BASIN MARGIN UPLAND ENVIRONMENT AT SOUTH PASS, WYOMING WAS A BIODIVERSITY HOTSPOT FOR LATE EARLY TO MIDDLE EOCENE RODENTS

Vertebrates from the Late Early to Middle Eocene basin margin at South Pass include a unique combination of upland and lowland taxa. Comparisons between this faunal assemblage and coeval basin-center faunas reveal distinctive and unique taxa as well as differences in first or last appearances for particular taxa. One explanation for these differences is that closely spaced and range restricted habitats are ideal conditions for promoting evolutionary innovation, including peripatric speciation. Yet to be explored in detail are potential differences in rodent biodiversity of Late Early to Middle Eocene basin margins vs. basin center deposits. I compared fossil rodent specimens recovered from the basin margin upland environment at South Pass in southwestern Wyoming to those from a coeval Wind River Basin fauna and a younger Bridger Basin fauna. The South Pass faunal assemblage (Br1a) had a higher taxonomic diversity with nine genera and 16 species compared to seven genera and 12 species found at Davis Ranch (Br1a) of the Wind River basin. Two species of Sciuravus, S. popi and S. eucristadens were unique to basin margin faunas (South Pass and Powder Springs); Acritoparamys francesci was unique to basin centers (Wind River and Bridger Basin). Other taxa were found in both environments, but at different times. For example, the oldest record of Uintaparamys parvus and U. bridgerensis is at South Pass (Br1a), where they are relatively rare; later, they are found in the Bridger Basin (Blacks Fork Member; Br1b and Br2) and are more abundant. On the other hand, Paramys delicatior and P. delicatus, which are relatively common at South Pass (Br1a), are less common in the Bridger Basin (Br1b), but become more numerous later, during the Middle Eocene (Br2-Br3). Based on these distribution patterns, it is possible that some rodent taxa originated in a basin margin environment and migrated to a basin center habitat, with fluctuations in colonization success over time. In summary, the South Pass rodent fauna is distinctive from coeval basin-center faunas, characterized by greater sciuravid species diversity, distinctive and unique taxa. These results support previous data, which suggest that marginal areas around the Green River basin were biodiversity hotspots for vertebrates during the Late Early to Middle Eocene.