Geomyini provides a curious combination of patterns that resemble in
part the Miocene genus _Dikkomys_ and in part the early and middle
Pliocene genus _Pliosaccomys_. 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 _Dikkomys_, and are distinctly different from
those of _Pliosaccomys_ 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 _Pliosaccomys_ 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 _Pliosaccomys_,
rather than _Dikkomys_. In this regard it should be pointed out that
the upper molars of _Dikkomys_ 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 _Dikkomys_ might have become united across the entire surface of
their protomeres as in _Pliosaccomys_. Even so, the two columns of M1
do initially join closer to their mid-points than they do in
_Pliosaccomys_, 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 _Pliosaccomys_ 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 _Pliosaccomys_ and the Geomyini become eventually
united, with wear, into a single column.
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, _Pliosaccomys_, but had not also
evolved the specializations of m1 and m2 that distinguish
_Pliosaccomys_. Therefore, the ancestor of the Geomyini differed from
the _Pliosaccomys_-Thomomyini lineage in its retention, unmodified, of
the primitive patterns in m1 and m2 that characterized the earliest
known Geomyines (_Dikkomys_). The same patterns are preserved in m1
and m2 of its modern descendents, the living Geomyini. In the
_Pliosaccomys_-Thomomyini lineage the pattern of m1 and m2 are
entirely different, as described above.
The earliest record of the Geomyini is the extinct genus _Pliogeomys_
(see Fig. 6) in the latest Hemphillian (middle Pliocene) and earliest
Blancan (late Pliocene). _Pliogeomys_ 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 _Pliogeomys_ 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 _Pliogeomys_ were
in the direct line of evolution. Separation of _Pliogeomys_ 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 _Pliogeomys_ are
narrow (see Fig. 7A) 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.
_Pliogeomys_ is in an intermediate stage in evolution, and was not so
advanced as was the main lineage at the time _Pliogeomys_ died out.
Its structure does provide clues as to phyletic development that took
place in the main lineage.
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 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 _Pliogeomys_, _Zygogeomys_ and
_Geomys_. The innermost groove is weakly developed in _Pliogeomys_,
suggesting that this character was in an intermediate stage of
evolution in the ancestral lineage at the time that _Pliogeomys_ 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.
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 _Pliosaccomys_). Resemblance of
dentitions in certain stages of wear in _Pliosaccomys_ and in
entoptychines led some investigators, for instance, Hibbard
(1953:357), to suggest that _Pliosaccomys_ descended from one of the
less specialized entoptychines, possibly _Grangerimus_ but probably
_Gregorymys_. 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 _Pliosaccomys_ 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 _Pliosaccomys_ 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.
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
_Entoptychus_ of the lower Miocene. In _Entoptychus_, 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 _Entoptychus_ and a sulcated upper incisor, a
single shallow groove usually near the median border of the tooth, is
found in the genus _Gregorymys_ 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.
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 _Entoptychus_ shows a greater degree of fossorial adaptation than
earlier genera of the subfamily. There is no reason to suppose that
the geomyine genus _Dikkomys_, which lived at the same times as the
entoptychines, had acquired any more advanced fossorial adaptations
than had the entoptychines.
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 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,
_loc. cit._), 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, _Pliosaccomys_,
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, _loc. cit._). 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.
The Madro-tertiary climax does not appear as a major flora until the
Miocene, but probably originated earlier. According to Axelroad (_loc.
cit._), 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.
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 _Dikkomys_ that survived
in association with the entoptychids throughout the 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.
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.
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, _Pliosaccomys_ differs but little from
_Dikkomys_. 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.
Plio-Pleistocene radiation of Geomyini
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 Fig. 6); 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: _Zygogeomys_, _Geomys_,
_Orthogeomys_, and _Pappogeomys_.
[Illustration: FIG. 6. Plio-Pleistocene radiation of the Tribe
Geomyini.]
_Morphotype_
The immediate, unknown, ancestor probably lived on the Central Plateau
of México. After the radiation began the ancestors of _Geomys_ and
_Zygogeomys_ extended their ranges northward.
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 _Pliogeomys_, _Zygogeomys_ and _Geomys_; (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.
_Specializations in Genera_
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.
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.)
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.
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.
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 _simultaneously_ 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 _castanops_-group of the
subgenus _Cratogeomys_). 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.
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 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 characterized by only
one pattern, but in one genus (_Pappogeomys_) 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 (Fig. 7D, F, H)
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).
[Illustration: FIG. 7. Molariform dentitions of the Tribe Geomyini.
Drawings illustrating enamel patterns characteristic of
_Pliogeomys_, _Zygogeomys_, and the subgenera of _Orthogeomys_
(_Orthogeomys_, _Heterogeomys_ and _Macrogeomys_). × 5.
A. _Pliogeomys buisi_, No. 29157 (UMMP), holotype, Buis Ranch
(Upper Middle Pliocene), Beaver Co., Oklahoma. Right lower,
p4-m2 (m3 unknown).
B and C. _Zygogeomys trichopus trichopus_, adult female, No. 51971
(FMNH), Mt. Tancítaro, 10,500 ft., Michoacán. Left upper
(B), P4-M3; right lower (C), p4-m3.
D and E. Subgenus _Orthogeomys_. _Orthogeomys grandis guerrerensis_,
adult female, No. 39807 (KU), 1/2 mi. E La Mira, 300 ft.,
Michoacán. Left upper (D), P4-M3; right lower (E), p4-m3.
F and G. Subgenus _Heterogeomys_. _Orthogeomys hispidus hispidus_,
adult female, No. 23975 (KU), 4 km. W Tlapacoyan, 700 ft.,
Veracruz. Left upper (F), P4-M3; right lower (G), p4-m3.
H and I. Subgenus _Macrogeomys_. _Orthogeomys heterodus
cartagoensis_, adult female, No. 60664 (KU), Rancho
Redando, Volcán Lrozá, Prov. San José, Costa Rica. Left
upper (H), P4-M3; right lower (I), p4-m3.
]