Biochronology and magnetostratigraphy of the Pliocene Panaca Formation, southeast Nevada
About 2,500 specimens of small mammal fossils were collected from 10 localities in the Pliocene Panaca Formation in Meadow Valley, southeast Nevada. The most common taxa are cricetid and heteromyid rodents, and lagomorphs. Twenty-seven species, including five new species, were recognized from the fauna. A new genus of microtine rodent, Nevadomys, including three new species, N. fejfari, N. lindsayi, and N. downsi, is recognized. Nevadomys was probably derived from the Hemphillian Paramicrotoscoptes. Repomys minor n. sp. is another new species of high-crowned cricetid that was detected in the Panaca local fauna. With abundant upper and lower third molars of Repomys, it was possible to amend the diagnoses of genus Repomys. The fifth new species recognized from the fauna is a shrew, Paranotiosorex panacaensis n. gen. and sp. Enamel microstructure of molars of the microtine rodents Mimomys panacaensis and Nevadomys n. gen. from the fauna were examined under the scanning electronic microscope. Mimomys panacaensis shows the same schmelzmuster among different populations. Nevadomys has a more primitive schmelzmuster compared to Mimomys panacaensis. Both its leading and trailing edges are entirely composed of radial enamel. The assemblage of small mammal fossils suggests that the Panaca local fauna represents the early Blancan land mammal age. The first appearance datum of Mimomys panacaensis is suggested as the beginning of Blancan land mammal age, and the absolute age of the lowest stratigraphic datum of M. panacaensis in the Panaca Formation is interpolated from magnetostratigraphy and is considered as the age of the Hemphillian-Blancan boundary. The magnetostratigraphy of the Panaca Formation in Meadow Valley is established based on four sections in the basin. The ash in the Rodent Hill section was dated at 4.64 +/- 0.03 Ma, which constrains the correlation of the reversed polarity magnetozone in the Rodent Hill section to the C3n.2r in the Geomagnetic Polarity Time Scale (GPTS). The other three sections are correlated to the Rodent Hill section based on the ashes. The Panaca Formation in Meadow Valley was deposited from 4.50--5.30 Ma, and the Hemphillian-Blancan boundary is around 4.95 Ma, close to the termination of Thvera Subchron in the GPTS.
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