2" id="Zygogeomys2"></a>Zygogeomys</p>
<p>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
<span class="pagenum"><a name="Page_565" id="Page_565">[565]</a></span>
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 <i>Zygogeomys</i> 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 (<i>Z. minor</i>),
middle Pleistocene (<i>Z. persimilis</i>), and Recent (<i>Z. trichopus</i>). 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
<a href="#Fig_7">Fig. 7B</a>), 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
<i>Orthogeomys</i> (<i>Orthogeomys cherriei costaricensis</i>). 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
(<i>Z. persimilis</i>).</p>
<p class="caption4"><a name="Geomys2" id="Geomys2"></a>Geomys</p>
<p><i>Geomys</i>, slightly more specialized than <i>Zygogeomys</i>, 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 <a href="#Fig_9">Fig. 9A</a>). All of these primitive features
are shared with <i>Zygogeomys</i>. In addition, three other trends, or
specializations, in evolution characterize the phyletic development
of <i>Geomys</i>. 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
<i>Geomys</i>, and at least one of the earlier species (<i>quinni</i>) 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
<span class="pagenum"><a name="Page_566" id="Page_566">[566]</a></span>
<i>G. bursarius</i>, 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 <i>Geomys</i>,
for example <i>G. quinni</i>, 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.</p>
<p>Remains of <i>Geomys</i> 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 <i>Geomys</i> are depicted in <a href="#Fig_8">Figure 8</a>; those estimates
are useful in discussing the phyletic development of the
genus. One of the earliest known species, <i>Geomys quinni</i>, ranges
from Upper Pliocene to the later stages of the Lower Pleistocene
(Aftonian interglacial deposits). The dentition of <i>G. quinni</i> is essentially
the same as in the living species except that open lateral
re-entrant angles are retained in the premolars. <i>Geomys paenebursarius</i>,
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 <i>Geomys</i> from the Rexroad
fauna (late Pliocene) and Saunders fauna (latest Aftonian)
may also be on the main line of evolution. Surprisingly, <i>Geomys
tobinensis</i> and <i>Geomys garbanii</i> of later Irvingtonian provincial age
are less specialized than either <i>Geomys quinni</i> or <i>Geomys paenebursarius</i>.
It is likely that <i>G. tobinensis</i> and the unnamed species
from the Dixon are closer to the main line of descent than <i>G.
paenebursarius</i> suggesting that the direct ancestral lineage of the
living species of <i>Geomys</i> was more conservative and less specialized
than <i>Geomys paenebursarius</i> of the Lower Pleistocene. <i>Geomys
quinni</i> and <i>G. paenebursarius</i> seem to have acquired specialized
dental features in the early Pleistocene. <i>Geomys quinni</i> was successful
on the Great Plains, and persisted into the late Blancan.
The main line may be represented in the early Pleistocene by
<span class="pagenum"><a name="Page_567" id="Page_567">[567]</a></span>
<i>Geomys paenebursarius</i> from the Hancock formation of the Texas
Trans-Pecos. The structure of <i>G. paenebursarius</i> 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 <i>Geomys</i> from the
Rexroad fauna.</p>
<div class="fig_center" style="width:529px">
<a name="Fig_8" id="Fig_8"></a>
<img src="images/fig_8.png" width="529" height="647" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 8.</span> Tentative arrangement of species of the genus <i>Geomys</i>, depicting
phylogenetic trends and probable relationships within the genus.</p>
</div>
<p>Isolated teeth, to which the name <i>Geomys bisulcatus</i> probably
applies, from Illinoian deposits on the Great Plains, show that the
dentition characteristic of the living <i>Geomys</i> had been developed
by that time. Actually, the Illinoian material is too fragmentary to
<span class="pagenum"><a name="Page_568" id="Page_568">[568]</a></span>
show clearly its taxonomic or phyletic affinities with the species of the
later Pleistocene. Even so, the two main stocks of living <i>Geomys</i>,
<i>G. bursarius</i> and <i>G. pinetis</i>, 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, <i>Geomys arenarius</i> clearly differentiated
from populations of <i>Geomys bursarius</i> 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.</p>
<p>In review, it seems that the Recent species, represented basically
by <i>bursarius</i> and <i>pinetis</i>, evolved from Illinoian species (<i>Geomys
bisulcatus?</i>), which descended in turn from the more primitive
species of the early Pleistocene, possibly <i>Geomys paenebursarius</i>
or possibly from descendants of the Saunders species. Actually
the Saunders species may prove to be <i>Geomys paenebursarius</i>. 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. <i>Geomys garbanii</i>, occurring at the periphery
of the range of the genus, is regarded as a sterile offshoot of the
primitive <i>tobinensis</i>-line of evolution.</p>
<p class="caption4"><a name="Orthogeomys2" id="Orthogeomys2"></a>Orthogeomys</p>
<p>This is one of the more specialized genera of the Geomyini. Save
for one record in the late Pleistocene (<i>Orthogeomys onerosus</i>),
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 <i>Orthogeomys</i> has closer affinities
with <i>Zygogeomys</i> than with any of the other genera, and that
<i>Orthogeomys</i> may have originated in an early dichotomy of primitive
<i>Zygogeomys</i> 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 <i>Zygogeomys</i>, 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 <a href="#Fig_7">Fig. 7F and H</a>). In <i>Orthogeomys</i>,
the trend in reduction of enamel is carried to its extreme
only in the subgenus <i>Orthogeomys</i>, where this plate has been completely
<span class="pagenum"><a name="Page_569" id="Page_569">[569]</a></span>
lost in most taxa (see <a href="#Fig_7">Fig. 7D</a>). The most significant trends
in <i>Orthogeomys</i>, 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 <i>Orthogeomys</i> and <i>Macrogeomys</i>,
and are less developed in the subgenus <i>Heterogeomys</i>.
Aside from the general dolichocephalic specializations, trends in
<i>Orthogeomys</i> 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.</p>
<p class="caption4"><a name="Pappogeomys2" id="Pappogeomys2"></a>Pappogeomys</p>
<p>The genus <i>Pappogeomys</i>, as it is conceived of in this study, is
comprised of two subgenera; one, <i>Pappogeomys</i>, is generalized and
primitive, and the other, <i>Cratogeomys</i>, is specialized, and includes
the most highly specialized of the modern pocket gophers. The
subgenus <i>Pappogeomys</i> is regarded as the ancestral lineage, and
the subgenus <i>Cratogeomys</i> 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 <i>Pappogeomys</i> changed little. It is known only
from late Pliocene fragments and from the living species. The ancestral
morphotype is preserved in <i>Pappogeomys</i>. 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 <i>Cratogeomys</i>, suggesting
that they were already developed in the early evolution of the
subgenus <i>Pappogeomys</i> before <i>Cratogeomys</i> diverged. Agreement
with <i>Geomys</i> is demonstrated by the lack of enamel on the posterior
wall of P4 (see <a href="#Fig_9">Fig. 9</a>) and by retention of the posterior enamel
plate on M1 and M2. In <i>Pappogeomys (Pappogeomys) alcorni</i> the
enamel from the posterior face of M1 has been lost from all but the
<span class="pagenum"><a name="Page_570" id="Page_570">[570]</a></span>
lingual fourth or so of the posterior wall (<a href="#Fig_9">Fig. 9E</a>). Reduction of
enamel in M1 provides an example of parallelism with the more
advanced subgenus <i>Cratogeomys</i>, discussed below.</p>
<p>There is no record as yet of the early evolution of the subgenus
<i>Cratogeomys</i>. The features that characterize the subgenus were
<span class="pagenum"><a name="Page_571" id="Page_571">[571]</a></span>
already well developed in the first known fossils which are from
Wisconsin deposits of the late Pleistocene. <i>Cratogeomys</i> is not a
homogenous assemblage; instead it is composed of two groups of
living species, the generalized <i>castanops</i> group and the specialized
<i>gymnurus</i> group. The <i>castanops</i> 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 <i>castanops</i> group has
acquired its peculiar specializations. Indeed, <i>P. merriami</i> of the
<i>castanops</i> group differs from the hypothetical stem more than does
<i>P. castanops</i>. Judging from the structure of the living species of the
subgenus <i>Cratogeomys</i> and from the primitive subgenus <i>Pappogeomys</i>,
the subgenus <i>Cratogeomys</i> 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.</p>
<div class="fig_center" style="width: 500px">
<a name="Fig_9" id="Fig_9"></a>
<img src="images/fig_9.png" width="424" height="689" alt="" title="" />
<p class="fig_caption"><span class="smcap">Fig. 9.</span> Molariform dentitions of the Tribe Geomyini. Drawings illustrating
enamel patterns characteristic of <i>Geomys</i> and <i>Pappogeomys</i> (including the
subgenera <i>Pappogeomys</i> and <i>Cratogeomys</i>). × 5.</p>
<div class="smaller">
<p class="key2sp1">A and B. <i>Geomys bursarius bursarius</i>, adult female, No. 46275 (KU), Elk
River, Sherborne Co., Minnesota. Left upper (A), P4-M3; right
lower (B), p4-m3.</p>
<p class="key2sp1">C and D. Subgenus <i>Pappogeomys</i>. <i>Pappogeomys bulleri albinasus</i>, 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.</p>
<p class="key2sp1">E and F. Subgenus <i>Pappogeomys</i>. <i>Pappogeomys alcorni</i>, adult female, No.
31051 (KU), holotype, 4 mi. W Mazamitla, 6600 ft., Jalisco. Left
upper (E), P4-M3; right lower (F), p4-m3.</p>
<p class="key2sp1">G and H. Subgenus <i>Cratogeomys</i>. <i>Pappogeomys gymnurus tellus</i>, adult female,
No. 31051 (KU), 1 mi. NE Tala, 4400 ft., Jalisco. Left
upper (G), P4-M3; right lower (H), p4-m3.</p>
</div>
</div>
<p>Loss of enamel is a trend common to all living genera of the
tribe Geomyini, but the greatest loss has occurred in <i>Cratogeomys</i>.
It has lost the plates on the posterior walls of M1 and M2 (<a href="#Fig_9">Fig. 9G</a>).
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 <i>Cratogeomys</i> 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
<span class="pagenum"><a name="Page_572" id="Page_572">[572]</a></span>
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 <i>gymnurus</i> 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 <i>castanops</i> group separated from the <i>gymnurus</i> group before the
common ancestor had developed the more extreme trends in platycephaly.
It is interesting to note that the subtriangular M3 (<a href="#Fig_9">Fig. 9G</a>)
postulated for the ancestral morphotype and that characterizes the
subgenus <i>Pappogeomys</i> is retained also in the <i>gymnurus</i> group.</p>
<p class="caption2"><a name="LITERATURE_CITED" id="LITERATURE_CITED"></a>
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<p class="center pmt2 pmb2"><i>Transmitted May 29, 1967.</i><br />
<img src="images/square.png" width="16" height="17" alt="[]" title="[]" />
</p>
<div class="trans_notes">
<p class="caption2">Transcriber's Notes</p>
<p>All obvious typographical errors were corrected. Minor changes were
made to standardize the text to match the most prevalent form used.</p>
<p class="caption2">Typographical Corrections</p>
<table summary="Corrections">
<tr>
<td class="brdb2">Page</td>
<td> </td>
<td class="brdb2">Correction</td>
</tr>
<tr>
<td><a href="#Page_477">477</a></td>
<td></td>
<td>cumberlandicus => cumberlandius</td>
</tr>
<tr>
<td><a href="#Page_535">535</a></td>
<td></td>
<td>breath => breadth</td>
</tr>
</table>
</div>
<div>*** END OF THE PROJECT GUTENBERG EBOOK 40282 ***</div>
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