EQUUS SCOTTI FROM THE TULE SPRINGS LOCAL FAUNA, SOUTHERN NEVADA: IMPLICATIONS FOR THE DIVERSITY AND BIOGEOGRAPHY OF LATE PLEISTOCENE EQUIDS IN WESTERN NORTH AMERICA

Late Pleistocene ground water discharge deposits in the upper Las Vegas Wash outside of Las Vegas, Nevada have yielded an abundant and diverse vertebrate fossil assemblage, the Tule Springs local fauna. More than 600 discrete localities from the Las Vegas Formation have been documented in this region. Stratigraphically ascending members of this formation span the last 250 ka, based on ¹⁴C dating of charcoal and luminescence dating of sediments. Most of these units have yielded fossils. The recovered fauna, dominated by Camelops and Mammuthus, also includes relatively common remains of extinct Equus and Bison as well as rare carnivorans such as Canis dirus, Smilodon, and Panthera along with abundant vertebrate microfaunal fossils. The Tule Springs local fauna is the largest open-site vertebrate fossil assemblage dating to the Rancholabrean North American Land Mammal Age in the Mojave Desert/southern Great Basin.

Recent field efforts yielded a partial skull, mandible, and metatarsal of a subadult individual of extinct large Equus. These specimens, the first from the Las Vegas Formation to be assignable to species, represent the large horse Equus scotti. Diagnostic features include large size, stout metapodials, and infundibula in the lower incisors. These fossils are directly associated with a radiocarbon date of 13.70 ka (thousands of calibrated ¹⁴C years before present). The fossils are therefore the youngest and most southerly record of this species in Nevada and among the youngest recorded anywhere in North America.

In addition to Equus scotti, two other species of Equus can be discerned in the Tule Springs local fauna: a small stout-limbed form and likely a small stilt-legged species. The presence of multiple species of horse in the Las Vegas region accords well with the fossil record from other late Pleistocene localities in the American southwest. However, the presence of E. scotti in the Tule Springs local fauna challenges inferences that the large late Pleistocene horse species E. occidentalis was present at multiple Mojave Desert fossil localities. This inference, based on the relative geographic proximity of the Mojave to more coastal sites such as Rancho La Brea and Diamond Valley Lake, where E. occidentalis is abundant, merits reconsideration.