Nebraska) to Recent. Numerous records, mostly isolated teeth, from
nearly all stratigraphic levels of the Pleistocene (for details, see
account of fossil record).
_Description._--Molars pear-shaped in cross-section, becoming abruptly
narrow at one end of the tooth. The teeth of the late Pleistocene
species _Thomomys vetus_ are less distinctly pear-shaped than other
referred species (see remarks in the description of the subgenus
_Pleisothomomys_).
Essentially on the basis of its significantly larger size and details
of the skull, Elliott (1903:190) proposed subgeneric recognition of
_Thomomys bulbivorus_ and described the subgenus _Megascapheus_ to
include it. Also the molars of _Thomomys bulbivorus_ usually have a
small enamel plate, both above and below, bordering the persistent
inflection on the protomere end of the tooth; each lateral plate is
isolated from the transverse plates on the anterior and posterior
walls of the tooth. In my opinion these features do not warrant
subgeneric recognition; however, these characters do distinctly
separate _Thomomys bulbivorus_ from other groups of species, and the
character of the molars suggests retention of a primitive trait.
Therefore, I propose that the unique structure of this species be
recognized by setting it apart in the _bulbivorus_ species-group.
_Referred species._--Ten species, three extinct, placed in three
species-groups (the numerous subspecies of this genus are listed in
Miller and Kellogg, 1955:276-332, and Hall and Kelson, 1959:412-447).
_bulbivorus_ species-group
_Thomomys bulbivorus_ (Richardson, 1829). Fauna Boreali-Americana,
1:206. Type from Columbia River, probably near Portland,
Oregon.
_umbrinus_ species-group
*_Thomomys scudderi_ Hay, 1921. Proc. U. S. Nat. Mus., 49:614.
Type from Fossil Lake beds, late Pleistocene, Oregon.
_Thomomys umbrinus_ (Richardson, 1829). Fauna Boreali-Americana,
1:202. Type from southern México, probably near Boca de Monte,
Veracruz.
_Thomomys bottae_ (Eydoux and Gervais, 1836). Mag. de Zool., Paris,
6:23. Type from coast of California, probably near Monterey.
*_Thomomys vetus_ Davis, 1937. Jour. Mamm., 18:156, May 12. Type
from Fossil Lake beds, late Pleistocene, Oregon.
_Thomomys townsendii_ (Bachman, 1839). Jour. Acad. Nat. Sci.
Philadelphia, 8:105. Type probably from near Nampa, Canyon Co.,
Idaho (erroneously given as "Columbia River").
_talpoides_ species-group
*_Thomomys microdon_ Sinclair, 1905. Bull. Dept. Geol. Univ.
California, 4:145-161. Type from Potter Creek Cave, late
Pleistocene, California.
_Thomomys monticola_ J. A. Allen, 1893. Bull. Amer. Mus. Nat. Hist.,
5:48, April 28. Type from Mt. Tallac, 7500 ft., El Dorado Co.,
California.
_Thomomys talpoides_ (Richardson, 1828). Zool. Jour., 3:518. Type
locality fixed at near Fort Carlton (Carlton House),
Saskatchewan River, Saskatchewan, Canada.
_Thomomys mazama_ Merriam, 1897. Proc. Biol. Soc. Washington,
11:214, July 15. Type from Anna Creek, 6000 ft., near Crater
Lake, Mt. Mazama, Klamath Co., Washington.
Tribe GEOMYINI, new tribe
_Genotype._--_Geomys_ Rafinesque, 1817.
_Chronologic and geographic range._--Known from late middle Pliocene
deposits to Recent. The range of living members extends from extreme
southern Manitoba and the southeastern United States south to southern
Panamá, and probably northern Colombia, South America.
_Diagnosis._--Size small to large (condylobasal length of skull 33.0
to 73.0 in adults, including both sexes); sexual dimorphism marked,
sometimes strongly, females being smaller than males, especially in
cranial dimensions; upper incisors invariably grooved, number and
position of grooves varying according to genus; cheek teeth
high-crowned and ever-growing, except in one primitive genus
(_Pliogeomys_); all three lower molars and M1 and M2 monoprismatic,
and elliptical in cross-section in final stages of wear (teeth of
young, subadult, and adult animals); primitive biprismatic patterns
(as known from Recent specimens) occurring only in pre-final stages of
wear (teeth of juveniles only); biprismatic patterns of lower molars
as in _Dikkomys_, and upper molars as in _Pliosaccomys_ (for detailed
description of these patterns, see account beyond of the phylogeny of
the Geomyinae); m3 becoming monoprismatic, anteroposteriorly
compressed and elliptical in cross-section like m1 and m2, but M3
remaining, with rare exceptions (see accounts of _Geomys_ and
_Pappogeomys_ beyond), at least partially biprismatic throughout life,
having one or both lateral inflections usually persisting (with
exceptions) and developing various occlusal shapes (subtriangular,
elongate, obcordate, suborbiculate, or quadriform) but never
elliptical.
Enamel of cheek teeth reduced to interrupted plates, with exception of
p4 in _Pliogeomys_; plate on posterior wall of P4 variable, occurring
completely across posterior surface in primitive members, but
progressively reduced to lingual side only or completely lost in
modern genera (see generic accounts beyond for detailed description);
both anterior and posterior plates usually retained in M1 and M2,
posterior plate sometimes reduced to lingual side or completely lost
(as in _Pappogeomys_) but anterior plate always completely retained;
M3 usually having three plates, one anterior and two lateral;
posterior plate wanting (sometimes lingual plate moved to posterior
position); plates retained completely across posterior walls of all
lower cheek teeth with no reduction, but anterior plates of m1-3
always lacking, except in primitive genus _Pliogeomys_ (only Geomyini
having both anterior and posterior enamel plates on lower molars).
Skull primitively generalized, but becoming specialized towards either
dolichocephaly (_Orthogeomys_) or platycephaly (_Pappogeomys_) in two
modern genera; skull highly specialized for fossorial life; mandible
stout and deep, angular process being high and diverging laterally at
right angles to ramus; masseteric ridge and fossa weakly developed in
primitive members, becoming well developed and massive in modern
genera; basitemporal fossa absent in primitive forms (_Pliogeomys_ and
early members of _Zygogeomys_); pelage usually soft, but harsh and
hispid in some genera; forefeet broad and massive, claws long and
stout for digging; body form remarkably fossorial.
The tribe Geomyini includes the most highly specialized members of the
subfamily Geomyinae.
Key to the Genera of the Tribe Geomyini
A Cheek teeth rooted; p4 with uninterrupted enamel loop; enamel
plates on both anterior and posterior walls of m1 and m2;
masseteric ridge weakly developed, low, not massive.
Genus _Pliogeomys_ p. 522
A´ Cheek teeth rootless, ever-growing; p4 with enamel investment
interrupted at ends of columns, consequently, forming four
isloted plates; enamel plate retained only on posterior wall
of m1 and m2, anterior wall without trace of enamel (except
rarely in pre-final stage of wear in _Geomys tobinensis_ of
middle Pleistocene); masseteric crest strongly developed and
massive.
B Enamel plate on posterior wall of P4, but usually
restricted to lingual end of tooth (usually absent in
subgenus _Orthogeomys_ of genus _Orthogeomys_);
M3 conspicuously bicolumnar, longer than wide owing to
elongation of posterior loph.
C Upper incisor bisulcate; skull generalized; rostrum
relatively narrow; length of labial enamel plate of
M3 decidedly less than length of lingual plate;
pelage soft and thick. Genus _Zygogeomys_ p. 523
C´ Upper incisor unisulcate; skull strongly
dolichocephalic; rostrum remarkably broad and massive;
length of lingual plate of M3 approximately equal to,
or greater than, length of labial plate; pelage harsh,
often hispid and scant. Genus _Orthogeomys_ p. 528
B´ Posterior wall of P4 without trace of enamel; M3 not
strongly bicolumnar, having shallow re-entrant fold on
labial side, and crown no longer than wide owing to
shortness of posterior loph.
D Upper incisor bisulcate; skull generalized; both
anterior and posterior walls of M1 and M2 having
complete enamel plates. Genus _Geomys_ p. 525
D´ Upper incisor unisulcate; skull generalized or
tending towards platycephaly; enamel plate on
posterior wall of M1 usually reduced to lingual
side or absent (complete only in one species,
_Pappogeomys bulleri_); enamel plate on posterior
wall of M2 also absent in advanced species
(subgenus _Cratogeomys_). Genus _Pappogeomys_ p. 532
Genus =Pliogeomys= Hibbard
1954. _Pliogeomys_ Hibbard, Michigan Acad. Sci., Arts and
Letters, 39:353.
_Genotype._--_Pliogeomys buisi_ Hibbard, 1954, from Buis Ranch local
fauna (middle Pliocene), Beaver County, Oklahoma.
_Chronologic range._--Latest Middle Pliocene, known only from the
highest part of the Hemphillian mammalian fauna (Buis Ranch local
fauna, Oklahoma). Professor Hibbard informs me (personal
communication) that he found the type, a right ramus, lying on the
surface near the base of the fossil beds. The isolated teeth of small
geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392)
may also be referable to this genus. The Saw Rock Canyon local fauna
may also be middle Pliocene in age but is considered to be from the
later part of the late Pliocene, and, therefore, somewhat younger than
the Buis Ranch local fauna (Hibbard, _op. cit._:342).
_Description and discussion._--The size of members of this small genus
of the Geomyinae is about the same as in smaller adults of _Geomys
bursarius_. According to Hibbard (_op. cit._:353), the holotype is
smaller than specimens from the Rexroad local fauna referred to
_Geomys quinni_ and larger than specimens referred to _Zygogeomys_ cf.
_minor_. The cheek teeth are rooted, and the crowns are as high as
those of living geomyids. The upper incisor is bisulcate, and the
inner groove is fine and indistinct in places.
Of the molariform dentition only the lower premolar and first two
lower molars are known. The enamel investment of p4 is complete, and
would not be subject to interruption at any stage of wear; the two
prisms are joined at their mid-points, and the isthmus of dentine is
relatively broad (as in _Pliosaccomys_) when compared with modern
pocket gophers of this tribe. Also, the re-entrant folds, rather than
having parallel sides, diverge broadly to the sides. The divergence is
especially noticeable in the labial fold. The lower deciduous premolar
would have formed essentially the same enamel pattern with wear as
observed in _Nerterogeomys_ [= _Zygogeomys_] cf. _minor_ (see Hibbard,
1954:fig. 5, A and B) and _Pliosaccomys dubius_ (see Wilson, 1936; pl.
1, fig. 1). Each molar is a single column in the final stages of wear;
pre-final stages are unknown. Anterior and posterior enamel plates are
present on m1 and m2 (m3 has not been recovered). The dentine tracts
of m1 are exposed over a relatively wide surface; therefore, the
enamel plates are distinctly separated. The tracts of dentine of m2
are much narrower than in m1 and the enamel plates are barely
separated at the anterolateral margin of the tooth. Possibly the
enamel band of m2 was continuous in an earlier stage of wear.
The mandible is stout and its general construction not unlike that in
modern geomyines. The capsule at the base of the angular process that
receives the terminal end of the lower incisor is well developed. The
base of the angular processes is preserved, and suggests that the
process was short and decidedly smaller than in living examples of the
tribe. The masseteric ridge is distinct but weakly developed, and not
at all massive as in living pocket gophers. The mental foramen is
immediately anterior, and slightly ventral, to the anterior extension
of the crest. The basitemporal fossa is absent as such, but its
position is marked by a slight depression.
_Specimens examined._--Two rami; nos. 29147 (holotype) and 33446;
several isolated teeth 30194 and 30195, including an upper incisor and
a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo.
_Referred species._--One.
*_Pliogeomys buisi_ Hibbard, 1954. Papers Michigan Acad. Sci.,
Arts, and Letters, 39:353. Type from Buis local fauna, latest
middle Pliocene, Beaver County, Oklahoma.
Genus =Zygogeomys= Merriam
1895. _Zygogeomys_ Merriam, N. Amer. Fauna, 8:195, January 31.
1942. _Nerterogeomys_ Gazin, Proc. U. S. Nat. Mus., 92:507
(type, _Geomys persimilis_ Hay, 1927).
_Type._--_Zygogeomys trichopus_ Merriam, 1895, from Nahuatzen,
Michoacán.
_Chronologic range._--Late Pliocene (Benson and Curtis Ranch local
faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent.
_Description and discussion._--The size is small to medium for the
subfamily Geomyinae. This genus is distinguished principally by the
retention of primitive features. In the living species, the skull is
generalized, rather than specialized toward either extreme
dolichocephaly or platycephaly. The angular process is short, barely
exceeding the lateral extensions of the mastoid process of the
squamosal. The rostrum is remarkably narrow in relation to its length.
The jugal is reduced and displaced ventrally, causing the maxillary
arm of the zygomata to articulate with the squamosal arm of the
zygomata along the dorsal border of the zygomatic arch (a feature
observed also in _Orthogeomys cherriei costaricensis_).
The upper incisor, recovered in material from the late Pliocene and
middle Pleistocene, is bisulcate as in the genus _Geomys_ and the
primitive genus _Pliogeomys_. The enamel plate across the posterior
wall of P4 is either complete (late Pliocene to late Pleistocene) or
restricted to the lingual half of the tooth (always restricted in
living species). The Pliocene specimens of the Rexroad local fauna
referred to _Nerterogeomys_ cf. _minor_ by Hibbard (1950:138-139) are
exceptional. In these specimens the length and position of the
posterior enamel plate is variable; however, all but one specimen had
persistant enamel. Evidently, in approximately 43 per cent of the
specimens, a complete enamel blade was present (see Paulson,
1961:139), and in the others (except the one without any enamel) the
plate was restricted to a small area of the ventral surface, usually
on the lingual side of the loph. Hibbard suggested that the decrease
in size of the plate, and its restriction to the lingual side, may be
a function of age. Hence, most adults would be characterized by the
reduced posterior plate on the upper premolar. Although age may be the
important factor, intragroup variation cannot be ruled out. It is of
interest to note that in all specimens from the Benson (type series of
_P. minor_) and Curtis Ranch local faunas, the former of late Pliocene
age and the latter of middle Pleistocene age, the enamel plates are
complete on the posterior face of the upper premolar. As mentioned
before, the specimens from Kansas may actually represent the
transitional stages of the early evolution of _Geomys_ in which the
posterior plate of P4 is entirely lost. The enamel pattern of p4 is
like that in other members of the tribe (excepting the genus
_Pliogeomys_). The re-entrant angles of P4 and p4 are widely open
(obtuse) in the examples recovered from late Pliocene and middle
Pleistocene deposits, representing retention of a trait that is
primitive in the Geomyini (see account of phylogeny).
M1 and M2 are elliptical in cross-section and each has an enamel plate
on both the anterior and posterior surface. In the living species (_Z.
trichopus_), the posterior enamel plate fails to reach the labial
margin of the tooth and is restricted to the lingual two-thirds of the
posterior surface; however, the enamel plates are complete in the late
Pliocene species (_Z. minor_) and the middle Pleistocene species (_Z.
persimilis_), being only slightly separated from the anterior plate by
narrow tracts of dentine on the ends of the tooth. M3 is partly
biprismatic in the living species, the two incompletely divided lophs
being separated by a distinct outer sulcus. The posterior loph is
elongated and forms a conspicuous heel paralleling the evolution of
this trait in the genus _Orthogeomys_; therefore, the crown is longer
than wide. The posterior part of the tooth is protected by two lateral
enamel plates; of the two, the lingual plate is especially long and
extends to the end of the heel. M3 has not been recovered in the
Pliocene species, but in the middle Pleistocene species (_Z.
persimilis_) M3 is subtriangular, no longer than wide, and the lateral
inflections are weakly developed. The trend towards elongation of M3
evidently occurred in late Pleistocene evolution of the genus. All
three of the inferior molars are elliptical, and only the posterior
enamel plate is present (as in all other genera of the tribe except
_Pliogeomys_).
The masseteric ridge of the mandible is well developed. In the late
Pliocene species _Z. persimilis_ and _Z. minor_ the mental foramen is
directly beneath the anterior extension of the masseteric ridge, but
in the living species, _Z. trichopus_, the foramen lies well anterior
to the ridge. The basitemporal fossa in the living species is well
developed and deep; in the Pliocene species it is usually distinct but
shallow (late Pliocene specimens of _Z. minor_).
_Referred species._--Three (two extinct and one living; the last has
two subspecies):
*_Zygogeomys minor_ (Gidley), 1922. U. S. Geol. Surv. Prof. Paper,
131:123, December 26. Type from Benson local fauna (late
Pliocene), Cochise County, Arizona; also known from the
Rexroad local fauna, Meade County, Kansas.
*_Zygogeomys persimilis_ Hay, 1927. Carnegie Inst. Washington
Publ., 136. Originally described by Gidley, 1922 (U. S. Geol.
Surv. Prof. Papers, 131:123, December 26) as _Geomys
parvidens_ which was preoccupied by _G. parvidens_ Brown,
1908. Type from Curtis Ranch local fauna (middle
Pleistocene), Cochise County, Arizona.
_Zygogeomys trichopus trichopus_ Merriam, 1895. N. Amer. Fauna,
8:196, January 31. Type from Nahuatzen, Michoacán.
_Zygogeomys trichopus tarascensis_ Goldman, 1938. Proc. Biol. Soc.
Washington, 51:211, December 23. Type from 6 mi. SE
Pátzcuaro, 8,000 ft., Michoacán.
Genus =Geomys= Rafinesque
1817. _Geomys_ Rafinesque, Amer. Monthly Mag., 2(1):45, November.
1817. _Diplostoma_ Rafinesque, Amer. Monthly Mag., 2(1):44-45,
November. Included species: _Diplostoma fusca_ Rafinesque
[= _Mus bursarius_ Shaw] and _Diplostoma alba_ Rafinesque
[= _Mus bursarius_ Shaw] from the Missouri River region.
1820. _Saccophorus_ Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66.
Type: _Mus bursarius_ Shaw, from upper Mississippi Valley.
1823. _Pseudostoma_ Say, Long's Expd. Rocky Mts., I, pp. 406. Type:
_Pseudostoma bursaria_ [= _Mus bursarius_ Shaw], from upper
Mississippi Valley.
1825. _Ascomys_ Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822),
p. 20., fig. 2. Type: _Ascomys canadensis_ Lichtenstein
[= _Mus bursarius_ Say], probably from upper Mississippi Valley.
1944. _Parageomys_ Hibbard, Bull. Geol. Soc. Amer., 55:735, June.
Type: _Parageomys tobinensis_ Hibbard, from Pleistocene, Cudahy
(Tobin) local fauna, Russell Co., Kansas.
_Type._--_Geomys pinetis_ Rafinesque, 1817, restricted to Screven
County, Georgia, in region of the pines.
_Chronologic range._--Late Pliocene faunas of Blancan age (Rexroad,
Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported
from numerous Pleistocene deposits of all stratigraphic levels,
especially from the Great Plains, where common today.
_Description and discussion._--Pocket gophers of this genus are
medium-sized geomyids; none is so small as the average-sized
_Thomomys_. The skull is generalized and lacks the dolichocephalic and
platycephalic specializations seen in the genera _Orthogeomys_ and
_Pappogeomys_, respectively. _Geomys_ closely resembles _Zygogeomys_,
but retains fewer of the primitive characters of the ancestral stock.
At the same time, _Geomys_ has several specializations. Even so, a
considerable amount of parallelism is evident in the phyletic trends
of the two genera.
The upper incisor of _Geomys_ is bisulcate as in _Pliogeomys_ and
_Zygogeomys_; the deeper grove is medial and the shallower grove lies
near the inner border of the tooth. The premolar, above and below, is
bicolumnar; and two columns are joined at their mid-points (deep
re-entrant angles separate the columns at the sides). A permanent
enamel plate protects the anterior face of the anterior loph, and
enamel bands outline each of the re-entrant folds. In p4 a complete
enamel plate covers the posterior surface of the posterior loph. All
of the enamel bands are interrupted by tracts of dentine, except in
the initial stages of wear of the occlusal surface of the newly
erupted tooth. For a short time in living _Geomys_, the enamel bands
are continuous as observed in juveniles of _Geomys bursarius major_
(KU 5628, 8531, and 41540). But, the enamel cap is thin and the
dentine tracts, which are high on the sides of the tooth, are soon
revealed by a minimum of wear on the crown. Therefore, the adult, or
final, pattern characterized by interrupted enamel plates emerges
early in life and remains throughout the life of the individual.
Evidence from fossil _Geomys_, especially from specimens from early
and late Pleistocene deposits, suggests that the final adult pattern
appears later, ontogenetically, than in Recent specimens. Some of the
fossil premolars in initial stages of wear have continuous and
uninterrupted bands of enamel. _Geomys quinni_ of the late Pliocene
and early Pleistocene has the interrupted pattern seen in late
Pleistocene and Recent _Geomys_. Also, in late Pliocene and early
Pleistocene species, the re-entrant folds diverge laterally and form
"open" angles. In later taxa (middle Pleistocene to Recent) the folds
are compressed and parallel-sided, and the "open" folds are found only
in the early stages of wear.
The posterior enamel plate of P4 disappears in the final stages of
wear as the interrupted enamel pattern is formed. In the late
Pleistocene and Recent _Geomys_, the loss of the posterior plate
occurs early in life, usually in the first phases of wear on the
occlusal surface of the newly erupted tooth, but in fossils of
_Geomys_ of corresponding ontogenetic age from the early and middle
Pleistocene, the posterior plate is retained in some individuals until
a later phase of wear, thereby delaying the appearance of the final
pattern. Indeed, in five or fewer per cent of the individuals (see
Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of
enamel is retained throughout life or at least until late in
adulthood. In _Geomys tobinensis_, for example, a thin, but
transversely complete, plate of enamel occurs all the way down to the
base of the loph (Paulson, _loc. cit._) and would persist throughout
life. In _Geomys garbanii_, a vestige on the lingual side of the
posterior surface of a fully adult specimen was noted by White and
Downs (_loc. cit._). Vestiges of the posterior plate occur less
frequently in living geomyids. Paulson (_loc. cit._) found a posterior
plate in one of 75 specimens of _Geomys bursarius dutcheri_. A young
(suture present between exoccipitals and supraoccipital) female of
_Geomys pinetis austrinus_ (KU 23358) has a vestige of the posterior
plate on the lingual side of the tooth as White and Downs (_loc.
cit._) observed in a specimen of _Geomys garbanii_. The enamel, I
suspect, tends to be thicker on the lingual than on the labial side of
the loph and extends farther down the lingual surface in some
individuals; therefore, wear on the occlusal surface erodes it down to
the dentine more rapidly on the labial than on the lingual side. The
tendency of enamel to be retained is a primitive feature.