LATE BLANCAN GAZELLE-HORSE NANNIPPUS PENINSULATUS (MAMMALIA, EQUIDAE) FROM SCURRY COUNTY, TEXAS, WITH IMPLICATIONS FOR BIOCHRONOLOGY

Ongoing excavations at Roland Springs Ranch Locality 1 (RSR-1) in Scurry County, Texas have produced skeletal remains representing a diverse assemblage of vertebrate life. Remains of small, tridactyl equids are common in the stream sediments of RSR-1. As the most common large mammal in the RSR-1 fauna, understanding these diminutive horses is important to determining paleo-community composition. Further, identification of horse remains is biochronologically significant. Lacking numeric age determinations, placing the RSR-1 fauna in time is dependent upon faunal correlation. Ongoing biochronology research is focused on seven taxa, one of which is the small horse. The RSR-1 small horse remains are identified as Nannippus peninsulatus based on a combination of cheek tooth crown height, enamel pattern form, and metapodial size. Highly cursorial with strongly hypsodont cheek teeth, N. peninsulatus is derived in its adaptations to open grassland. An enigmatic precursor, N. beckensis, is known only from the late Pliocene locality of Beck Ranch, also in Scurry County. The RSR-1 sample is distinguished from the slightly less advanced N. beckensis by increased crown height, absence of protostylids, and increased metapodial elongation. Nannippus peninsulatus is characteristic of the late Pliocene-early Pleistocene (Blancan Land Mammal Age) fauna of North America. The presence of N. peninsulatus at RSR-1 provides temporal separation from Beck Ranch, and indicates an age of less than ~3.5 million years ago. Although not conclusive, this age supports the early findings of the biochronology research of an earliest Pleistocene age for the RSR-1 fauna.