Fig. 8. Tentative arrangement of species of the genus Geomys, depicting phylogenetic trends and probable relationships within the genus.
Isolated teeth, to which the name Geomys bisulcatus probably applies, from Illinoian deposits on the Great Plains, show that the dentition characteristic of the living Geomys had been developed by that time. Actually, the Illinoian material is too fragmentary to show clearly its taxonomic or phyletic affinities with the species of the later Pleistocene. Even so, the two main stocks of living Geomys, G. bursarius and G. pinetis, had certainly been differentiated by Sangamon time. The other living species evidently evolved from one or the other of these two stocks in a period of isolation from the main population, probably in either the Wisconsin or post-Wisconsin. For example, Geomys arenarius clearly differentiated from populations of Geomys bursarius that were isolated by the eastward retreat of the main population from the southwestern United States as that region became more arid in the post-Wisconsin.
In review, it seems that the Recent species, represented basically by bursarius and pinetis, evolved from Illinoian species (Geomys bisulcatus?), which descended in turn from the more primitive species of the early Pleistocene, possibly Geomys paenebursarius or possibly from descendants of the Saunders species. Actually the Saunders species may prove to be Geomys paenebursarius. At any rate, three trends that took place during the Pleistocene stage of evolution, in the direction of the modern species, were an increase in size, progressive loss of the posterior enamel plate on P4, and a decrease in the vertical depth of the enamel cap as a result of which the dentine is reached in the initial phases of attrition on the tooth of a juvenile. Geomys garbanii, occurring at the periphery of the range of the genus, is regarded as a sterile offshoot of the primitive tobinensis-line of evolution.
This is one of the more specialized genera of the Geomyini. Save for one record in the late Pleistocene (Orthogeomys onerosus), there is no fossil history of the genus upon which to reconstruct its phylogeny; therefore, its phyletic development must be estimated by comparing it and the primitive morphotype of the tribe. Results of that comparison suggest that Orthogeomys has closer affinities with Zygogeomys than with any of the other genera, and that Orthogeomys may have originated in an early dichotomy of primitive Zygogeomys stock instead of descending from the ancestral stock of the tribe. Except for the unisulcate incisors and the longer posterior loph on the third upper molars, the teeth of the two genera do not differ significantly. As in Zygogeomys, the enamel blade on the posterior wall of P4 has been reduced to a short plate restricted to the lingual third of the tooth (see [Fig. 7F and H]). In Orthogeomys, the trend in reduction of enamel is carried to its extreme only in the subgenus Orthogeomys, where this plate has been completely lost in most taxa (see [Fig. 7D]). The most significant trends in Orthogeomys, and the principal basis for recognizing the genus, are the dolichocephalic specializations of the skull, as described elsewhere, and the adaptive traits that have equipped the genus for living in tropical environments. The dolichocephalic features are more sharply defined in the subgenera Orthogeomys and Macrogeomys, and are less developed in the subgenus Heterogeomys. Aside from the general dolichocephalic specializations, trends in Orthogeomys include: Increase in size; loss of the median one of the two grooves on the anterior face of the upper incisor in the ancestral stock; increase in the anteroposterior length of each of the cheek teeth, as well as the aforementioned elongation of the posterior loph of M3; compression of the lateral angles of the premolars; and the remarkable increase in the size of the rostrum.
The genus Pappogeomys, as it is conceived of in this study, is comprised of two subgenera; one, Pappogeomys, is generalized and primitive, and the other, Cratogeomys, is specialized, and includes the most highly specialized of the modern pocket gophers. The subgenus Pappogeomys is regarded as the ancestral lineage, and the subgenus Cratogeomys is regarded as an early offshoot, probably in the early Pleistocene, that became progressively more specialized in the course of its subsequent evolution. In the same period of time, the subgenus Pappogeomys changed little. It is known only from late Pliocene fragments and from the living species. The ancestral morphotype is preserved in Pappogeomys. Primitive characters are: (1) Small size; (2) skull generalized and smoothly rounded; (3) temporal ridges separate (not uniting into a sagittal crest); (4) enamel plates retained on both anterior and posterior walls of M1 and M2; (5) M3 bilophate, its posterior loph short. Basic specializations are few and include loss of the inner groove from the anterior face of the upper incisor; anteroposterior compression of the lateral re-entrant folds of the premolars; and loss of enamel from the posterior wall of P4. All three features have been perpetuated in the advanced subgenus Cratogeomys, suggesting that they were already developed in the early evolution of the subgenus Pappogeomys before Cratogeomys diverged. Agreement with Geomys is demonstrated by the lack of enamel on the posterior wall of P4 (see [Fig. 9]) and by retention of the posterior enamel plate on M1 and M2. In Pappogeomys (Pappogeomys) alcorni the enamel from the posterior face of M1 has been lost from all but the lingual fourth or so of the posterior wall ([Fig. 9E]). Reduction of enamel in M1 provides an example of parallelism with the more advanced subgenus Cratogeomys, discussed below.
There is no record as yet of the early evolution of the subgenus Cratogeomys. The features that characterize the subgenus were already well developed in the first known fossils which are from Wisconsin deposits of the late Pleistocene. Cratogeomys is not a homogenous assemblage; instead it is composed of two groups of living species, the generalized castanops group and the specialized gymnurus group. The castanops group may be survivors of the ancestral lineage that diverged in two different stages in the phyletic development of the main line. Even so, the castanops group has acquired its peculiar specializations. Indeed, P. merriami of the castanops group differs from the hypothetical stem more than does P. castanops. Judging from the structure of the living species of the subgenus Cratogeomys and from the primitive subgenus Pappogeomys, the subgenus Cratogeomys featured five major trends: (1) Increase in size; (2) formation of sagittal crest by union of the temporal impressions; (3) increase in rugosity and angularity of the skull; (4) progressive development of platycephalic specializations, including the elongation of the angular process of the mandible; (5) complete loss of enamel plates from the posterior wall of M1 and M2. Each trend is thought to be adaptive.
