Pediomydae

Figure 130

The pediomyid affinities of the right upper molar, AMNH 77375, from Clambank Hollow, are indicated by the prominence of stylar cusp D, the narrowing of the stylar shelf labial to the paracone, and the presence of an anterolingual cingulum which often occurs as a variation in later Maestrichtian species, particularly in P. elegans. AMNH 77375 (fig. 130) is small and as transverse as M 3 of P. elegans Clemens (1966, p. 37). A ridge connects the apex of the paracone to the parastyle. It is likely that AMNH 77375 belongs with the other small lower and upper molars assigned to Pediomys clemensi but such an association cannot be demonstrated. DISCUSSION: Pediomys clemensi is a relatively rare member of the fauna from the Judith River Formation, and is closest morphologically to P. elegans from the Lance Formation of Wyoming. Pediomys elegans is the Maestrichtian species that Clemens (1966, p. 34) regarded as most primitive of those then known. Only one species of Pediomys appears to be present as ascertained from the upper molars. Unfortunately, there is not a single complete upper molar of Pediomys; thus there is no way of determining whether the three upper molar fragments belong to the same species. The pediomyid stylar shelf is radically different from that of other marsupials not only in the nonuniform width of the stylar shelf but also in the relative development of the stylar cusps. Buccal to the paracone, the stylar shelf narrows considerably and is little more than a narrow cingulum. The shelf in some later Maestrichtian species may even be absent and discontinuous in this area. Posteriorly, the shelf widens buccal to the metacone. Stylar cusp B is feebly formed and situated closer to cusp C than to cusp A. Cusp D is the largest stylar cusp. The paracone is more labial than the metacone.