STABLE ISOTOPE PALEOECOLOGY AND VEGETATION STRUCTURE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM FROM MAMMALIAN TOOTH ENAMEL

Major changes in faunal and floral compositions occurred with extreme warming during the Paleocene-Eocene Thermal Maximum (PETM) in North America. Here we use stable isotopes in mammalian tooth enamel to test whether climate change and extreme floral turnover during the PETM resulted in changes in forest structure or niche partitioning in mammals. Most fossils are from the southern Bighorn Basin (>260 samples), augmented with a small late Paleocene dataset from the northern basin. We infer forest structure using the distribution of carbon isotopes, whereby negative outliers are indicative of closed forest understory, and by comparison with modern floras, adjusted for atmospheric changes in carbon isotopes. The outlier approach suggests closed canopy forest in the latest Paleocene followed by open forests in the PETM and post-PETM. Comparison with modeled modern environments suggests moderately dense, wet open forests in the late Paleocene and post-PETM, with slightly drier and more open forest in the PETM. Although this PETM interpretation relies heavily on an atmospheric correction, a lack of negative outliers in a very large sample (172 individuals) provides strong support for open forests in the PETM. Most terrestrial taxa maintain the same relative positions in carbon isotope space through the PETM, suggesting fairly stable niche usage. Of the endemic taxa, the pantodont Coryphodon has the most negative carbon values, consistent with a semiaquatic or riparian niche. 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 and post-PETM faunas the immigrant Sifrhippus (Equidae) is isotopically indistinguishable from the small phenacodontids. The artiodactyl Diacodexis has the highest carbon values in the PETM, which decrease slightly in the post-PETM where it is indistinguishable from the small phenacodontids and Sifrhippus. Relative upward shifts in oxygen isotope values occur in Coryphodon and Diacodexis in the post-PETM fauna suggesting possible change in niche usage. However, the fairly stable niche usage implied by carbon isotopes through the PETM is notable, given the dramatic changes seen in faunal and floral compositions.