A GREAT BASIN MIOCENE BIOTA FROM THE ESMERALDA FORMATION OF NEVADA AS AN ECOLOGICAL CONSTRAINT ON THE TIMING OF SIERRAN UPLIFT

The Esmeralda Formation is a Miocene, terrestrial unit exposed at the north end of Fish Lake Valley, east of the White-Inyo Range of eastern California-western Nevada. Previous work has been conducted throughout the Great Basin to determine the timing of the Sierra Nevada uplift, including isotopic work to establish the timing of a major rain shadow. These studies have recorded the first-order topographic changes of the developing mountain range at a macro scale, but localized surface uplift events have not yet been detailed. Several significant faunas have been documented throughout the Great Basin, including a diverse assemblage from the Esmeralda Formation. This fauna provides an ecological constraint on the timing of the Sierra uplift. In addition, a well-preserved, petrified grove is known from the site, containing oak trees, which are no longer found within the Great Basin. Several ash beds are associated with the fossils, limiting their deposition to between 22 and 11 Ma. Previous studies of related units further constrain deposition to 14.5 – 15.0 Ma.

This study has thus far found the remains of Gomphotheriume, camelid, and equid megafauna at this locality. Three unusual gomphothere skulls collected in 1968, 1986, and 2014 may be the only representatives of their species collected in North America, G. minor. A pair of large, hypsodont teeth were recovered during our first survey, potentially representing rhinocerids. Several taxa of microfauna were also collected, representing individuals of prehistoric beaver and other rodents.

Two prehistoric beaver incisors yielded preliminary isotopic data that agree with their status as xylophagous mammals inhabiting a marshy, dry-summer continental region. δ¹³C values fall within the expected range for a browser (-10.7 to -15.3‰), while δ¹⁸O values have a range of -10.4 to -14.2‰, which is notably more depleted than are found in the modern Great Basin. These data support the interpretation that the Great Basin was cooler and wetter during the Miocene, and provide insight into the paleoecology and habitat response of a region made discontinuous by a physical barrier. The biota of the Esmeralda Formation represents a fauna and flora which have no analog in the modern Great Basin, and are thus instructive in terms of constraining topographic uplift of the Sierra Nevada.