A new stable isotope record of Neogene paleoenvironments and mammalian paleoecologies in the western Great Plains during the expansion of C4 grasslands

Abstract Stable isotopes in horse teeth from the North American Great Plains show that equids began to incorporate C 4 plants in their diets about 6.5 million years ago as C 4 grasslands expanded. However, the ecological response of many other ungulates to this expansion is poorly documented. We use stable carbon isotopes in mammalian tooth enamel to test whether other ungulates adapted by incorporating C 4 vegetation in their diets. The openness of habitats before the expansion of C 4 grasslands is uncertain, with plant phytolith studies suggesting a patchy environment with open-habitat grasses and forest, and pollen and macrofloral studies suggesting more forested habitats. To address this problem we use a model that predicts carbon isotope values for tooth enamel for a variety of environments, based on values in modern plants. Carbon and oxygen isotopes were analyzed in medium to large herbivores from three late Miocene and three Pliocene local faunas comprising a total of 13 and 7 genera, respectively, and 59 and 42 individuals. Results indicate that before the expansion of C 4 grasslands, taxa with high-crowned teeth were consuming predominantly C 3 vegetation. In contrast, by the late Pliocene most taxa studied were consuming a component of C 4 vegetation and only the peccary Platygonus sp. had a pure C 3 diet. C 4 consumption increased in the late Pliocene (~ 3.0 Ma) Big Springs local fauna probably in response to increased C 4 biomass. Most landscapes in the late Miocene of Nebraska were open, such as woodland-savanna or C 3 grassland, although low carbon values from the Pratt Slide local fauna suggest a denser, presumably forested area. This general pattern suggests an expansion of open-habitats no later than 12 Ma (early Clarendonian). Through the Miocene–Pliocene transition there was an overall shift to lower δ 18 O enamel values, which parallels the long-term decrease in global mean annual temperature inferred from the marine record. Our results indicate that major changes in the diets of medium and large herbivores broadly corresponded with increased C 4 biomass and cooling climate from the latest Miocene to the late Pliocene.