Cratogeomys/Geomys —Yellow-faced or Plains Pocket Gopher // Cratogeomys castanops—Yellow-faced Pocket Gopher // Cratogeomys sansimonensis—San Simon Pocket Gopher // Geomys—Eastern Pocket Gophers // Thomomys—Western Pocket Gophers
Pocket gophers are fossorial animals spending most of their lives underground. The external sign of their occurrence in an area is the presence of mounds of dirt pushed to the surface during the construction of their burrows. Openings to the surface usually are blocked by plugs of soil. Although most of their diet of plants is procured from underground, individuals may venture onto the surface to harvest plants in the immediate vicinity of a burrow opening, and during breeding season, surface travel in search of mates is necessary. Apparently they are highly vulnerable during such activities, for occurrence as fossils entombed in caves is common as is presence in modern owl pellets.
As befits burrowing mammals, they have massive, solidly-built skeletons, large front claws, small ears, small eyes, and a short tail with sparse hair.
Geomyids are notoriously antisocial, not only usually occurring as one individual per burrow system, but generally allopatric on a microgeographic scale. Although the local geographic ranges of two species may interfinger, actual overlap is rare. With each species having its own ecological preferences, presence of two or more species in a single fossil site is an indication that two habitats are within range of predators, or that climatic/ecologic conditions have shifted during deposition, or both.
Three genera of pocket gophers occur in the Wisconsin of the region. With proper material, the three genera can easily be told from one another. Discriminating between species within a genus may be difficult, however (see generic accounts). Not unexpectedly for fossorial animals, a number of extinct species is known.
Fig. 1. Ventral views of Cratogeomys castanops (left), Geomys arenarius (center), and Thomomys talpoides (right). Scale in mm.
Pocket gophers are remarkably variable in intraspecific size and coloration. Size usually is correlated to considerable degree with soil depth and type, and coloration correlates with soil color (an indication of predation pressure against those individuals that do not blend well with the background).
Figures 2-4 show the differences in the upper cheek dentition between the genera. The P4 of Thomomys is distinctive compared to that of either of the other genera (P4 of the latter appears to be too variable to distinguish between the two). The shape of M1 and M2 of Thomomys also differs notably from those of the other genera. Note the presence of enamel on the posterior of the P4 of Thomomys and its lack in the other two genera. In addition, Cratogeomys lacks enamel on the posterior of M1 and M2, whereas Geomys retains enamel on those two teeth.
Figs. 2-4. Upper right cheek dentition of three genera of pocket gophers. Fig. 1, Thomomys bottae; Fig. 2, Cratogeomys castanops; Fig. 3, Geomys arenarius. Not to scale.
In addition to the differences in upper cheek tooth rows, the upper incisors of the three genera differ (Fig. 5). Cratogeomys has a single longitudinal groove per upper incisor, Geomys has two, and Thomomys has none or a very faint groove near the medial edge of the tooth. Since upper incisors tend to preserve well, these differences allow generic identification (rabbits and non-geomyid rodents with longitudinal grooves either differ significantly in size or details of the grooving is different).
Fig. 5. Incisors of three genera of pocket gophers. On the left, Thomomys (represented here by T. talpoides) has a very faint longitudinal groove toward the medial edge of the incisor (pointed to by the white line). In the center, Geomys (in the form of G. arenarius) has two grooves, one in the same position as in Thomomys and a more pronounced one in the center of the tooth. On the right, the single, central groove is characteristic of Cratogeomys. Scale in mm.
Since virtually all of our region supports one species or another, identification to the family "Geomyidae" is useless for interpretation. However, the current distributions of Cratogeomys and Geomys are limited to the region east of Arizona; various species of Thomomys occur throughout the region.
Last Update: 6 Jul 2014