PALEOECOLOGICAL ANALYSIS OF HERBIVORES FROM THE OPTIMA LOCAL FAUNA OF OKLAHOMA (MIOCENE; LATE HEMPHILLIAN)

The Optima Local Fauna of Guymon, Oklahoma represents an important glimpse into the ecological transition between savannah and grassland during the Late Miocene of what is now the Great Plains of North America. Though dominated by horses, herbivores from the Optima are morphologically diverse, bearing adaptations for both browsing and grazing lifestyles. In this study, we used carbon isotope and mesowear analyses to investigate niche partitioning between primary consumers from V52, a large fossil assemblage housed at the Sam Noble Museum of Natural History. Carbon isotope analysis was performed on enamel taken from 10 taxa, representing five perissodactyls (Teleoceras hicksi, Dinohippus interpolatus, Neohipparion eurystyle, Nannippus ingenuus, and Astrohippus ansae), four artiodactyls (Texoceros guymonensis, Pediomeryx hemphillensis, Megatylopus matthewi, and Platygonus sp.), and a single proboscidean (Mammut sp.). The four horses tested were significantly more enriched in 𝛿¹³C_VPDB-LSVEC (mean=-6.3‰, n=49) than the other six taxa (mean=-9.5‰, n=65), implying that grasses had become their primary food source by the Late Hemphillian. Distinctions within these two groups could not be further differentiated from the isotope data alone. In order to further distinguish between browsers and grazers in the data set, mesowear values were compared between eight ungulates from the Optima fauna (all except Platygonus sp.). Five observers independently scored mesowear variables, including cusp shape, relief, and an overall mesowear numerical score (MNS) for each specimen. The median value of each variable was used in subsequent analyses and compared to a dataset of modern grazers and browsers, using a Kruskal-Wallis test and a cluster analysis. Astrohippus, Dinohippus, Nannippus, and Neohipparion were recovered as grazers, while Texoceros, Pediomeryx, and Megatylopus were recovered as mixed feeders. MNS for Teleoceraswas most similar to the mixed feeders, while cusp shape and relief was most similar to the grazers, so we interpret Teleocerasas a mixed feeder/grazer. These data taken together support the presence of a complex and transitional ecosystem dominated by grazing horses, but still occupied by a plethora of browsing/mixed-feeding ungulates.