MESOWEAR ANALYSIS OF OLIGOCENE TO EARLY MIOCENE HERBIVORES IN NORTH AMERICA

Cooling and drying climate trends in North America during the Oligocene epoch (33.9-23.0 Ma) coincided with a shift in paleoenvironment, with grasses increasing in prevalence as forests receded in the central United States. Here, I investigate the influence of this change in flora on the diet of herbivorous mammals in the Great Plains region. The sharpness and relief of molars (mesowear) has been used to infer a fossil herbivore’s diet, with less abrasive diets (such as browsing soft vegetation) resulting in taller, sharper cusps, while more abrasive diets (such as grazing on grasses) are indicated by lower, rounded cusps. Mihlbachler et al. (2011) developed a mesowear ruler ranking mesowear on a scale of 0 (tall, sharp cusps) to 6 (flat cusps) for use in evaluating mesowear in fossil herbivores. Using this mesowear ruler, I collected mesowear data from 211 total specimens (115 Oligocene, 96 Miocene) from several extinct herbivore families, including the Merycoidodontidae, Equidae, Rhinoceratidae, Camelidae, and Paleomerycidae.

While mesowear scores were generally low (< 3) in most specimens from all ages, Miocene herbivores tended to have higher mesowear scores than Oligocene taxa. For example, Oligocene merycoidodonts (e.g., Merycoidodon and Leptauchenia) from the Orellan and Whitneyan North American Land Mammal Ages (NALMA; 33.9-30.8 Ma) had an average mesowear score of less than 1, while the Miocene merycoidodont Merychyus averaged over 2. A similar trend was observed in equids, with Oligocene horses (i.e., Mesohippus and Miohippus) having an average mesowear score of approximately 0.4, while the average mesowear score of Miocene horses (e.g., Parahippus) from the Hemingfordian NALMA (20.43-15.97 Ma) was 2.25. Overall, the results of this study suggest that while no Oligocene or early Miocene taxon exhibits mesowear patterns signifying a grazing diet, North American herbivores of the early Miocene had slightly more abrasive diets than Oligocene herbivores.