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 (_Aplodontia_). Inasmuch as the
morphology and taxonomy of the entoptychines were discussed in detail
by Cope (1884) and reviewed later by Wood (_loc. cit._), there is no
need to recount the details here. According to Wood (_op. cit._,
27-28), _Pleurolicus_ 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, _Pleurolicus_ is less specialized
than the other genera and occurs in deposits of both the Great Plains
and the Pacific Coast. _Gregorymys_, also little specialized, occurred
only on the Great Plains. The more specialized genera, _Grangerimus_
and _Entoptycus_, evidently appeared somewhat later than _Pleurolicus_
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), _Grangerimus_ is known only from the
Pacific coast. _Entoptycus_ was restricted to the Pacific Coast (John
Day fauna).
_Entoptycus_ 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 _Entoptycus_. Rootless, ever-growing cheek teeth,
discontinuous enamel patterns, and monoprismatic molars were not
evolved in the subfamily Geomyinae until the late Pliocene.
Phyletic Trends in Subfamily Geomyinae
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 Figure 3. 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.
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.
_Dikkomys_, the earliest known genus of the tribe Dikkomyini, can be
taken as a starting point of evolution for the subfamily Geomyinae.
The Pliocene genus _Pliosaccomys_ is the only other known geomyine
having primitive features closely resembling those of _Dikkomys_. The
relatively close but previously unrecognized relationship between
_Dikkomys_ and _Pliosaccomys_ can be understood when patterns of wear
on the occlusal surfaces of the cheek teeth are taken into account. It
appears that _Pliosaccomys_ descended from _Dikkomys_-like stock, if
not _Dikkomys_ itself. Although _Dikkomys_ is towards the beginning of
this phyletic sequence and _Pliosaccomys_ towards the end of the
sequence, the primitive features shared by the two provide a
generalized morphotype for the subfamily Geomyinae.
In the molariform dentition, an almost complete series of stages of
wear in _Pliosaccomys_ has been preserved, and those of _Dikkomys_ can
be reconstructed with reasonable accuracy from those that are known
(see Fig. 4):
(1) In the initial stage of wear in _Dikkomys_ the anterior and
posterior columns are separated by an intervening valley (Fig. 4A),
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 _Dikkomys_, but
does occur in both geomyines and entoptychines in all stages of
evolution, and it must have also occurred in _Dikkomys_ in order for
the next two stages, which are preserved, to have developed.
(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 (Fig. 4B), 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.
[Illustration: FIG. 3. 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, _Pleurolicus_, _Gregorymys_ and _Dikkomys_ are
illustrated as ranging into the Hemingfordian, rather than being
confined to the Arikareean (see MacDonald, 1963, and Black, 1961).]
(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 (Fig. 4C). 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.
(4) In the final stage of wear (Fig. 4D), 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 Fig. 4C
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.
The occlusal pattern, at least in M1 and M2, in the final stages of
wear in _Dikkomys_ resembles that in the subfamily Entoptychinae, but
the U-pattern develops on only the first and probably the second molar
in _Dikkomys_ 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 _Dikkomys_ until the
Hemingfordian (_D. woodi_), upper Rosebud, and specimens of _D.
matthewi_ 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.
Essentially the same patterns of wear characterize the genus
_Pliosaccomys_, 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:
(1) The first phases of wear produced the pattern (Fig. 4E and I)
described for _Dikkomys_ in the previous account (Fig 4A).
(2) A small additional amount of wear produced the 2nd stage (Fig. 4F
and J) 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 _Dikkomys_. 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 _Pliosaccomys_
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.
(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
(Fig. 4G and K). The inflection appears to have persisted in the upper
teeth (Fig. 4H), but evidently with slight wear, disappeared in the
lower teeth (Fig. 4L). 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 _Pliosaccomys_. In _Dikkomys_ the two columns
never united into a single column, and a bilophodont occlusal pattern
persisted throughout life.
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 _Dikkomys_ to _Pliosaccomys_, 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 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 _Thomomys_ than it is to that of _Dikkomys_. Other
specializations in the dentition of _Pliosaccomys_, especially in m1
and m2 where the H-pattern has been completely eliminated from the
sequence of wear, are too far advanced for _Pliosaccomys_ 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 _Dikkomys_. 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 _Pliosaccomys_ probably were closely allied otherwise, and
both probably had attained the highly specialized fossorial
adaptations characterizing all modern pocket gophers, before the
divergence of _Pliosaccomys_ and the Geomyini took place.
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 Fig. 3). One, the most primitive of the two, gave rise to the
Thomomyini lineage that eventually evolved into _Thomomys_.
_Pliosaccomys_ 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 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
_Thomomys bulbivorus_); (6) forefoot small, less modified for digging
than in the Geomyini.
[Illustration: FIG. 4. Drawings of the molariform dentitions of
_Dikkomys_ and _Pliosaccomys_ (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.
A-D. _Dikkomys woodi_, right lower tooth-row, including p4-m3.
Patterns based on No. P26284 (FMNH) from Upper Rosebud (Middle
Miocene), Shannon Co., South Dakota (B above).
E-H. _Pliosaccomys dubius_, 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.
I-L. _Pliosaccomys dubius_, 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.
]
The lineage of the Thomomyini is essentially rectilinear and without
the major branching seen in the tribe Geomyini. The one genus,
_Thomomys_, 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 (Fig. 5, K and L) 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
Fig. 5L) 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.
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
Fig. 5, I and J), 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 bilophodont patterns
appear in both the Dikkomyini and Geomyini before the monoprismatic
pattern is developed.
[Illustration: FIG. 5. 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.
A and E. _Geomys bursarius majusculus_, No. 2948 (KU), Douglas Co.,
Kansas. Right upper (A) including DP4-M3; lower left (E)
including dp4-m3.
B and F. _Pappogeomys bulleri burti_, 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).
C and G. _Pappogeomys bulleri albinasus_, No. 31044 (KU), 10 mi. S
and 8 mi. W Guadalajara, Jalisco. Right upper (C) including
P4-M3; right lower (G) including p4-m3.
D and H. _Pappogeomys bulleri albinasus_, 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.
I and J. _Thomomys talpoides bridgeri_, No. 6865 (KU), 2 mi. up Mink
Creek, Pocatella, Bannock Co., Idaho. Left upper (I),
DP4-M3; left lower (J), dp4-m3.
K and L. _Thomomys talpoides fossor_, No. 13205 (KU), Wasson Ranch,
3 mi. E Creede, Mineral Co., Colorado. Right lower (K),
p4-m3; left upper (L), P4-M3.
]
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 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
Fig. 5, A and E), 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.
In the lower molars, the pattern characteristic of _Dikkomys_ (Fig.
4C) is preserved without significant modification, as illustrated in
an immature specimen of _Geomys_ (see Fig. 5E). 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 _Dikkomys_.
After the two columns have become united at their mid-points, a
secondary union is formed at the edge of their protomeres, thus
enclosing the enamel fossette as illustrated in Figure 5E (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 Fig. 5E). 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.
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 Fig. 5B), 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 _Pliosaccomys_ (see Fig.
4H). 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 Fig. 5B),
therefore forming a U-pattern similar to that developed in Ml and M2
of _Pliosaccomys_. 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 Fig. 5C). 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 Fig. 5D). M3's of _Dikkomys_ and
_Pliosaccomys_ 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 _Dikkomys_.