retaining a permanent labial re-entrant fold in most species (with
exceptions in _Pappogeomys bulleri_ and some old adults of _P.
castanops_). Primitively the occlusal surface of M3 is subtriangular
(subgenus _Pappogeomys_), but in the _castanops_ species-group of the
advanced subgenus _Cratogeomys_, the posterior loph usually is reduced
and the occlusal surface is quadriform or obcordate. Curiously, the
trend towards reduction of the posterior loph is reversed in one
subspecies (_P. merriami fulvescens_) and, the loph has elongated into
a pronounced heel in some specimens, resembling the condition in
_Orthogeomys_. The entire range of variation occurs in _P. m.
fulvescens_. The subtriangular pattern is retained in the most
specialized species of _Cratogeomys_ where that pattern is associated
with extreme platycephaly in the _gymnurus_ species-group. In most
species the posterior loph supports two lateral plates, the outer one
always bordering the labial re-entrant fold. In _Pappogeomys bulleri_
and in the _castanops_ species-group, the outer re-entrant fold of M3
tends to be obsolete, and the tooth becomes quadriform or
suborbiculate in some individuals and loses the bilophodont pattern
that characterizes other species. The lingual enamel plate is
displaced to the posterior surface of the tooth, and one or both
plates may disappear with advancing age. Consequently, only the
anterior enamel plate remains in some adults, and constitutes the
maximum degree of reduction of enamel on M3 in the Geomyinae. In many
adults of _Pappogeomys bulleri_, the enamel investment of the
posterior loph is complete and the two lateral plates are connected,
without interruption around the posterior apex of the tooth, evidently
representing the retention of a primitive character of the ancestral
lineage.
The m3 of _P. bensoni_ from the late Pliocene is distinguished by
minute lateral inflections suggesting the primitive biprismatic
pattern. Also the posterior enamel plates of m1 and m2 are remarkably
long, extending around the ends of the tooth. The associated upper
incisor was unisulcate as in the modern species, and the basitemporal
fossa of the mandible is well developed and deep.
The lower jaw is stout and relatively short. The masseteric ridge is
well developed and has an especially thick crest. The basitemporal
fossa is deep. In most living species, the pelage is soft and dense,
but in one species, _Pappogeomys fumosus_, the hairs are coarse and
hispid somewhat as in _Orthogeomys_.
Key to the Subgenera of _Pappogeomys_
A Enamel plates completely developed across posterior walls of
M1 and M2, except in one species (_P. alcorni_) having enamel
restricted to lingual fourth in M1; sagittal crest lacking
owing to impressions of temporal muscles remaining separated
(even in old adults); zygomata slender, and without platelike
expansion at lateral angle. Subgenus _Pappogeomys_ p. 534
A´ Enamel lacking on posterior walls of M1 and M2; pronounced
sagittal crest developed in adults of both sexes by union
of temporal impressions at middorsal line; zygomata stout
and wide, with lateral angle expanded into broad plate.
Subgenus _Cratogeomys_ p. 535
Subgenus =Pappogeomys= Merriam
1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.
_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope
Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.
_Chronologic range._--Late Pliocene (Benson local fauna, Arizona) to
Recent, but no specimens known from Pleistocene.
_Description._--Small, approximately same size as small subspecies of
_Thomomys umbrinus_ but forefeet larger and claws longer; skull of
generalized shape, broad, relatively short, smoothly rounded, not
especially compressed dorso-ventrally; zygomatic breadth great but not
exceeding mastoid breadth; zygomata relatively slender for geomyid and
lacking platelike expansions at lateral angles; rostrum relatively
narrow; sagittal crest lacking, owing to impressions of temporal
muscles remaining separated; angular process of mandible not
especially elongated; enamel plates extending completely across
posterior wall of M1 and M2, except in one species, _P. alcorni_,
where posterior plate of M1 remains only on lingual fourth of
posterior wall (remainder of plate lacking); with wear, plates
sometimes exceptionally thin completely across posterior face of M2
and especially M1 in a few individuals of _P. bulleri_ much as Paulson
(1961:138-139) describes in extinct _Geomys tobinensis_; one or both
plates rarely disappear in final stages of attrition in old
individuals resulting in same dental pattern found in _Cratogeomys_;
M1 and M2 retaining enamel plate on anterior wall throughout life; M3
usually subtriangular in cross-section but sometimes suborbiculate or
ovoid, crown slightly bilophodont owing to shallowness of labial
re-entrant angle in modern species; posterior loph of M3 not
especially elongated and crown not significantly longer than wide;
both lateral enamel plates of M3 usually well developed and
approximately equal in length, occasionally plates reduced in length
and rarely one or both plates are lost with wear in old individuals;
patch of whitish or buffy hairs surrounding nose of most individuals.
The primitive character of the lower dentition, as described in the
species account above, suggest that _Cratogeomys_ [= _Pappogeomys_]
_bensoni_ Gidley should be referred to the subgenus _Pappogeomys_
rather than _Cratogeomys_. Only the upper dentition would make
positive identification possible; however, reference to the subgenus
_Pappogeomys_ seems to be the best arrangement at this time.
_Referred species._--Three (one extinct):
*_Pappogeomys bensoni_ (Gidley), 1922. U. S. Geol. Surv. Prof.
Papers, 131:123. Type from Benson local fauna (late Pliocene),
Cochise County, Arizona.
_Pappogeomys alcorni_ Russell, 1957. Univ. Kansas Publ. Mus. Nat.
Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.
_Pappogeomys bulleri_ Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6,
vol. 10:196, August. Type from "near Talpa," west slope of
Sierra Madre de Mascota, Jalisco.
Subgenus =Cratogeomys= Merriam
1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.
1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.
Type: _Geomys gymnurus_ Merriam, 1892.
_Type._--_Geomys merriami_ Thomas, 1893, from "Southern México,"
probably in Valley of México.
_Chronologic range._--Late Pleistocene, from Wisconsin deposits (San
Josecito Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave,
New Mexico, local faunas) to the Recent.
_Description._--Size medium to large; skull becoming angular and
rugose with age, and tending towards platycephaly and dorso-ventral
compression; zygomata stout, each bearing platelike expansion at
anterolateral angle into which anterior end of jugal becomes morticed;
breadth across zygomata great relative to length of skull; rostrum
relatively broad; squamosals expanding medially with age eventually
growing over lateral parts of parietals, and sometimes also expanding
laterally displacing postglenoid notch; sagittal crest well developed
in adults of both sexes, but especially high and bladelike in males;
lambdoidal crest prominent in all but young animals, having dorsal
outline broadly convex posteriorly in most species but strongly
sinuous in _gymnurus_-group; enamel plate on posterior wall of P4
absent; enamel plates present only on anterior walls of M1 and M2; M3
variform in occlusal shape (as described in species account), either
subtriangular (_gymnurus_-group), quadriform or obcordate
(_castanops_-group, with exceptions as noted before); lateral plates
of M3 usually present in all species, labial plate approximately as
long as lingual plate in _gymnurus_-group (like that in subgenus
_Pappogeomys_) or distinctly shorter in _castanops_-group (labial
plate scarcely extending beyond border of labial re-entrant fold); one
or both lateral plates tending to disappear with wear in
_castanops_-group, with lingual plate usually disappearing first;
breadth across angular processes clearly more than breadth across
zygomatic processes, especially in _gymnurus_-group.
_Remarks._--In the species of the _castanops_-group the skulls can be
spoken of as generalized and the least platycephalic of the subgenus.
Indeed, the species of the _castanops_-group are hardly more
specialized in this respect than is the subgenus _Pappogeomys_.
In these skulls the breadth across the squamosal processes is less
than that across the zygomatic arches, although the two dimensions
are almost equal in some examples of _P. merriami_ of the
_castanops_-group (where squamosal breadth varies from 85 to 98% of
zygomatic breadth). In the species having marked platycephalic skulls
(_gymnurus_ species-group) the breadth across the squamosal processes
equals or exceeds the breadth across the zygomatic arches (squamosal
breadth rarely 97 to 99% of zygomatic breadth), except in _P. zinseri_
and _P. tylorhinus zodius_.
The variable character of the third upper molar as between species
suggests that this tooth is presently undergoing active evolution. The
structure of this tooth, although differing between taxa, is
remarkably stable in other kinds of Geomyini. The most remarkable
modification of M3 in _Cratogeomys_ is the obcordate pattern
developed in _P. merriami_ of the _castanops_-group. The posterior
loph and entire tooth is shortened somewhat resembling in shape that
of _Thomomys_. Moreover, the posterior loph is twisted labially;
consequently, its posterior surface now forms the labial border of the
weakly defined posterior loph. Owing to the torsion, the lingual
enamel plate has been rotated to the posterior surface of the tooth.
Therefore, the tooth is provided with two transverse enamel plates,
including the plate on the anterior wall of the tooth. The labial
plate is greatly reduced, its total surface being restricted to the
small labial inflection. The highly specialized obcordate M3 is not
found in the most specialized platycephalic skulls characteristic of
the _gymnurus_ species-group. Instead the _gymnurus_-group retains the
primitive subtriangular pattern without significant modification.
_Referred species._--Seven:
_castanops_ species-group
_Pappogeomys castanops_ (Baird, 1852). Report Stanbury's Exp'd. to
Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's
Fort," near present town of Las Animas, Colorado.
_Pappogeomys merriami_ (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6,
12:271, October. Type from "southern Mexico," probably Valley of
México (see Merriam, 1895:152).
_gymnurus_ species-group
_Pappogeomys fumosus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
7:165, September 29. Type from 3 mi. W Colima, Colima.
_Pappogeomys gymnurus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
7:166, September 29. Type from Zapotlan (Ciudad Guzman),
Jalisco.
_Pappogeomys neglectus_ (Merriam, 1902). Proc. Biol. Soc.
Washington, 15:68, March 22. Type from Cerro de la Calentura,
about 8 mi. NW Pinal de Amoles, Querétaro.
_Pappogeomys tylorhinus_ (Merriam, 1895). N. Amer. Fauna, 8:167,
January 31. Type from Tula, Hidalgo.
_Pappogeomys zinseri_ (Goldman, 1939). Jour. Mamm., 20:91, February
15. Type from Lagos, Jalisco.
PHYLOGENY OF THE GEOMYIDAE
The fossil record of the Geomyidae provides a sequence of morphotypes,
each representing a stage in the phyletic development of the family.
Most of the preserved specimens probably represent the stufenreihe
rather than the ahnenreihe, as Simpson (1953:219-220) points out. Even
so, the stufenreihe closely approximates the general trend of
evolution, and the level of structural organization in the different
stages of phyletic development may be ascertained. The actual
ancestral series of most lineages probably will remain unknown, but
hopefully some of the existing gaps will be filled by future
discoveries. From the established record, several clearly defined
lineages can be distinguished; in fact the sequence of origin, pattern
of evolution, and specializations, of the principal lineages are
reasonably well expressed.
Primitive Morphotype
In the earliest known geomyids from the Upper Oligocene and Lower
Miocene, the premolars and molars are biprismatic and bilophodont. In
rodents, this is itself a specialized pattern, and is thought to have
evolved from a more primitive sextituberculate prototype by the union
of individual cusps, and probably also cuspules, forming the two
transverse enamel lophs. The primitive, common ancestor of the
Geomyidae and Heteromyidae with sextituberculate teeth in the early
Tertiary is unknown.
As soon as geomyids attained the early bilophodont stage of evolution,
the basic morphological structure of the family was established. The
family probably first became clearly distinguished from other
Geomyoidea at this stage. In the early bilophodont stages of
evolution, owing to the relatively deep valley between them, the two
columns probably failed to unite in the normal cycle of wear, as they
do in all later geomyids. _Griphomys_ described by Wilson (1940:93)
from the late Eocene of California, has a bilophate pattern in which
the anterior and posterior lophs are separated by a persistent
transverse valley. The occlusal pattern of _Griphomys_ closely
resembles a stage through which the ancestors of the early Miocene
geomyids must have passed in their pre-Miocene evolution, as Wilson
suggests (1949:115-116). Although he (1940:95; 1949:110-118)
tentatively referred _Griphomys_ to the superfamily Geomyoidea and
Simpson (1945:80) went so far as to refer it to the family Geomyidae,
with a notation of _incertae sedis_, its exact relationship to the
pocket gophers is uncertain. However, the structure of the molariform
dentition of _Griphomys_ does not exclude it from the phyletic
ancestry of the Geomyidae. In subsequent stages of evolution the
anterior and posterior columns become united. Thereby part of the
valley floor between the transverse prisms was progressively elevated,
to the stage where attrition on the occlusal surface would unite the
two columns. On the unworn enamel cap of living geomyids the two
transverse enamel folds are separated by a shallow but well defined
valley, briefly reflecting the ancient ancestral pattern.
Union of the lophs may have been either at the mid-points of the two
columns or at the edge of their protomeres. [A protomere is the half
of a tooth containing the protocone or protoconid--lingual side of
upper tooth and labial side of lower tooth. The paramere is the
opposite half of a given tooth--labial side of upper tooth and
lingual side in lower tooth. See Miller and Gidley, 1918:434.] Union
of the columns at the mid-points would have produced the figure-8
occlusal pattern (or H-pattern), which is characteristic of the early
Miocene Geomyinae (_Dikkomys_). Union of the two columns at the
protomeres would have produced the U-shaped pattern of the
Entoptychinae, which also occurred in the early Miocene and were
contemporary with the earliest Geomyinae. Since pre-Miocene geomyids
are unknown, the actual phyletic development of the dentition is a
matter of speculation. Probably the development of the two divergent
lineages, one leading to the Entoptychinae and the other to the
subfamily Geomyinae, occurred in the Oligocene (as depicted in Fig.
3). Of the two lineages, the subfamily Geomyinae, in my view, is the
more primitive and less specialized. Support for this view is
furnished by a reconstruction of the pattern of occlusal wear in
_Dikkomys_ and _Pliosaccomys_, especially on the first and second
molars.
In _Dikkomys_, the anterior and posterior column first unite near
their mid-points in the first stages of wear thus producing a figure-8
shaped (H-shaped) occlusal pattern in the premolar and all three
molars. Evidently in the first two upper molars, the columns unite
closer to their lingual margins than their mid-points, but at any rate
both outer and inner re-entrant folds are evident at this stage of
wear. With continued attrition on m1 and m2 of _Dikkomys_, the
anterior and posterior columns secondarily unite at the edge of their
labial margins thus enclosing a fossette of enamel in the labial half
of the tooth. The lateral coalescence at the ends of the protomeres
occurs because of the shallow vertical depth of the labial re-entrant
fold, and the fossette itself does not reach the base of the crown and
with continued wear it too would disappear, but not until the last
stages of wear, at least in _Dikkomys matthewi_. The lingual
re-entrant fold is deep, and therefore, persistent through all stages
of wear. Although the amount of wear required for its effacement would
be great, the occlusal configuration of the first and second lower
molars in _Dikkomys_ could be eventually ground down to a U-pattern as
in the entoptychids. Only one upper molar of _Dikkomys_, the first,
has been recovered (see Wood, 1936:23, fig. 32B). Although the tooth
is in an early stage of wear, the lingual valley is minute. Less
attrition than required in m1 and m2 would progressively reduce the
lingual fold until it too would essentially form a U-pattern, perhaps
retaining a slight lingual inflection. Hence, the first upper molar
becomes a mirror image of the first lower molar, and the second upper
molar probably had the same pattern as the first (at least it does so
in _Pliosaccomys_). Both of the lateral re-entrant folds of the
premolar are deep vertically, and consequently would not disappear
with occlusal wear. Therefore, the H-pattern of the premolars is
retained throughout life.
The m3 (M3 unknown for _Dikkomys_ or _Pliosaccomys_) also has deep
lateral folds; hence, it too retains the H-pattern in all stages of
attrition, although the isthmus between the two prisms may become
wider in the final phases of wear (as it does in _Pliosaccomys_).
In _Pliosaccomys_, the stages of wear are essentially the same as
those described for _Dikkomys_, except that the anterior and posterior
loph of the first and second molars tend to unite closer to one side
of the tooth, lingual side in upper molars and labial in lower. Only a
slight inflection of the re-entrant fold is evident on the side of
union, and the inflection disappears in the first phases of wear as
the columns unite. Concomitant with the lateral shift in the initial
point of coalescence of the transverse lophs, the occlusal penetration
of the re-entrant fold from the opposite side increases in horizontal
depth, and the fold extends medially more than half way across the
occlusal surface, thus forming a pattern essentially like that of the
entoptychids. The U-pattern in _Pliosaccomys_ appears in the initial
stages of wear without going through an earlier H-pattern as is the
case in its Miocene ancestors of the genus _Dikkomys_, unless the
minute inflection is considered as indicative of that stage. The two
columns of the premolar and m3 are joined near their mid-points as in
_Dikkomys_; therefore, they retain their primitive H-pattern, a
feature unique to the Geomyinae.
The evolutionary trend toward an ontogenetically earlier U-pattern in
the first two molars in the primitive lineage of the Geomyinae
suggests that the U-pattern characteristic of the Entoptychinae was
simply an earlier tendency toward the same specialization that
occurred later in the subfamily Geomyinae. If so, early entoptychines
would have been characterized by an H-pattern in the first stages of
attrition, like _Dikkomys_, and later developed union at the edge of
the protomeres. However, in the entoptychines, all the molariform
dentition, and not merely the first and second molar, became
specialized; consequently the U-pattern was produced on the occlusal
surfaces of each of the cheek teeth. As in _Pliosaccomys_, the
transitional phase, in which the two columns were united at their
mid-points, was eventually eliminated from the pattern of wear and
only the U-pattern, that now appeared in the initial stages of wear,
was retained. In the entoptychines of the early Miocene there is no
suggestion of the H-pattern that characterizes the Geomyinae, except
in the position of the cusps before wear in the lower molars of
_Pleurolicus sulcifrons_, which, according to Wood (1936:6), suggests
the H-pattern. In earlier unknown Oligocene stages of evolution, the
prisms possibly united first at their mid-points, and the columns may
have joined at the side of the tooth only in the terminal stages of
wear. The U-pattern of pre-Miocene entoptychines, therefore, may have
become the dominant occlusal pattern only in the later stages of
phyletic development.
According to the recently expressed views of several paleontologists,
the Entoptychinae constitute the primitive lineage of the family and
the early Geomyinae constitute a specialized offshoot of the
entoptychine ancestral assemblage. The structure of the Entoptychinae,
especially of the less advanced genera, closely approximates that of
the hypothetical primitive morphotype. But, according to my view, the
subfamily Geomyinae constitutes the ancestral assemblage and its
structure is essentially that of the primitive morphotype of the
family. At any rate the structure of the early geomyines more closely
approximates the structure of the ancestral stock than the more
divergent entoptychines. Therefore, the genus _Dikkomys_ of the early
Miocene, the first known geomyine, is considered to be a generalized
geomyid, and, although it is a contemporary of the more specialized
entoptychid assemblage, is considered to be more closely allied to the
ancestral stock.
The entoptychines were the dominant and most highly differentiated
geomyids of the early and middle Miocene. Nevertheless, they became
extinct in the middle Miocene, and the geomyines of that time survived
and later gave rise to the modern pocket gophers. Therefore, the early
history of the family Geomyidae is characterized by an early radiation
and trend toward specialization, followed by survival of the less
specialized Geomyinae and extinction of the more specialized
Entoptychinae.
Entoptychid Radiation
The most abundant geomyids of the early and middle Miocene, the
Entoptychinae, consisted of at least 24 species (see Wood, 1936:4-25)
classified in four genera: _Pleurolicus_, _Gregorymys_, _Grangerimus_,
and _Entoptychus_. The genera were essentially contemporaneous (see
Figure 3). Even so, the subfamily was morphologically varied, pointing
to an earlier origin in the Oligocene (actually a part of the John Day
Fauna, including _Pleurolicus_ may be correlated with late Oligocene
Whitneyian age) followed by a relatively rapid radiation including all
four genera in the early Miocene. Two genera, _Pleurolicus_ and
_Gregorymys_, continued into the Middle Miocene (Hemingfordian). This
divergence, specialization, and subsequent radiation suggest that the
entoptychines evolved into a new major adaptive zone, in the sense
described by Simpson (1945:199-206).
The radiation is correlated geographically and temporally with the
southward retreat of the Neotropical flora of the Tertiary from the
western United States and southward movement of the Arctic flora of
the Tertiary (see Axlerod, 1950; Berry, 1937:31-46; Chaney,
1947:139-148; and Kendeigh, 1961:280-283). In the early Tertiary the
Neotropical-tertiary geoflora occurred northward to at least 49°
latitude in western North America, and the boreal Arctic-tertiary
flora was restricted to a circumpolar zone. The southward and eastward
shift of the Neotropical-tertiary flora, associated with the drying
and chilling of the continent, began in the middle or late Oligocene
and was concurrent with the divergence and radiation of the
Entoptychinae. Beginning in late Oligocene and continuing at least
into middle Miocene, most of the region in which the entoptychines
occurred was occupied by the Arcto-tertiary geoflora of which the
temperate forest division contributed the dominate plant associations.
The maples, chestnuts, dogwoods, beeches, walnuts, oaks, elms,
birches, and sycamores of that flora were the forerunners of today's
eastern deciduous forest. It is my view that the entoptychines became
adapted to the conditions of this paleoecological environment and
radiated rapidly in the Arikareean when the major change occurred in
climax vegetation. The ancestral stock of the Geomyinae was not so
successful in the Arcto-tertiary climax, and most of it probably was
displaced southward along with the tropical flora.