Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae Part 10

_Pappogeomys fumosus_ (Merriam, 1892). Proc. Biol. Soc. Was.h.i.+ngton, 7:165, September 29. Type from 3 mi. W Colima, Colima.

_Pappogeomys gymnurus_ (Merriam, 1892). Proc. Biol. Soc. Was.h.i.+ngton, 7:166, September 29. Type from Zapotlan (Ciudad Guzman), Jalisco.

_Pappogeomys neglectus_ (Merriam, 1902). Proc. Biol. Soc.

Was.h.i.+ngton, 15:68, March 22. Type from Cerro de la Calentura, about 8 mi. NW Pinal de Amoles, Queretaro.

_Pappogeomys tylorhinus_ (Merriam, 1895). N. Amer. Fauna, 8:167, January 31. Type from Tula, Hidalgo.

_Pappogeomys zinseri_ (Goldman, 1939). Jour. Mamm., 20:91, February 15. Type from Lagos, Jalisco.

PHYLOGENY OF THE GEOMYIDAE

The fossil record of the Geomyidae provides a sequence of morphotypes, each representing a stage in the phyletic development of the family.

Most of the preserved specimens probably represent the stufenreihe rather than the ahnenreihe, as Simpson (1953:219-220) points out. Even so, the stufenreihe closely approximates the general trend of evolution, and the level of structural organization in the different stages of phyletic development may be ascertained. The actual ancestral series of most lineages probably will remain unknown, but hopefully some of the existing gaps will be filled by future discoveries. From the established record, several clearly defined lineages can be distinguished; in fact the sequence of origin, pattern of evolution, and specializations, of the princ.i.p.al lineages are reasonably well expressed.

Primitive Morphotype

In the earliest known geomyids from the Upper Oligocene and Lower Miocene, the premolars and molars are biprismatic and bilophodont. In rodents, this is itself a specialized pattern, and is thought to have evolved from a more primitive s.e.xt.i.tuberculate prototype by the union of individual cusps, and probably also cuspules, forming the two transverse enamel lophs. The primitive, common ancestor of the Geomyidae and Heteromyidae with s.e.xt.i.tuberculate teeth in the early Tertiary is unknown.

As soon as geomyids attained the early bilophodont stage of evolution, the basic morphological structure of the family was established. The family probably first became clearly distinguished from other Geomyoidea at this stage. In the early bilophodont stages of evolution, owing to the relatively deep valley between them, the two columns probably failed to unite in the normal cycle of wear, as they do in all later geomyids. _Griphomys_ described by Wilson (1940:93) from the late Eocene of California, has a bilophate pattern in which the anterior and posterior lophs are separated by a persistent transverse valley. The occlusal pattern of _Griphomys_ closely resembles a stage through which the ancestors of the early Miocene geomyids must have pa.s.sed in their pre-Miocene evolution, as Wilson suggests (1949:115-116). Although he (1940:95; 1949:110-118) tentatively referred _Griphomys_ to the superfamily Geomyoidea and Simpson (1945:80) went so far as to refer it to the family Geomyidae, with a notation of _incertae sedis_, its exact relations.h.i.+p to the pocket gophers is uncertain. However, the structure of the molariform dent.i.tion of _Griphomys_ does not exclude it from the phyletic ancestry of the Geomyidae. In subsequent stages of evolution the anterior and posterior columns become united. Thereby part of the valley floor between the transverse prisms was progressively elevated, to the stage where attrition on the occlusal surface would unite the two columns. On the unworn enamel cap of living geomyids the two transverse enamel folds are separated by a shallow but well defined valley, briefly reflecting the ancient ancestral pattern.

Union of the lophs may have been either at the mid-points of the two columns or at the edge of their protomeres. [A protomere is the half of a tooth containing the protocone or protoconid--lingual side of upper tooth and l.a.b.i.al side of lower tooth. The paramere is the opposite half of a given tooth--l.a.b.i.al side of upper tooth and lingual side in lower tooth. See Miller and Gidley, 1918:434.] Union of the columns at the mid-points would have produced the figure-8 occlusal pattern (or H-pattern), which is characteristic of the early Miocene Geomyinae (_Dikkomys_). Union of the two columns at the protomeres would have produced the U-shaped pattern of the Entoptychinae, which also occurred in the early Miocene and were contemporary with the earliest Geomyinae. Since pre-Miocene geomyids are unknown, the actual phyletic development of the dent.i.tion is a matter of speculation. Probably the development of the two divergent lineages, one leading to the Entoptychinae and the other to the subfamily Geomyinae, occurred in the Oligocene (as depicted in Fig.

3). Of the two lineages, the subfamily Geomyinae, in my view, is the more primitive and less specialized. Support for this view is furnished by a reconstruction of the pattern of occlusal wear in _Dikkomys_ and _Pliosaccomys_, especially on the first and second molars.

In _Dikkomys_, the anterior and posterior column first unite near their mid-points in the first stages of wear thus producing a figure-8 shaped (H-shaped) occlusal pattern in the premolar and all three molars. Evidently in the first two upper molars, the columns unite closer to their lingual margins than their mid-points, but at any rate both outer and inner re-entrant folds are evident at this stage of wear. With continued attrition on m1 and m2 of _Dikkomys_, the anterior and posterior columns secondarily unite at the edge of their l.a.b.i.al margins thus enclosing a fossette of enamel in the l.a.b.i.al half of the tooth. The lateral coalescence at the ends of the protomeres occurs because of the shallow vertical depth of the l.a.b.i.al re-entrant fold, and the fossette itself does not reach the base of the crown and with continued wear it too would disappear, but not until the last stages of wear, at least in _Dikkomys matthewi_. The lingual re-entrant fold is deep, and therefore, persistent through all stages of wear. Although the amount of wear required for its effacement would be great, the occlusal configuration of the first and second lower molars in _Dikkomys_ could be eventually ground down to a U-pattern as in the entoptychids. Only one upper molar of _Dikkomys_, the first, has been recovered (see Wood, 1936:23, fig. 32B). Although the tooth is in an early stage of wear, the lingual valley is minute. Less attrition than required in m1 and m2 would progressively reduce the lingual fold until it too would essentially form a U-pattern, perhaps retaining a slight lingual inflection. Hence, the first upper molar becomes a mirror image of the first lower molar, and the second upper molar probably had the same pattern as the first (at least it does so in _Pliosaccomys_). Both of the lateral re-entrant folds of the premolar are deep vertically, and consequently would not disappear with occlusal wear. Therefore, the H-pattern of the premolars is retained throughout life.

The m3 (M3 unknown for _Dikkomys_ or _Pliosaccomys_) also has deep lateral folds; hence, it too retains the H-pattern in all stages of attrition, although the isthmus between the two prisms may become wider in the final phases of wear (as it does in _Pliosaccomys_).

In _Pliosaccomys_, the stages of wear are essentially the same as those described for _Dikkomys_, except that the anterior and posterior loph of the first and second molars tend to unite closer to one side of the tooth, lingual side in upper molars and l.a.b.i.al in lower. Only a slight inflection of the re-entrant fold is evident on the side of union, and the inflection disappears in the first phases of wear as the columns unite. Concomitant with the lateral s.h.i.+ft in the initial point of coalescence of the transverse lophs, the occlusal penetration of the re-entrant fold from the opposite side increases in horizontal depth, and the fold extends medially more than half way across the occlusal surface, thus forming a pattern essentially like that of the entoptychids. The U-pattern in _Pliosaccomys_ appears in the initial stages of wear without going through an earlier H-pattern as is the case in its Miocene ancestors of the genus _Dikkomys_, unless the minute inflection is considered as indicative of that stage. The two columns of the premolar and m3 are joined near their mid-points as in _Dikkomys_; therefore, they retain their primitive H-pattern, a feature unique to the Geomyinae.

The evolutionary trend toward an ontogenetically earlier U-pattern in the first two molars in the primitive lineage of the Geomyinae suggests that the U-pattern characteristic of the Entoptychinae was simply an earlier tendency toward the same specialization that occurred later in the subfamily Geomyinae. If so, early entoptychines would have been characterized by an H-pattern in the first stages of attrition, like _Dikkomys_, and later developed union at the edge of the protomeres. However, in the entoptychines, all the molariform dent.i.tion, and not merely the first and second molar, became specialized; consequently the U-pattern was produced on the occlusal surfaces of each of the cheek teeth. As in _Pliosaccomys_, the transitional phase, in which the two columns were united at their mid-points, was eventually eliminated from the pattern of wear and only the U-pattern, that now appeared in the initial stages of wear, was retained. In the entoptychines of the early Miocene there is no suggestion of the H-pattern that characterizes the Geomyinae, except in the position of the cusps before wear in the lower molars of _Pleurolicus sulcifrons_, which, according to Wood (1936:6), suggests the H-pattern. In earlier unknown Oligocene stages of evolution, the prisms possibly united first at their mid-points, and the columns may have joined at the side of the tooth only in the terminal stages of wear. The U-pattern of pre-Miocene entoptychines, therefore, may have become the dominant occlusal pattern only in the later stages of phyletic development.

According to the recently expressed views of several paleontologists, the Entoptychinae const.i.tute the primitive lineage of the family and the early Geomyinae const.i.tute a specialized offshoot of the entoptychine ancestral a.s.semblage. The structure of the Entoptychinae, especially of the less advanced genera, closely approximates that of the hypothetical primitive morphotype. But, according to my view, the subfamily Geomyinae const.i.tutes the ancestral a.s.semblage and its structure is essentially that of the primitive morphotype of the family. At any rate the structure of the early geomyines more closely approximates the structure of the ancestral stock than the more divergent entoptychines. Therefore, the genus _Dikkomys_ of the early Miocene, the first known geomyine, is considered to be a generalized geomyid, and, although it is a contemporary of the more specialized entoptychid a.s.semblage, is considered to be more closely allied to the ancestral stock.

The entoptychines were the dominant and most highly differentiated geomyids of the early and middle Miocene. Nevertheless, they became extinct in the middle Miocene, and the geomyines of that time survived and later gave rise to the modern pocket gophers. Therefore, the early history of the family Geomyidae is characterized by an early radiation and trend toward specialization, followed by survival of the less specialized Geomyinae and extinction of the more specialized Entoptychinae.

Entoptychid Radiation

The most abundant geomyids of the early and middle Miocene, the Entoptychinae, consisted of at least 24 species (see Wood, 1936:4-25) cla.s.sified in four genera: _Pleurolicus_, _Gregorymys_, _Grangerimus_, and _Entoptychus_. The genera were essentially contemporaneous (see Figure 3). Even so, the subfamily was morphologically varied, pointing to an earlier origin in the Oligocene (actually a part of the John Day Fauna, including _Pleurolicus_ may be correlated with late Oligocene Whitneyian age) followed by a relatively rapid radiation including all four genera in the early Miocene. Two genera, _Pleurolicus_ and _Gregorymys_, continued into the Middle Miocene (Hemingfordian). This divergence, specialization, and subsequent radiation suggest that the entoptychines evolved into a new major adaptive zone, in the sense described by Simpson (1945:199-206).

The radiation is correlated geographically and temporally with the southward retreat of the Neotropical flora of the Tertiary from the western United States and southward movement of the Arctic flora of the Tertiary (see Axlerod, 1950; Berry, 1937:31-46; Chaney, 1947:139-148; and Kendeigh, 1961:280-283). In the early Tertiary the Neotropical-tertiary geoflora occurred northward to at least 49 lat.i.tude in western North America, and the boreal Arctic-tertiary flora was restricted to a circ.u.mpolar zone. The southward and eastward s.h.i.+ft of the Neotropical-tertiary flora, a.s.sociated with the drying and chilling of the continent, began in the middle or late Oligocene and was concurrent with the divergence and radiation of the Entoptychinae. Beginning in late Oligocene and continuing at least into middle Miocene, most of the region in which the entoptychines occurred was occupied by the Arcto-tertiary geoflora of which the temperate forest division contributed the dominate plant a.s.sociations.

The maples, chestnuts, dogwoods, beeches, walnuts, oaks, elms, birches, and sycamores of that flora were the forerunners of today's eastern deciduous forest. It is my view that the entoptychines became adapted to the conditions of this paleoecological environment and radiated rapidly in the Arikareean when the major change occurred in climax vegetation. The ancestral stock of the Geomyinae was not so successful in the Arcto-tertiary climax, and most of it probably was displaced southward along with the tropical flora.

The skeleton in the entoptychines is not so strongly fossorial as in 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 p.r.o.nounced 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 l.a.b.i.al 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, Th.o.m.omyini, and Geomyini (for full discussion of cla.s.sification, see previous account). The phylogeny proposed by me is ill.u.s.trated 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 Th.o.m.omyini and Geomyini are considerably more specialized than the ancestral Dikkomyini from which they evolved.

The Geomyini are clearly more specialized than the Th.o.m.omyini, suggesting closer affinity between the Th.o.m.omyini and the Dikkomyini than between the Geomyini and the Dikkomyini. The specializations in the dent.i.tion and the a.s.sociated changes in the skull of the Th.o.m.omyini 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 relations.h.i.+p 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 dent.i.tion, 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.

[Ill.u.s.tration: FIG. 3. Diagram depicting geologic range and probable phyletic relations.h.i.+ps 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 relations.h.i.+ps 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 ill.u.s.trated 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 (l.a.b.i.al 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 l.a.b.i.al 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 l.a.b.i.al 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 l.a.b.i.al 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 dent.i.tions, 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 pa.s.sed 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 s.h.i.+fted 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 l.a.b.i.al 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 ill.u.s.trated 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 _Th.o.m.omys_ than it is to that of _Dikkomys_. Other specializations in the dent.i.tion 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, Th.o.m.omyini and Geomyini (see Fig. 3). One, the most primitive of the two, gave rise to the Th.o.m.omyini lineage that eventually evolved into _Th.o.m.omys_.

_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 Th.o.m.omyini are less advanced than in the other specialized lineage (tribe Geomyini). Primitive traits retained in the tribe Th.o.m.omyini (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 _Th.o.m.omys bulbivorus_); (6) forefoot small, less modified for digging than in the Geomyini.

[Ill.u.s.tration: FIG. 4. Drawings of the molariform dent.i.tions 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 dent.i.tions 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).