Pliosaccomys, on the other hand, represents the terminal stages of a long trend that began with the Dikkomys-like Geomyinae of the early Miocene. In this lineage, the rate of evolution in the dentition and the skull was slow; therefore, the differences between early Miocene (Dikkomys) and middle Pliocene (Pliosaccomys) are not great and the two are united into the tribe Dikkomyini. The Dikkomyini is the ancestral geomyinen trunk from which the modern groups have diverged.
The Pliocene ancestor of Thomomys is unknown but probably resembled Pliosaccomys, with which it may have been a contemporary. Thomomys is the least specialized of the modern Geomyinae, and, consequently, shows the most resemblance to the ancestral tribe. The specializations of Thomomys, however, clearly preclude its reference to the tribe Dikkomyini; therefore, it is set apart in the monotypic tribe Thomomyini. That tribe has not undergone an adaptive radiation comparable to that of the tribe Geomyini or that of the Entoptychinae in the early Miocene. Here, for the first time, Thomomys is set apart in classification from the other living pocket gophers.
Merriam's genera Orthogeomys, Heterogeomys, and Macrogeomys are closely related. Each of these taxa is retained as a subgenus of a single genus, Orthogeomys. Some species of Macrogeomys seem to be more closely allied to the subgenus Orthogeomys and others to the subgenus Heterogeomys. A revision of the genus is needed; it might show that the currently recognized subgenera are artificial, and that a different arrangement of the species would more clearly express their evolutionary relationships. The subgenus Heterogeomys seems to be the most nearly uniform of the subgenera, and it is the least specialized. Radiation within the genus may have begun relatively recently, but the many special adaptations for tropical environments suggest that the genus has been in the Neotropical Zone a long time. Therefore, discovery of an early dichotomy from the common ancestral stock of the tribe would come as no surprise.
Nerterogeomys Gazin here is arranged as a junior synonym of Zygogeomys. Both are less specialized than any of the other Geomyini, except Pliogeomys. The single living species (Zygogeomys tricopus) is obviously a relic. Its range is small. The two subspecies differ only in minor features. The living species does have a few unique characteristics, only to be expected in the surviving species of a long phyletic lineage. Some of these are specializations. Otherwise, Zygogeomys and Nerterogeomys are closely related and the latter is best placed as a synonym of the former. Both are admittedly closely related to Geomys. Zygogeomys and Geomys share several characters, particularly primitive ones; there is considerable parallelism, especially marked in Irvingtonian species of Geomys. Nevertheless, Geomys is more specialized, particularly in the dentition, and it has developed some Pappogeomys-like specializations. Zygogeomys has retained more of the primitive characters of the tribe. A strong case could be made for recognizing only one genus, Geomys, containing Zygogeomys as one of two subgenera. Nevertheless, the characters separating Zygogeomys and Geomys are of considerable importance and I consider the two kinds to be distinct genera.
The species of Geomys, both living and extinct, form a distinct and well-marked group. The genus is less primitive in most respects than Zygogeomys and Orthogeomys and it is less specialized than Pappogeomys, excluding the ancestral stock (subgenus Pappogeomys). Some specimens of species of Irvingtonian age (Geomys tobinensis and Geomys garbanii, especially the former) retain primitive enamel plates as does Zygogeomys; but this is true of only a small percentage of the individuals. Also the adult dental pattern developed somewhat later in ontogeny in these middle Pleistocene species of Geomys than in either Recent or late Pliocene and early Pleistocene representatives (Geomys paenebursarius, Geomys quinni) of the genus. Whether these features represent a stage in the evolution of the late Pleistocene and Recent species or a terminal stage in members of a sterile and primitive branch of the main line of evolution of Geomys is uncertain. At present I favor the latter explanation, and view G. paenebursarius and G. quinni as early progressive species that evolved dental specializations that were maintained in the main line of phylogeny.
Hibbard proposed the generic name Parageomys (1944:55), but later regarded it as a subgenus of Geomys (1956:182) that includes those species retaining continuous enamel bands until relatively late in ontogeny; no other differences have been noted. When the early phylogeny of Geomys is better understood, Parageomys may serve as a subgeneric taxon in which the primitive species of Geomys can be grouped, but as of now Parageomys is arranged as a synonym of Geomys.
Pappogeomys and Cratogeomys also form a natural group. Their close relationship is best reflected in formal taxonomy by including them in the same genus. Their dissimilarities are of the sort that separate a primitive ancestral lineage from a divergent and progressively more specialized assemblage. The fossil record is inadequate, and I can only speculate that Cratogeomys diverged from primitive Pappogeomys-stock in the earlier Pleistocene, at least before the end of the Irvingtonian. Cratogeomys probably originated on the Mexican Plateau and probably underwent its subsequent evolution there. The living species of the subgenus Pappogeomys are evidently relics of the ancestral stock of the genus. Hooper (1946:397), I think correctly, considered Platygeomys as congeneric with Cratogeomys, although the highest degree of specialization of the genus is attained in those species formerly classed in the genus Platygeomys. Even so, in my opinion, the differences are insufficient to warrant even subgeneric recognition.