SUMMARY

The Pliocene Panaca Fm. in southeast Nevada has yielded abundant small mammal fossils. Including the cricetids and other small mammals, more than 20 species were identified in the Panaca l.f. from Meadow Valley. The cricetids, dominated by Mimomys panacaensis, Nevadomys n.gen., and Repomys, suggest the very early Blancan NALMA for the Panaca l.f..

Cricetids are the major group of rodents in the Panaca l.f., including the subfamilies Sigmodontinae, Arvicolinae, and Prometheomyinae. The low-crowned cricetids are represented by Peromyscus hagermanensis and a species in the genus Onychomys. The high-crowned cricetid in sigmodontine cricetids is Repomys, including R. panacaensis and a new species, R. minor n. sp. The two species are morphologically similar to each other, with different sizes and the occlusal outline of the first lower molar. R. minor is significantly smaller than R. panacaensis. With a large sample of R. panacaensis, especially the last upper and lower molars, the diagnoses of the genus Repomys and the species R. panacaensis are emended.

Arvicolines are abundant in the Panaca l.f.. Mimomys panacaensis was recognized from three other localities in the Fm. in addition to its type locality. A new genus, Nevadomys, is erected for the arvicoline-like rodents from four localities in Meadow Valley. This new genus is distinctive from Mimomys by its combination of the reduction of enamel islet on the first lower molar and a large posterior enamel islet on the last upper molar. Different from M. panacaensis, Nevadomys was considered endemic to North America. A phylogeny of Paramicrotoscoptes-Nevadomys was suggested, and Nevadomys was placed in the same subfamily as Paramicrotoscoptes. Nevadomys includes three species, N. fejfari, N. lindsayi, and N. downsi. N. fejfari is characterized by its two-rooted condition on the last upper molar. N. lindsayi is distinctive on its large size and three-rooted condition on the last upper molar. N. downsi is characterized by its four-lobed anteroconid complex on the first lower molar. N. fejfari was probably derived from N. lindsayi through decreasing size and fusion of the two anterior roots on the last upper molar.

The enamel microstructures at the level of schmelzmuster are examined under the SEM for Mimomys panacaensis and three species of Nevadomys. M. panacaensis from Locality 9702 presents very similar schmelzmuster to that of M. panacaensis from the type locality, indicating that there is no significant difference in the schmelzmuster among different populations of this species. Nevadomys has a more primitive schmelzmuster than M. panacaensis, with only radial enamel on the leading and trailing edges of alternating triangles. In N. fejfari and N. lindsayi, the leading edge of anteroconid complex of the first lower molar is entirely occupied by radial enamel, whereas in N. downsi, traces of discrete lamellar enamel were seen close to the dentine on this leading edge.

The presence of derived horse Equus and a primitive arvicoline rodent Mimomys panacaensis indicate that the Panaca l.f. represents the early Blancan land mammal age. The evolutionary stage of Nevadomys supports this inference.