High-Resolution Mammal Biostratigraphy and Response to Transient Climate Change at the Paleocene-Eocene Carbon Isotope Excursion

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The Paleocene-Eocene epoch boundary at 55.5 million years before present coincides with the sudden onset of an 85,000-year greenhouse event of transient climate change, the Paleocene-Eocene thermal maximum or PETM, that involved release of massive amounts of light carbon into the atmosphere from shallow marine methane reservoirs. The resulting negative carbon isotope excursion [CIE] was first recognized in the deep sea, and then on land in the northern Bighorn Basin, Wyoming. In the Bighorn Basin the CIE coincides with a "Wa-0" faunal immigration event including the first appearance of modern orders of mammals such as Artiodactyla, Perissodactyla, and Primates in North America, and with an abrupt but short-lived diminution in size of some mammalian lineages. Research here involves high resolution biostratigraphic investigation of continental mammals in the PETM interval. A 50-m-thick stratigraphic section containing the CIE and Wa-0 mammalian fauna is being studied around the margin of the Bighorn and Clarks Fork basins. Specific objectives include measurement of stratigraphic sections documenting marker beds and lateral facies changes in this interval; mapping of distinctive marker beds tying sections together using differential GPS with meter-scale precision; location of known fossils relative to measured and mapped bed units; location of productive new lithologies; quantification of body size change in all lineages crossing the Wa-0 interval; preparation and description of previously collected and new specimens of Wa-0 mammals focusing on the newly-discovered Meniscotherium priscum interval at the base of Wa-0; and collection of additional faunal specimens (gar scales, Coryphodon teeth, etc.) of known bed level for high resolution study of oxygen isotopes and paleotemperature through the continental CIE/Wa-0 interval. The overall objective is better understanding of biotic response to short-term climate change.