STABLE ISOTOPE RECORD OF PALEOECOLOGICAL CHANGE IN THE LATE NEOGENE OF NEBRASKA 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 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 we 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.