MAMMALIAN PALEOECOLOGY THROUGH THE PALEOCENE-EOCENE THERMAL MAXIMUM: AN ISOTOPIC PERSPECTIVE

Major faunal reorganization occurred during the Paleocene-Eocene Thermal Maximum (PETM) in North America with the arrival of the first members of Perissodactyla, Artiodactyla, Euprimates, and other mammalian clades. Floral turnover was also extreme, resulting in local extirpation of most plant species, which returned post-PETM (Wa-1). Here we use stable isotopes in mammalian tooth enamel to test whether faunal reorganization and floral change forced taxa into new habitats. Fossils are mostly from the southern Bighorn Basin (>260 samples), augmented with a late Paleocene sample from the northern basin. Results show that most taxa maintain the same relative positions in carbon isotope space through the PETM. Of the endemic taxa, the pantodont Coryphodon has the most negative mean carbon values, consistent with a wet/dense habitat. The small phenacodontid condylarths Ectocion and Copecion have high carbon values consistent with feeding in open areas, while the large phenacodontid Phenacodus has intermediate values. In both PETM (Wa-0) and Wa-1 faunas, the immigrants Sifrhippus (Equidae) and Diacodexis (Artiodactyla) also have high carbon values and are statistically indistinguishable from the small phenacodontids. Of the best sampled taxa, three-point moving averages for Coryphodon and Ectocion-Copecion are non-overlapping through the PETM sequence. Moving averages for Sifrhippus are also non-overlapping with Coryphodon through Wa-0 and Wa-1. Together, this suggests ecological stability for these taxa, as far as can be ascertained using carbon isotopes. However, shifts do occur in oxygen isotopes from the PETM to Wa-1 where oxygen values increase significantly in both Coryphodon and Diacodexis relative to the rest of the fauna. Both taxa were probably consuming water from more evaporated sources in Wa-1. The shift in Coryphodon suggests that it moved to a drier/more open habitat in Wa-1. Diacodexis has the highest mean oxygen values of the Wa-1 fauna suggesting that it was living in the driest, most open areas where water in fruit and leaves would be the most evaporated. Although stable isotopes imply a somewhat stable mammalian ecology through the PETM, changes in body size and species abundance are well documented, and a multi-proxy approach is needed to best understand PETM paleoecology.