Part 12

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.

Loss of enamel is a trend common to all living genera of the tribe Geomyini, but the greatest loss has occurred in _Cratogeomys_. It has lost the plates on the posterior walls of M1 and M2 (Fig. 9G). If the lateral plates of M3 are considered as one functional plate and the lateral plates on either side of P4 together as two transverse plates, then, the transverse cutting blades in _Cratogeomys_ number seven in the upper and seven in the lower cheek teeth compared with 10 in the upper and seven in the lower in the primitive morphotype. Indeed, in some species of the subgenus, one or both of the lateral plates on M3 is also lost, usually in old age, resulting in even greater reduction of enamel. Loss of enamel from the posterior walls of the upper molars may be associated with changes in the mechanics of mastication from anteroposterior planing to anterotransverse shearing, as discussed elsewhere. Merriam (1895:95-96) argues convincingly that the posterior cutting blades of the upper molars would hinder efficient shearing action of the teeth; hence, selection would favor their reduction and eventual loss. Changes in the shape of the skull also seem to be correlated with the shift from a planing to a shearing type of mastication. More efficient shearing action, which depends upon lateral movement of the jaw, can be developed if the functional muscles insert farther laterally than is possible in the generalized type of skull. Therefore, platycephalic specializations involved lateral expansion of the braincase and mandible. Pronounced lateral expansion has been developed only in the _gymnurus_ group of species, suggesting that the dental specializations evolved earlier in the evolution of the subgenus than did the platycephalic specializations of the skull, and that the _castanops_ group separated from the _gymnurus_ group before the common ancestor had developed the more extreme trends in platycephaly. It is interesting to note that the subtriangular M3 (Fig. 9G) postulated for the ancestral morphotype and that characterizes the subgenus _Pappogeomys_ is retained also in the _gymnurus_ group.

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