NEW STABLE ISOTOPE RECORD OF PALEOECOLOGICAL CHANGE IN THE LATE NEOGENE OF THE WESTERN GREAT PLAINS FROM ENAMEL IN LARGE MAMMALS

An expansion of C₄ grasslands occurred between 6 and 8 million years ago in the Great Plains of North America, as evidenced by a marked shift to more positive stable carbon isotope values in both paleosol carbonates and horse tooth enamel. One important question related to this expansion of C₄ grasslands is how larger mammals were able to adapt to this dramatic environmental change, e.g., were some browsing mammals able to adapt by switching to C₄ grasses? To explore this problem we present a preliminary compilation of stable carbon (δ¹³C) and oxygen (δ ¹⁸O) isotope values from a variety of mammals from 6 localities and 5 time intervals in Nebraska. These intervals range in age from the late Clarendonian to the late Blancan land-mammal ages (late Miocene through late Pliocene). Carbon isotopic results show that large mammals like rhinoceroses (Teleoceras, Aphelops), camels (Megatylopus, Procamelus, Alforjas, Hemiauchenia), and horses (Cormohipparion, Neohipparion, Pseudhipparion, Pliohippus, Calippus), consumed C_3­ plants in the late Miocene, as expected. All δ¹³C enamel values are below -8‰ (VPDB) for these taxa, which is consistent with a diet of pure C₃ vegetation. A shift to more positive carbon isotope values occurs in the Blancan, indicating that some camels and horses were consuming C₄ vegetation, while others continued with a pure C₃ diet. The range of δ¹³C values increased from -11‰ to -7.5‰ in the Hemphillian, to -10.5‰ to -4‰ in the Blancan. By the late Blancan (late Pliocene), some camels and horses had δ¹³C values approaching 0‰, indicating a diet consisting almost entirely of C₄ vegetation. Larger samples from these localities will provide a broader understanding of how mammals were able to adapt to changing climate and the late Neogene expansion of C₄ grasslands in the western Great Plains. They will provide a basis for comparison with other areas in North America and the world.