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,GregorymysandDikkomysare illustrated as ranging into the Hemingfordian, rather than being confined to the Arikareean (see MacDonald, 1963, and Black, 1961).
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,GregorymysandDikkomysare 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 inFig. 4Cwould 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 inDikkomysresembles that in the subfamily Entoptychinae, but the U-pattern develops on only the first and probably the second molar inDikkomysand 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 ofDikkomysuntil the Hemingfordian (D. woodi), upper Rosebud, and specimens ofD. matthewifrom 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 genusPliosaccomys, 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 (p. 4Eand I) described forDikkomysin 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 inDikkomys. 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 thePliosaccomyslineage 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 inPliosaccomys. InDikkomysthe 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 fromDikkomystoPliosaccomys, 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 retentionof 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 ofThomomysthan it is to that ofDikkomys. Other specializations in the dentition ofPliosaccomys, especially in m1 and m2 where the H-pattern has been completely eliminated from the sequence of wear, are too far advanced forPliosaccomysto 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 inDikkomys. 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 ofPliosaccomysprobably were closely allied otherwise, and both probably had attained the highly specialized fossorial adaptations characterizing all modern pocket gophers, before the divergence ofPliosaccomysand 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 (seeFig. 3). One, the most primitive of the two, gave rise to the Thomomyini lineage that eventually evolved intoThomomys.Pliosaccomysis 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 slightrudimentary 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 inThomomys bulbivorus); (6) forefoot small, less modified for digging than in the Geomyini.
Fig. 4.Drawings of the molariform dentitions ofDikkomysandPliosaccomys(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.
Fig. 4.Drawings of the molariform dentitions ofDikkomysandPliosaccomys(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.
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 (seeFig. 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 (seeFig. 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 ofbilophodont patterns appear in both the Dikkomyini and Geomyini before the monoprismatic pattern is developed.
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.
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.
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 thecrown 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 inFig. 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 ofDikkomys(Fig. 4C) is preserved without significant modification, as illustrated in an immature specimen ofGeomys(seeFig. 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 genusDikkomys. After the two columns have become united at their mid-points, a secondary union is formed at the edge of theirprotomeres, thus enclosing the enamel fossette as illustrated inFigure 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 inFig. 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 (seeFig. 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 genusPliosaccomys(seeFig. 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 (seeFig. 5B), therefore forming a U-pattern similar to that developed in Ml and M2 ofPliosaccomys. 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 (seeFig. 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 (seeFig. 5D). M3's ofDikkomysandPliosaccomysare 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 genusDikkomys.
Judging from the pre-final stages of wear, the dentition of theGeomyini provides a curious combination of patterns that resemble in part the Miocene genusDikkomysand in part the early and middle Pliocene genusPliosaccomys. 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 ofDikkomys, and are distinctly different from those ofPliosaccomyswhere 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 ofPliosaccomysand 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 ofPliosaccomys, rather thanDikkomys. In this regard it should be pointed out that the upper molars ofDikkomysare 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 ofDikkomysmight have become united across the entire surface of their protomeres as inPliosaccomys. Even so, the two columns of M1 do initially join closer to their mid-points than they do inPliosaccomys, 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 inPliosaccomysor 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 bothPliosaccomysand 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 distinguishPliosaccomys. Therefore, the ancestor of the Geomyini differed from thePliosaccomys-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 thePliosaccomys-Thomomyini lineage the pattern of m1 and m2 are entirely different, as described above.
The earliest record of the Geomyini is the extinct genusPliogeomys(seeFig. 6) in the latest Hemphillian (middle Pliocene) and earliest Blancan (late Pliocene).Pliogeomysis 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 inPliogeomysare 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 ifPliogeomyswere in the direct line of evolution. Separation ofPliogeomysfrom 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 ofPliogeomysare narrow (seeFig. 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.Pliogeomysis in an intermediate stage in evolution, and was not so advanced as was the main lineage at the timePliogeomysdied 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 anteriorand 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 inPliogeomys,ZygogeomysandGeomys. The innermost groove is weakly developed inPliogeomys, suggesting that this character was in an intermediate stage of evolution in the ancestral lineage at the time thatPliogeomyssplit 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 (inPliosaccomys). Resemblance of dentitions in certain stages of wear inPliosaccomysand in entoptychines led some investigators, for instance, Hibbard (1953:357), to suggest thatPliosaccomysdescended from one of the less specialized entoptychines, possiblyGrangerimusbut probablyGregorymys. 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 inPliosaccomysis 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 toPliosaccomyspassed 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 genusEntoptychusof the lower Miocene. InEntoptychus, 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 genusEntoptychusand a sulcated upper incisor, a single shallow groove usually near the median border of the tooth, is found in the genusGregorymysof 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 ofEntoptychusshows a greater degree of fossorial adaptation than earlier genera of the subfamily. There is no reason to suppose that the geomyine genusDikkomys, 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 bringabout 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 forDikkomysthat survived in association with the entoptychids throughout theearly 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,Pliosaccomysdiffers but little fromDikkomys. 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 (seeFig. 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, andPappogeomys.
Fig. 6.Plio-Pleistocene radiation of the Tribe Geomyini.
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 ofGeomysandZygogeomysextended 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 inPliogeomys,ZygogeomysandGeomys; (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 bladesimultaneouslymust 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 (thecastanops-group of the subgenusCratogeomys). 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 shapesof 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 ischaracterized 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).
Fig. 7.Molariform dentitions of the Tribe Geomyini. Drawings illustrating enamel patterns characteristic ofPliogeomys,Zygogeomys, and the subgenera ofOrthogeomys(Orthogeomys,HeterogeomysandMacrogeomys). × 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. SubgenusOrthogeomys.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. SubgenusHeterogeomys.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. SubgenusMacrogeomys.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.
Fig. 7.Molariform dentitions of the Tribe Geomyini. Drawings illustrating enamel patterns characteristic ofPliogeomys,Zygogeomys, and the subgenera ofOrthogeomys(Orthogeomys,HeterogeomysandMacrogeomys). × 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. SubgenusOrthogeomys.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. SubgenusHeterogeomys.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. SubgenusMacrogeomys.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.
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. SubgenusOrthogeomys.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. SubgenusHeterogeomys.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. SubgenusMacrogeomys.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.
The subcordate form is characterized by pronounced anteroposterior 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 thesechanges, 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 subgenusPappogeomysor the smaller subspecies ofGeomys bursarius. The recorded range of variation in condylobasal length is 36.1 to 45.5 inPappogeomys 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 (subgenusCratogeomys) and 75.0 (subgenusOrthogeomys). These are more than twice the minima observed inPappogeomys 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 ofP4, 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 inZygogeomysare 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 (seeFig. 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 ofOrthogeomys(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 thanZygogeomys, 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 (seeFig. 9A). All of these primitive features are shared withZygogeomys. In addition, three other trends, or specializations, in evolution characterize the phyletic development ofGeomys. 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 ofGeomys, 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 speciesG. 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 ofGeomys, for exampleG. 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 ofGeomysare 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 ofGeomysare depicted inFigure 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 ofG. quinniis 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 ofGeomysfrom the Rexroad fauna (late Pliocene) and Saunders fauna (latest Aftonian) may also be on the main line of evolution. Surprisingly,Geomys tobinensisandGeomys garbaniiof later Irvingtonian provincial age are less specialized than eitherGeomys quinniorGeomys paenebursarius. It is likely thatG. tobinensisand the unnamed species from the Dixon are closer to the main line of descent thanG. paenebursariussuggesting that the direct ancestral lineage of the living species ofGeomyswas more conservative and less specialized thanGeomys paenebursariusof the Lower Pleistocene.Geomys quinniandG. paenebursariusseem to have acquired specialized dental features in the early Pleistocene.Geomys quinniwas successful on the Great Plains, and persisted into the late Blancan. The main line may be represented in the early Pleistocene byGeomys paenebursariusfrom the Hancock formation of the Texas Trans-Pecos. The structure ofG. paenebursariusindicates that it is in or close to the main line of descent, and probably evolved from one of the more primitive late Pliocene species ofGeomysfrom the Rexroad fauna.