How the Herbivorous Mammalian Diet Changed When C4 Grasses Expanded in North American Great Plains

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

Authors:

Kita et al

Abstract:

Stable isotopes in horse teeth from the North American Great Plains show that equids began to incorporate C4 plants in their diets about 6.5 million years ago as C4 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 C4 vegetation in their diets. The openness of habitats before the expansion of C4 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 C4 grasslands, taxa with high-crowned teeth were consuming predominantly C3 vegetation. In contrast, by the late Pliocene most taxa studied were consuming a component of C4 vegetation and only the peccary Platygonus sp. had a pure C3 diet. C4 consumption increased in the late Pliocene (~ 3.0 Ma) Big Springs local fauna probably in response to increased C4 biomass. Most landscapes in the late Miocene of Nebraska were open, such as woodland-savanna or C3 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 δ18O 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 C4 biomass and cooling climate from the latest Miocene to the late Pliocene.