Diversity and climate change in the middle-late Wasatchian (early Eocene) Willwood Formation, central Bighorn Basin, Wyoming

Densely distributed mammal samples from the Willwood Formation in the central part of Wyomingʹs Bighorn Basin span episodes of major climate change. Previous work has focused on dramatic, short-term (< 200 kyr) faunal response to the Paleocene–Eocene Thermal Maximum and two periods of rapid faunal turnover 1–2 Myr later (Biohorizons A and B) that were probably also related to climate change. This project compares long-term diversity trends (> 0.5 Myr) during a ~ 1 Myr period of relatively static cool temperatures and a subsequent ~ 1 Myr period of warming that begins with the Biohorizon B event and ends with the Early Eocene Climatic Optimum (EECO) in which the Earth reached the hottest temperatures of the Cenozoic. Geographic Information Systems is used to create spatially-constrained subsamples to limit and explore the effects of the species–area relationship on multiple aspects of diversity, including the alpha (average locality) and beta (locality differentiation) components of species richness and the evenness and relative dominance of speciesʹ abundance distributions. Results demonstrate previously unrecognized spatial bias in the upper levels of the section that complicate the interpretation of faunal response to the EECO. Evenness is strongly related to alpha richness, whereas beta richness and dominance vary independently. Evenness increases significantly and richness becomes significantly more variable during warming. Richness also increases, but the rise in alpha richness is not significant and the rise in beta richness is equivocal. The dominance of the fauna by two abundant lineages escalates consistently regardless of changes in temperature. Diversity is not obviously affected across the Biohorizon B event, indicating that further study is necessary.