Fig. 9. Molariform dentitions of the Tribe Geomyini. Drawings illustrating enamel patterns characteristic of Geomys and Pappogeomys (including the subgenera Pappogeomys and Cratogeomys). × 5.
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.
Alston, E. R.
1876. On the classification of the order Glires. Proc. Zool. Soc. London, 1876:61-98, 1 pl., June.
Alvarez, T.