Biostratigraphy, paleoecology and synchronized evolution in the early Eocene mammalian fauna of the central Bighorn Basin, Wyoming

This project investigated the pattern of mammalian faunal evolution in the early Eocene Willwood Formation of the central Bighorn Basin, Wyoming. Preliminary analyses included an examination of depositional rate, in which the absolute dates of the beginning and end of the section were constrained and it was demonstrated that sediment accumulation rate increased from the bottom to the top of the section. Comparative biostratigraphic analysis confirmed the existence of a 70m difference in thickness between two measured sections within the central Bighorn Basin and identified the most appropriate biostratigraphic zonation for the central Bighorn Basin sediments.; An assessment of the biotic and potential taphonomic patterns in the dataset concluded that bias introduced through uneven sample sizes was the most difficult to control. Other analyses showed that local and large-scale differences in preservation had a significant impact on faunal composition. Biological interactions, including resource partitioning and predation, were evident at the family level, and two broad types of community composition were identified. Further analysis showed that the lateral gradient of depositional environments, reflecting fluvial sub-habitats, also influenced species distributions.; Patterns of species first and last appearances and turnover were compiled from the raw data, from a confidence interval analysis and from an analytical method developed in the text specifically to correct for uneven sampling distribution. Significance tests were used to assess the departure of the observed turnover from predicted background rates. Turnover was significantly high in two episodes corresponding to biohorizons A and B of Schankler (1980). Biohorizon A was an event of longer duration but much less intensity than biohorizon B and was related to an abrupt decrease in mean annual temperature. Biohorizon B was a much shorter and more intense event including both evolutionary change and immigration/emigration. Biohorizon B was probably the result of the crossing of a biotic threshold of conditions including climate warming, basin drying and habitat fragmentation related to tectonic activity. Compositional change in the fauna related to neither of these events was demonstrated to conform to the predictions of the variability selection hypothesis.