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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 13C-enriched carbon isotope compositions from large fossil mammal tooth enamel and paleosol carbonates. Prior to this expansion, habitats were comprised of exclusively C3 vegetation. To explore this problem I present a compilation of bulk stable carbon (δ13C) and oxygen (δ18O) isotope values from a variety of large mammals from 6 localities that span from the late Clarendonian to the late Blancan in Nebraska. As expected, late Miocene taxa had δ13C values indicating that they exclusively exploited C3 vegetation with habitats ranging from open forests in northern Nebraska to C3 grasslands in southwest Nebraska. A shift to higher δ13C values occurs in Pliocene camelids, equids, and gomphotheres suggesting that they began to consume C4 vegetation. In contrast, a peccary (Platygonus sp.) still consumed only C3 vegetation indicating that C3 niches were still present. Through this Miocene-Pliocene transition there is an overall shift to lower δ18O values in tooth enamel suggesting a long-term decrease in mean annual temperature. This agrees with previous work in the Great Plains and the marine realm. In addition, δ18O values from serial samples suggest that seasonality decreased from the Miocene to the Pliocene. These data agree with previous studies indicating global change in climate and vegetation from the late Miocene to the Pliocene.
Adviser: Ross Secord