This project investigates how vegetation structure responded to global warming of 5-10 C and increased atmospheric CO2 during the Paleocene-Eocene Thermal Maximum (PETM, approximately 55.8 million years ago), and whether this in turn affected the fauna feeding on this vegetation. Canopy structure in the PETM has important implications for the dispersal and subsequent evolution of the earliest representatives of several modern mammalian clades. Three dimensional vegetation structure is important for many reasons. It affects the albedo of land surfaces, hydrologic cycling, atmospheric circulation near the earth's surface, and carbon storage. All of these affect climate and biogeochemical cycles on a global scale. Vegetation also forms the habitat in which terrestrial organisms move and acquire food, and it fundamentally influences their locomotor adaptations and diet. This project tests the hypothesis that vegetation structure changed from open- to closed-canopy forests during the PETM. This has important implications for the global dispersal and ensuing radiations of the oldest known true primates (Euprimates), perissodactyls (odd-toed ungulates), and artiodactyls (even-toed ungulates), which first appeared in North America during the PETM. The earliest euprimates were highly specialized arborealists and their dispersal may have been facilitated by the development of a closed canopy. Vegetation growing in the understory of closed canopy forests has lower 13C/12C ratios than vegetation growing in open environments. Mammals that consume understory vegetation record this 'canopy-effect' in their mineralized tissues (like teeth). This project infers canopy structure by analyzing stable carbon isotope ratios (13C/12C) in mammalian tooth enamel. Fossil teeth are sampled for ~16 species from intervals before, during, and after the PETM. The teeth come from the southeastern Bighorn Basin (SBB) where the PETM is constrained biostratigraphically and geochemically by a carbon isotope excursion. This area contains the only macrofloras known from the PETM, which aids paleoenvironmental and paleoclimatic interpretations. The project also samples teeth and bone in extant mammalian faunas from the Neotropics to better constrain isotopic parameters that are used to interpret canopy structure and resource partitioning. The canopy effect is well documented in Old World faunas, especially from Africa, but is poorly constrained in Neotropical faunas. However, faunas in the late Paleocene and PETM may have been more analogous to modern Neotropical faunas, in that they lacked obligate folivores, which could diminish the canopy effect and narrow the range of faunal isotopic variability. Thus, study of Neotropical faunas should strengthen interpretations.
Broader impacts include the training of undergraduate students in field and laboratory techniques, and in database creation, management, and web interface. Project results will be broadly disseminated through professional meetings, publications, the FLMNH website on-line searchable database, and public lectures. Study of environmental change during the PETM may ultimately be useful for predicting the long-term consequences of global warming and is of growing interest to policy makers and the public.
