FOSSIL PRESERVATION IN THE LAS VEGAS FORMATION: TAPHONOMIC MODES AND DEPOSITIONAL ENVIRONMENTS

The Las Vegas Formation (LVF) is a middle Pleistocene to early Holocene sequence of groundwater discharge (GWD) deposits that represent ancient spring ecosystems that occupied the much of the Las Vegas Valley between ~573 ka and 8.2 ka. Fluctuations in climate led to repeated expansion and desiccation of these ecosystems, and evidence for various groundwater-fed hydrologic regimes including marshes, wet meadows, spring pools, and spring-fed streams are contained within the LVF sediments. The LVF preserves a diverse vertebrate fauna, designated as the Tule Springs Local fauna (TSLF), which dates to ~100 ka to 12.5 ka, and is composed of a variety of taxa including mammals, birds, reptiles, amphibians, and fish. The primary goals of this project are to: (1) Characterize the taphonomic modes of preservation of the large mammal taxa of the TSLF; (2) Examine the relationship between facies and taphonomic modification; and (3) determine if vertebrate fossil preservation in the LVF is primarily controlled by depositional setting or by stratigraphic member/age of deposition. Large mammals (>5 kg body size) were included in this analysis; small mammals and non-mammalian vertebrates are affected differently by taphonomic filters than large mammals, and were not considered for this study. The most abundant large mammal taxa in the TSLF are Mammuthus sp., Camelops sp., Equus sp., and Bison sp.; carnivore fossils are rare but include Smilodon fatalis and Canis dirus. A total of 28,842 bone fragments from museum collections were analyzed, yielding a total number of identified specimens (NISP) of 717. We are currently examining and describing surface modification, fragmentation, and weathering of specimens to interpret the taphonomic history of vertebrate remains from the LVF. The results of this study will clarify relationships between fossil preservation, depositional environment, and hydrologic regime in the upper Las Vegas Wash during the Late Pleistocene epoch and provide context for the preservation of vertebrate remains in GWD deposits elsewhere in the Mojave Desert.