A PRELIMINARY STABLE ISOTOPE ECOLOGY BASED ON TOOTH ENAMEL FOR LATEST EOCENE AND EARLIEST OLIGOCENE PERISSODACTYLS (MAMMALIA) FROM THE WHITE RIVER GROUP OF NORTHWESTERN NEBRASKA

We analyzed carbon and oxygen isotopes in tooth enamel carbonate of perissodactyls (odd-toed ungulates) from the White River Group of northwestern Nebraska to infer diet and habitat. The taxa examined include three genera that occur above and below the Eocene-Oligocene boundary: Miohippus (an equid), and Subhyracodon and Hyracodon (rhinos). We also include two genera that became extinct at the end of the Eocene: Brontops and Trigonias (rhino). Brontops was the last surviving member of the Brontotheriidae, a long lived family including the largest land-mammals of the North American Eocene, and as such was of particular ecological importance.

The δ¹³C values of cheek teeth range from -12.3‰ to -6.5‰ (V-PDB) and are consistent with pure C3 diets, assuming a δ¹³C value of atmospheric CO₂ of ~-5.5‰ at this time. In the latest Eocene, Hyracodon, Subhyracodon, and Trigonias yielded statistically indistinguishable average δ¹³C values (~-9.8‰). These values suggest that they shared a similar habitat, and are consistent with open forest/woodland. In contrast, Miohippus and Brontops yielded values (-8.3‰ & -11.8‰, respectively) that are significantly different from each other and from the rhinos. These values are consistent with a wet (riparian?), fairly closed habitat for Brontops and an open habitat for Miohippus. These data support niche partitioning between these sympatric taxa. δ¹⁸O values (V-SMOW) help to further separate these taxa, with Subhyracodon and Trigonias having similarly low values (22.0‰, 22.6‰), which are significantly different from Miohippus (24.0‰) and Hyracodon (25.0‰), but not from Brontops (23.0‰). This suggests that Brontops, Subhyracodon, and Trigonias were obligate drinkers and may have also fed in microhabitats where water was readily available to plants. In contrast Miohippus and Hyracodon may have been eating leaves in more open areas where plants were subject to water stress. δ¹⁸O values increase in the Oligocene for the three range-through taxa but the increases are too small to be significant. δ¹³C values also increase in Hyracodon and Subhyracodon in the Oligocene. Together these shifts suggest drier, more open habitats in the early Oligocene. Notably, no taxon plots in the isotopic space formerly occupied by Brontops, suggesting that its microhabitat was lost to drying in the Oligocene.