Part 20

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.

A and B. Geomys bursarius bursarius, adult female, No. 46275 (KU), Elk River, Sherborne Co., Minnesota. Left upper (A), P4-M3; right lower (B), p4-m3.

C and D. Subgenus Pappogeomys. Pappogeomys bulleri albinasus, adult female, No. 31002 (KU), W side La Venta, 13 mi. W and 4 mi. N Guadalajara, Jalisco. Left upper (C), P4-M3; right lower (D), p4-m3.

E and F. Subgenus Pappogeomys. Pappogeomys alcorni, adult female, No. 31051 (KU), holotype, 4 mi. W Mazamitla, 6600 ft., Jalisco. Left upper (E), P4-M3; right lower (F), p4-m3.

G and H. Subgenus Cratogeomys. Pappogeomys gymnurus tellus, adult female, No. 31051 (KU), 1 mi. NE Tala, 4400 ft., Jalisco. Left upper (G), P4-M3; right lower (H), p4-m3. ]

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.

LITERATURE CITED

ALSTON, E. R.

1876. On the classification of the order Glires. Proc. Zool. Soc. London, 1876:61-98, 1 pl., June.

ALVAREZ, T.

1964. Nota sobre restos oseos de mamiferos del Reciente, encontrados cerca de Tepeapulco, Hidalgo, Mexico. Publ. Inst. Nac. Antro, e Hist., 15:1-15.

1965. Catálago Paleomastozoológico Mexicano. Publ. Inst. Nac. Antro, e Hist., 17:1-70.

AXLEROD, D. I.

1950. Studies in late Tertiary paleobotany. Carnegie Inst. Washington Publ., 590:1-322.

1958. Evolution of the Madro-tertiary geoflora. Bot. Rev., 24:433-509.

BADER, R. S., and TECHTER, D.

1959. A list and bibliography of the fossil mammals of Illinois. Nat. Hist. Misc., Chicago Acad. Sci., 172:1-8, October 30.

BAIRD, S. F.

1858. Mammals. Part I. General report upon the zoology of the several Pacific railroad routes. Repts., explorations and surveys for a railroad route from the Mississippi River to the Pacific Ocean. Washington, D. C., vol. 8, pp. xlviii + 757, 35 figs., 43 pls., July 14.

BARBOUR, H., and SCHULTZ, C. B.

1937. An early Pleistocene fauna from Nebraska. Amer. Mus. Nat. Hist. Novit., 942:1-10, September 10.

BERRY, E. W.

1937. Tertiary floras of North America. Bot. Rev., 3:31-46.

BLACK, C. C.

1961. Rodents and lagomorphs from the Miocene Fort Logan and Deep River Formations of Montana. Postilla, Yale Peabody Museum, 48:1-20, 6 figs., January 16.

BRANDT, J. F.

1855. Beiträge zur mähern Kenntiss der Säugethiere Russlands. Acad. Imp. des Sciences St. Petersburg, 7:79-336, 11 pls.

BROWN, B.