Reconstructing the Paleoecology and Biogeography of Rhinoceroses (Mammalia: Rhinocerotidae) in the Great Plains of North America, Leading Up to Their Extinction in the Early Pliocene

Members of the family Rhinocerotidae first appeared in the middle Eocene and were one of most successful mammal groups of the Oligocene and Miocene in North America. Their extinction in the early Pliocene has been attributed to several causes, including cooling climate, an expansion of C4 grasslands, and faunal turnover favoring high-crowned, open habitat-adapted mammalian taxa. This study tests whether the extinction of North American rhinoceroses in the Great Plains was abrupt or gradual by examining changes in their paleogeographic distribution in a series of time-slices through the Barstovian, Clarendonian, and Hemphillian North American land-mammal ages. It further examines body size changes in rhinoceroses in the late Miocene through early Pliocene epochs, and uses stable isotope data to test whether or not rhinoceroses were able to adapt to the expansion of C4 grasslands in the late Miocene of the Great Plains. Results indicate that rhinoceros abundance and geographic distribution remained fairly stable through the Miocene until the late Hemphillian when a rapid decline in abundance occurred, based on data compiled from museum collections and online databases. The decline corresponds closely with the expansion of C4 grasslands in the Great Plains. Stable carbon isotopes from the tooth enamel of the two most common rhinoceros Miocene genera, Aphelops and Teleoceras, indicate that both remained almost exclusively C3 feeders during the C4 expansion. Measurements from lower cheek teeth, used as proxies for body mass, indicate significant increases in size in both Aphelops and Teleoceras from the Barstovian to Hemphillian. The increase is greater in Aphelops than in Teleoceras. In the late Hemphillian, however, body size increase in Aphelops remained static and decreased slightly in Teleoceras. This may be a result of lowered C3 biomass as C4 vegetation replaced C3 plants. This study suggests that the expansion of C4 grasslands and the failure of rhinoceroses to incorporate C4 vegetation into their diets, were primary causes of rhinoceros extinction. Rhinoceros tooth enamel δ18O values generated for this study decrease latitudinally, indicating a strong latitudinal δ18O gradient, probably reflecting a latitudinal temperature gradient. This suggests that these large herbivores effectively tracked δ18O values in environmental water and are potentially useful for paleoclimate reconstructions. Advisor: Ross Secor