New Stable Isotope Record of Paleoecological Change in the Late Neogene of the Western Great Plains from Enamel in Large Mammals

An expansion of C 4 grasslands occurred between 6 and 8 million years ago in the Great Plains of North America, as evidenced by a marked shift to more 13 C-enriched carbon isotope compositions from large fossil mammal tooth enamel and paleosol carbonates. Prior to this expansion, habitats were comprised of exclusively C 3 vegetation. To explore this problem I present a compilation of bulk stable carbon (δ 13 C) and oxygen (δ 18 O) 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 δ 13 C values indicating that they exclusively exploited C 3 vegetation with habitats ranging from open forests in northern Nebraska to C 3 grasslands in southwest Nebraska. A shift to higher δ 13 C values occurs in Pliocene camelids, equids, and gomphotheres suggesting that they began to consume C 4 vegetation. In contrast, a peccary (Platygonus sp.) still consumed only C 3 vegetation indicating that C 3 niches were still present. Through this Miocene-Pliocene transition there is an overall shift to lower δ 18 O 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, δ 18 O 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.