compression, and retention of a distinct labial re-entrant fold. The
posterior loph apparently has been rotated in such a way that what was
previously its posterior border now lies on the outer margin of the
tooth; therefore, the axis of the posterior loph is strongly oblique
in relation to the anteroposterior bearing of the maxillary tooth-row,
and the median enamel plate also has been rotated and so lies
transversely across the posterior wall of the tooth. Owing to the
rotation of the posterior loph, the apex of the obcordate tooth is at
its lingual side. The subcordate type is illustrated by Merriam (_loc.
cit._) in Figures 27 (3 and 4), 28 (a and b), 34 (3 and 4), and 35 (5,
6, and 7). The suborbicular and quadriform types are less specialized
than the two described above. Both are characterized by reduction,
often obliteration, of the bicolumnar pattern of the subtriangular
ancestral form, especially marked by the decrease in depth of the
lateral re-entrant folds and the decrease in length of the posterior
projection of the posterior loph. With these changes, the tooth
becomes essentially monocolumnar, its occlusal surface oval in outline
in one and squarish in shape in the other. Occlusal views of the
suborbicular form are presented by Merriam (_loc. cit._) in Figure 33
(1, 5, 6, 7, 11, and 12) and the quadriform tooth is depicted in
Figure 29. Grooved upper incisors are characteristic of the living
Geomyini, but variation occurs in the number of grooves, and, if only
one groove is present, its position on the anterior face of the tooth
varies. Except for the previously mentioned (p. 480) abnormal tooth
having three grooves, incisors with no more than two grooves are found
in these pocket gophers, and this number of grooves is taken to be
primitive. Loss of one or the other of the two grooves of the
bisulcate pattern, therefore, is regarded as specialization. However,
complete loss of both grooves never occurs in the Geomyini. Each of
the four major lineages is characterized by one of the three patterns
of grooving, and the particular groove-pattern is remarkably stable in
each group.
Shape of skull varies from dolichocephalic to platycephalic. The
morphology of each has been described in foregoing accounts. The
dolichocephalic skull is highly specialized for planing, a grinding
action of the teeth; whereas, the platycephalic skull is highly
specialized for shearing, a slicing action of the teeth. Of course,
concomitant specializations of the dentition, as described above, are
closely associated with both specialized trends in the skull. Most
kinds of living Geomyini have generalized skulls that show no tendency
toward either of the specialized conditions.
Increase in size of body and skull is seen in most Pleistocene
lineages of the Geomyini. Judging from the smallness of the skull in
late Pliocene species, representing the base of three of these
lineages, the ancestral species of the living assemblage were no
larger than the living species of the subgenus _Pappogeomys_ or the
smaller subspecies of _Geomys bursarius_. The recorded range of
variation in condylobasal length is 36.1 to 45.5 in _Pappogeomys
bulleri_, including both adult males and females. Probably the skulls
of the ancestral species were not significantly larger. Maximum
dimensions of males in living species are 74.5 (subgenus
_Cratogeomys_) and 75.0 (subgenus _Orthogeomys_). These are more than
twice the minima observed in _Pappogeomys bulleri_.
Zygogeomys
This is the least specialized and most primitive of the four lineages,
has a generalized type of skull, two grooves on the anterior face of
each upper incisor, an enamel plate on the posterior wall of P4, open
or divergent lateral re-entrant angles on the premolars, and a
bicolumnar and elongated M3. All of these features are primitive and
essentially as in the ancestral morphotype. No other modern genus
retains so much of the primitive structure. Phyletic trends in
_Zygogeomys_ are not well documented in the fossil record; and only a
few fossils are known and they are fragmentary as discussed before.
The genus is represented in the late Pliocene (_Z. minor_), middle
Pleistocene (_Z. persimilis_), and Recent (_Z. trichopus_). The living
species is a relict population in the mountains of Central México.
Judging from the known material, the phyletic trends in the genus have
been increase in size, reduction of enamel on the posterior face of P4
(occurring only in the living species) where a short enamel plate is
retained on the lingual side of the tooth (see Fig. 7B), loss of the
outer fourth of the enamel blade on the posterior wall of M1 and M2
(also occurring only in the living species), development of a more
pronounced heel on the M3 by progressive elongation of the posterior
loph, reduction in size of the jugal and its displacement ventrally,
which allows the maxillary and squamosal bones to meet along the
dorsal border of the zygomatic arch. The last specialization is seen
in at least one taxon of _Orthogeomys_ (_Orthogeomys cherriei
costaricensis_). In my opinion, too much weight has been given to this
feature in past classifications. Reduction of enamel in the upper
dentition evidently occurred in the late Pleistocene, since the
posterior plates on the upper cheek teeth were complete in specimens
from the middle Pleistocene (_Z. persimilis_).
Geomys
_Geomys_, slightly more specialized than _Zygogeomys_, must also be
regarded as one of the most primitive of the living genera. Primitive
features that have been retained are the generalized type of skull,
the bisulcate pattern of grooves on the upper incisor, and the
retention of enamel plates on both the anterior and posterior walls
of M1 and M2 (see Fig. 9A). All of these primitive features are
shared with _Zygogeomys_. In addition, three other trends, or
specializations, in evolution characterize the phyletic development of
_Geomys_. One major trend is toward loss of the enamel plate from the
posterior wall of P4. No trace of enamel remains on the posterior wall
of this tooth in late Pleistocene or Recent species of _Geomys_, and
at least one of the earlier species (_quinni_) was also characterized
by loss of this enamel plate. Secondly, M3 retains only a vestige of
the primitive bicolumnar pattern after the initial stages of wear. In
most Recent specimens, especially of the species _G. bursarius_, the
lateral re-entrant fold and the heel of M3 are small, and the
re-entrant inflection is hardly evident. The lateral fold is more
frequently well-developed in Irvingtonian species than in living
species (White and Downs, 1961:13), illustrating progressive loss of
the bicolumnar pattern in Pleistocene evolution. A third trend
involves the modification of the lateral folds of the premolars.
Primitively the angles of these folds are broadly open or divergently
V-shaped, and some of the earliest species of _Geomys_, for example
_G. quinni_, have retained this feature throughout life. Nevertheless,
the main trend is toward progressive compression of the folds
resulting in their walls being more nearly perpendicular, and
parallel, to the long axis of the tooth. Obtuse re-entrant angles
persist in premolars of young individuals of Irvingtonian species, but
the adults are characterized by well-compressed folds, as in Recent
species.
Remains of _Geomys_ are abundant, especially from Pleistocene deposits
of the Great Plains, but in most instances specific assignment is
difficult or impossible since only isolated teeth or fragments of
skulls have been preserved. Estimates of phyletic relationships of the
known species of _Geomys_ are depicted in Figure 8; those estimates
are useful in discussing the phyletic development of the genus. One of
the earliest known species, _Geomys quinni_, ranges from Upper
Pliocene to the later stages of the Lower Pleistocene (Aftonian
interglacial deposits). The dentition of _G. quinni_ is essentially
the same as in the living species except that open lateral re-entrant
angles are retained in the premolars. _Geomys paenebursarius_, also of
the early Pleistocene, is a smaller species and seems to be more
directly in the line of evolution of the modern species. As yet
unnamed smaller species of _Geomys_ from the Rexroad fauna (late
Pliocene) and Saunders fauna (latest Aftonian) may also be on the main
line of evolution. Surprisingly, _Geomys tobinensis_ and _Geomys
garbanii_ of later Irvingtonian provincial age are less specialized
than either _Geomys quinni_ or _Geomys paenebursarius_. It is likely
that _G. tobinensis_ and the unnamed species from the Dixon are closer
to the main line of descent than _G. paenebursarius_ suggesting that
the direct ancestral lineage of the living species of _Geomys_ was
more conservative and less specialized than _Geomys paenebursarius_ of
the Lower Pleistocene. _Geomys quinni_ and _G. paenebursarius_ seem to
have acquired specialized dental features in the early Pleistocene.
_Geomys quinni_ was successful on the Great Plains, and persisted into
the late Blancan. The main line may be represented in the early
Pleistocene by _Geomys paenebursarius_ from the Hancock formation of
the Texas Trans-Pecos. The structure of _G. paenebursarius_ indicates
that it is in or close to the main line of descent, and probably
evolved from one of the more primitive late Pliocene species of
_Geomys_ from the Rexroad fauna.
[Illustration: FIG. 8. Tentative arrangement of species of the genus
_Geomys_, depicting phylogenetic trends and probable relationships
within the genus.]
Isolated teeth, to which the name _Geomys bisulcatus_ probably
applies, from Illinoian deposits on the Great Plains, show that the
dentition characteristic of the living _Geomys_ had been developed by
that time. Actually, the Illinoian material is too fragmentary to
show clearly its taxonomic or phyletic affinities with the species of
the later Pleistocene. Even so, the two main stocks of living
_Geomys_, _G. bursarius_ and _G. pinetis_, had certainly been
differentiated by Sangamon time. The other living species evidently
evolved from one or the other of these two stocks in a period of
isolation from the main population, probably in either the Wisconsin
or post-Wisconsin. For example, _Geomys arenarius_ clearly
differentiated from populations of _Geomys bursarius_ that were
isolated by the eastward retreat of the main population from the
southwestern United States as that region became more arid in the
post-Wisconsin.
In review, it seems that the Recent species, represented basically by
_bursarius_ and _pinetis_, evolved from Illinoian species (_Geomys
bisulcatus?_), which descended in turn from the more primitive species
of the early Pleistocene, possibly _Geomys paenebursarius_ or possibly
from descendants of the Saunders species. Actually the Saunders
species may prove to be _Geomys paenebursarius_. At any rate, three
trends that took place during the Pleistocene stage of evolution, in
the direction of the modern species, were an increase in size,
progressive loss of the posterior enamel plate on P4, and a decrease
in the vertical depth of the enamel cap as a result of which the
dentine is reached in the initial phases of attrition on the tooth of
a juvenile. _Geomys garbanii_, occurring at the periphery of the range
of the genus, is regarded as a sterile offshoot of the primitive
_tobinensis_-line of evolution.
Orthogeomys
This is one of the more specialized genera of the Geomyini. Save for
one record in the late Pleistocene (_Orthogeomys onerosus_), there is
no fossil history of the genus upon which to reconstruct its
phylogeny; therefore, its phyletic development must be estimated by
comparing it and the primitive morphotype of the tribe. Results of
that comparison suggest that _Orthogeomys_ has closer affinities with
_Zygogeomys_ than with any of the other genera, and that _Orthogeomys_
may have originated in an early dichotomy of primitive _Zygogeomys_
stock instead of descending from the ancestral stock of the tribe.
Except for the unisulcate incisors and the longer posterior loph on
the third upper molars, the teeth of the two genera do not differ
significantly. As in _Zygogeomys_, the enamel blade on the posterior
wall of P4 has been reduced to a short plate restricted to the lingual
third of the tooth (see Fig. 7F and H). In _Orthogeomys_, the trend in
reduction of enamel is carried to its extreme only in the subgenus
_Orthogeomys_, where this plate has been completely lost in most taxa
(see Fig. 7D). The most significant trends in _Orthogeomys_, and the
principal basis for recognizing the genus, are the dolichocephalic
specializations of the skull, as described elsewhere, and the adaptive
traits that have equipped the genus for living in tropical
environments. The dolichocephalic features are more sharply defined in
the subgenera _Orthogeomys_ and _Macrogeomys_, and are less developed
in the subgenus _Heterogeomys_. Aside from the general dolichocephalic
specializations, trends in _Orthogeomys_ include: Increase in size;
loss of the median one of the two grooves on the anterior face of the
upper incisor in the ancestral stock; increase in the anteroposterior
length of each of the cheek teeth, as well as the aforementioned
elongation of the posterior loph of M3; compression of the lateral
angles of the premolars; and the remarkable increase in the size of
the rostrum.
Pappogeomys
The genus _Pappogeomys_, as it is conceived of in this study, is
comprised of two subgenera; one, _Pappogeomys_, is generalized and
primitive, and the other, _Cratogeomys_, is specialized, and includes
the most highly specialized of the modern pocket gophers. The subgenus
_Pappogeomys_ is regarded as the ancestral lineage, and the subgenus
_Cratogeomys_ is regarded as an early offshoot, probably in the early
Pleistocene, that became progressively more specialized in the course
of its subsequent evolution. In the same period of time, the subgenus
_Pappogeomys_ changed little. It is known only from late Pliocene
fragments and from the living species. The ancestral morphotype is
preserved in _Pappogeomys_. Primitive characters are: (1) Small size;
(2) skull generalized and smoothly rounded; (3) temporal ridges
separate (not uniting into a sagittal crest); (4) enamel plates
retained on both anterior and posterior walls of M1 and M2; (5) M3
bilophate, its posterior loph short. Basic specializations are few and
include loss of the inner groove from the anterior face of the upper
incisor; anteroposterior compression of the lateral re-entrant folds
of the premolars; and loss of enamel from the posterior wall of P4.
All three features have been perpetuated in the advanced subgenus
_Cratogeomys_, suggesting that they were already developed in the
early evolution of the subgenus _Pappogeomys_ before _Cratogeomys_
diverged. Agreement with _Geomys_ is demonstrated by the lack of
enamel on the posterior wall of P4 (see Fig. 9) and by retention of
the posterior enamel plate on M1 and M2. In _Pappogeomys (Pappogeomys)
alcorni_ the enamel from the posterior face of M1 has been lost from
all but the lingual fourth or so of the posterior wall (Fig. 9E).
Reduction of enamel in M1 provides an example of parallelism with the
more advanced subgenus _Cratogeomys_, discussed below.
There is no record as yet of the early evolution of the subgenus
_Cratogeomys_. The features that characterize the subgenus were
already well developed in the first known fossils which are from
Wisconsin deposits of the late Pleistocene. _Cratogeomys_ is not a
homogenous assemblage; instead it is composed of two groups of living
species, the generalized _castanops_ group and the specialized
_gymnurus_ group. The _castanops_ group may be survivors of the
ancestral lineage that diverged in two different stages in the
phyletic development of the main line. Even so, the _castanops_ group
has acquired its peculiar specializations. Indeed, _P. merriami_ of
the _castanops_ group differs from the hypothetical stem more than
does _P. castanops_. Judging from the structure of the living species
of the subgenus _Cratogeomys_ and from the primitive subgenus
_Pappogeomys_, the subgenus _Cratogeomys_ featured five major trends:
(1) Increase in size; (2) formation of sagittal crest by union of the
temporal impressions; (3) increase in rugosity and angularity of the
skull; (4) progressive development of platycephalic specializations,
including the elongation of the angular process of the mandible; (5)
complete loss of enamel plates from the posterior wall of M1 and M2.
Each trend is thought to be adaptive.
[Illustration: FIG. 9. Molariform dentitions of the Tribe Geomyini.
Drawings illustrating enamel patterns characteristic of _Geomys_
and _Pappogeomys_ (including the subgenera _Pappogeomys_ and
_Cratogeomys_). × 5.
A and B. _Geomys bursarius bursarius_, adult female, No. 46275 (KU),
Elk River, Sherborne Co., Minnesota. Left upper (A), P4-M3;
right lower (B), p4-m3.
C and D. Subgenus _Pappogeomys_. _Pappogeomys bulleri albinasus_,
adult female, No. 31002 (KU), W side La Venta, 13 mi. W and
4 mi. N Guadalajara, Jalisco. Left upper (C), P4-M3; right
lower (D), p4-m3.
E and F. Subgenus _Pappogeomys_. _Pappogeomys alcorni_, adult
female, No. 31051 (KU), holotype, 4 mi. W Mazamitla, 6600
ft., Jalisco. Left upper (E), P4-M3; right lower (F),
p4-m3.
G and H. Subgenus _Cratogeomys_. _Pappogeomys gymnurus tellus_,
adult female, No. 31051 (KU), 1 mi. NE Tala, 4400 ft.,
Jalisco. Left upper (G), P4-M3; right lower (H), p4-m3.
]
Loss of enamel is a trend common to all living genera of the tribe
Geomyini, but the greatest loss has occurred in _Cratogeomys_. It has
lost the plates on the posterior walls of M1 and M2 (Fig. 9G). If the
lateral plates of M3 are considered as one functional plate and the
lateral plates on either side of P4 together as two transverse plates,
then, the transverse cutting blades in _Cratogeomys_ number seven in
the upper and seven in the lower cheek teeth compared with 10 in the
upper and seven in the lower in the primitive morphotype. Indeed, in
some species of the subgenus, one or both of the lateral plates on M3
is also lost, usually in old age, resulting in even greater reduction
of enamel. Loss of enamel from the posterior walls of the upper
molars may be associated with changes in the mechanics of mastication
from anteroposterior planing to anterotransverse shearing, as
discussed elsewhere. Merriam (1895:95-96) argues convincingly that the
posterior cutting blades of the upper molars would hinder efficient
shearing action of the teeth; hence, selection would favor their
reduction and eventual loss. Changes in the shape of the skull also
seem to be correlated with the shift from a planing to a shearing type
of mastication. More efficient shearing action, which depends upon
lateral movement of the jaw, can be developed if the functional
muscles insert farther laterally than is possible in the generalized
type of skull. Therefore, platycephalic specializations involved
lateral expansion of the braincase and mandible. Pronounced lateral
expansion has been developed only in the _gymnurus_ group of species,
suggesting that the dental specializations evolved earlier in the
evolution of the subgenus than did the platycephalic specializations
of the skull, and that the _castanops_ group separated from the
_gymnurus_ group before the common ancestor had developed the more
extreme trends in platycephaly. It is interesting to note that the
subtriangular M3 (Fig. 9G) postulated for the ancestral morphotype and
that characterizes the subgenus _Pappogeomys_ is retained also in the
_gymnurus_ group.
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