The Optima Local Fauna represents an important glimpse into the ecological transition between savannah and grassland during the late Miocene (Hemphillian) of what is now the southcentral Great Plains of North America. Though dominated by horses, herbivores from the Optima are morphologically diverse, bearing adaptations for both browsing and grazing lifestyles. Likewise, the carnivorans show similar ranges of size and presumed dietary behavior. In this study, we used carbonate isotope, mesowear, and tooth breakage and wear analyses to investigate the dietary complexity of mammals from a single site collected by the Oklahoma Museum of Natural History. Seventeen taxa were analyzed, including five perissodactyls (Teleoceras hicksi, Dinohippus interpolatus, Neohipparion eurystyle, Nannippus ingenuus, and Astrohippus ansae), four artiodactyls (Texoceros guymonensis, Pediomeryx hemphillensis, Megatylopus matthewi, and Platygonus sp.), a single proboscidean (Mammut sp.), two rodents (Dipoides indet. and Umbogaulus monodon), and five carnivorans (Agriotherium schneideri, Amphimachairodus coloradensis, Borophagus secundus, Eucyon davisi, Pliotaxidea cf. nevadensis). Both stable isotope analysis and dental mesowear indicate a broad dietary partitioning occurred among the Optima herbivores, where the artiodactyls were identified as mixed feeders and the perissodactyls were recovered as grazers. In the carnivorans, the large felid Amphimachairodus coloradensis was a hypercarnivore with limited tooth breakage and an enriched δ¹³C signature, indicating low carcass utilization and a prey preference for horses. The canids had a more generalized diet, with B. secundus showing a greater proportional consumption of carcasses through a higher tooth breakage rate. The large ursid Agriotherium schneideri is here interpreted as an omnivore based on depleted δ¹³C values. Overall, we found evidence for a diversity of dietary niches in both carnivores and herbivores during the late Hemphillian in Oklahoma, likely driven by the expansion of grasslands in the region.

Key words: Mammalia, grassland, mesowear, savanna, stable isotopes, tooth breakage, Neogene, North America.

Joseph A. Frederickson [fredericksoj@uwosh.edu], Weis Earth Science Museum, University of Wisconsin Oshkosh Fox Cities Campus, 1478 Midway Rd, Menasha, WI 54952, USA; Oklahoma Museum of Natural History, 2401 Chautauqua Ave., University of Oklahoma, Norman, OK 73072, USA. Joshua E. Cohen [jcohen7@pace.edu] (corresponding author), Oklahoma Museum of Natural History, 2401 Chautauqua Ave., University of Oklahoma, Norman, OK 73072, USA; Department of Biology, Pace University, One Place Plaza, New York, NY 10038, USA. Michael H. Engel [ab1635@ou.edu], School of Geosciences, Mewbourne College of Earth and Energy, University of Oklahoma, 100 E. Boyd St, SEC 710, Norman, OK 73019, USA. Tyler C. Hunt [thunt@bio.fsu.edu], Department of Biological Sciences, Florida State University, 319 Stadium Drive, Tallahassee, FL 32304, USA. Greg A. Wilbert [greg.a.wilbert-1@ou.edu] and Nicholas J. Czaplewski [nczaplewski@ou.edu], Oklahoma Museum of Natural History, 2401 Chautauqua Ave., University of Oklahoma, Norman, OK 73072, USA. Olga S. Castañeda [castanedao@student.swosu.edu], Department of Biology, Southwestern Oklahoma State University, 100 W Campus Drive, Weatherford, OK 73096, USA.