PALEOSOL MATURITY, MAMMAL DISTRIBUTION, AND MICROHABITATS AT EARLY EOCENE FOSSIL VERTEBRATE LOCALITIES OF THE WILLWOOD FORMATION (LOWER EOCENE), BIGHORN BASIN, WYOMING

Vertebrate fossil-bearing localities of the Willwood Formation in the Bighorn Basin of northwestern Wyoming date to the lower Eocene (56-53 Ma), just after the Paleocene-Eocene Thermal Maximum (PETM). Fossil mammals from Willwood sediments include the oldest known occurrences of artiodactyls, perissodactyls, and true primates in North America. The sediments that make up localities where the fossils are recovered consist of sandstones and mudstones upon which paleosols (ancient soils) formed. These paleosols can be characterized in stages based on their relative maturity (time to form). Paleosol maturity is directly correlated with relative distance to active stream channels during deposition and soil formation, and may also be indicative of microhabitats within the paleo-landscape. In order to test this hypothesis, I examined fossil vertebrate specimens from the CSU Anthropology Department Paleontology Field School Lab collection collected from three localities in the Willwood Formation (Bighorn Basin, lower Eocene) which have similar meter levels, are in close spatial proximity to one another, and have distinct paleosol stages. When comparing the taxonomic distribution of vertebrate fossils on the ordinal and generic levels from these localities I have found that localities with similar paleosol maturity have similar faunal make-up to one another and also differ significantly from localities with different paleosol stages. This provides support for the hypothesis of a relationship between faunal distribution and paleosol maturity, and if substantiated on a larger scale has implications for the interpretation of the radiation and speciation of modern mammalian orders.