| The White River Group (WRG) preserves the Eocene-Oligocene climate transition (EOCT), an interval of global cooling and drying during the onset of Antarctic glaciation. In the Great Plains, a shift from forested conditions to drier woodland-savanna biomes is hypothesized to have occurred at this time. I test this hypothesis through the analyses of several paleoenvironmental proxies on the teeth of 12 WRG ungulate species: stable carbon and oxygen isotopes from tooth enamel, and mesowear and microwear texture. The EOCT shift toward more open habitats and lower vegetation density under drying climates should have resulted in an increase in mean carbon isotope values in vegetation and increase in abrasive ingesta in ungulates. These trends, in turn, should be reflected in the carbon isotope values of WRG ungulate teeth, as well as in their patterns of meso- and microwear. Data gathered for this study are grouped into two time-averaged faunas: one from the Chadronian North American land-mammal age (NALMA) (latest Eocene) and one from the Orellan NALMA (earliest Oligocene). Isotopic results suggest that both faunas inhabited drier, open canopied biomes, such as woodland-savanna or scrubland, and that wetter habitats became restricted in extent during the Orellan due to decreasing rainfall. Mesowear results suggest that the diets of several taxa living in open habitats during the Chadronian were already abrasive enough to suggest mixed-feeding, and that all but one range-through taxon had static diets through this interval. Only the oreodont Merycoidodon sp. shows evidence for having had a more abrasive diet during the Orellan than it did during the Chadronian, suggesting that the expansion of open habitats led to greater utilization of grasses by this oreodont. Results from microwear texture analyses of extant taxa suggest that it is a useful tool in characterizing diets for WRG species. These results provide a better understanding of the effects of climate change on mammals in Nebraska during the EOCT. |
