A REVISED LATE-MIOCENE FAUNAL ASSEMBLAGE, INCLUDING AN OCCURRENCE OF EXCEPTIONAL PRESERVATION OF INSECT LARVAE, FROM THE ESMERALDA FORMATION OF FISH LAKE VALLEY, NEVADA

The Esmeralda Formation of western Nevada records deposition of a Late Miocene (14.5-15 Ma) terrestrial, normal fault bound basin. Recent work by our team has recently discovered exceptionally preserved insect larvae, proboscideans, felids, camelids, equids, as well as microvertebrates. This fauna is preserved in a stratigraphic sequence consistent with deposition in a swampy, poorly drained fluvial to lacustrine setting. The Miocene fauna of Fish Lake Valley have been studied by previous researchers, however, there has been a hiatus for the past several decades. This study takes out new findings to revise the known faunal record of this geographic region, immediately east of the Sierra Nevada, and works to elucidate the significance, both biologically and geologically, of this fauna.

Of particular note is the silicification of soft tissue insect larvae. It appears these are the larval form of Calliphoridae (blow flies) which are known today as a variety of carrion fly. The numerous volcaniclastic horizons within the Esmeralda Formation were potentially the source of the mobile silica. Geothermal activity in the area was likely active through the Neogene to facilitate the rapid silicification of larval anatomy. These larvae are preserved in what was the nasal turbinates of a proboscidean, likely a gomphothere. In addition to the invertebrate record, petrified trees that contain silicified seed caches in hollows. Many of these trees have been leveled by sediment gravity flows.

Over the past 5 years, teams from UNLV, the Las Vegas Natural History Museum, and the American Museum of Natural History have found the remains of horses, camels, rhinos, gomphotheres, cats, dogs, beavers, and horned gophers. Each subsequent trip has yielded new finds. This faunal assemblage, in addition to the sedimentological data, suggests that the climate in western Nevada was more equitable than it is today. This assemblage suggests that atmospheric contact with the Pacific was more consistent, with a less significant rain shadow effect, and the presence of a lower Sierra Nevada during the Late Miocene.