Environmental Change in the Great Plains: An Isotopic Record from Fossil Horses

Abstract

Carbon and oxygen isotope ratios of fossil horse tooth enamel from Nebraska and Texas show evidence for late Neogene environmental changes in the Great Plains. The earliest unambiguous C4 dietary signal among Texas equids coincides with the age of the classic late Hemphillian–age Coffee Ranch fauna, which we suggest is ∼6.6 Ma based on volcanic ash correlations. C4 vegetation was present in the diets of a small fraction of late Hemphillian equids in Nebraska and was thereafter ordinary in the diets of both Nebraska and Texas equids. There is no unequivocal evidence for abundant C4 vegetation in the diets of pre–late Hemphillian equids, and we suggest that the ensuing dietary change reflects C4‐biomass expansion in the latest Miocene. Carbon isotope ratios of post‐Hemphillian horses in Nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co‐occur with horse remains, indicating that the two proxies (carbon isotopes and presence/absence of tortoises) record complementary environmental phenomena. The average δ18O values of late Hemphillian and younger fossil localities in Nebraska trend toward bimodal distribution, but more data are needed to confirm this pattern. Oxygen isotope ratios of Barstovian and Clarendonian horses are significantly enriched in 18O relative to Hemphillian horses, which in turn are significantly enriched relative to Blancan and Irvingtonian horses. A large portion of this oxygen isotope decrease appears to have taken place during late Hemphillian time. Secular variation in the Nebraska δ18O record correlates with changes in ungulate diversity, the disappearance of crocodilians in Nebraska, and global change in the latest Miocene.