Paleobiogeography of Latest Cretaceous and Early Paleocene Mammals from North America

Abstract

Nearly all of what is known of the patterns of latest Cretaceous and early Paleogene mammalian evolution (from approximately 69 to 57 million years ago) is documented in stratigraphic sequences from the Western Interior of North America. Throughout much of the latest Cretaceous and early Paleogene this region was tectonically active, with the emergence of the Rocky Mountains and Western Interior foreland basin. Several major marine transgressions also occurred during this interval and, at times, the Western Interior epicontinental seaway bisected the continent. Moreover, the latest Cretaceous and early Paleogene is marked by episodes of rapid climatic warming and cooling, and intensive volcanism. That mammals were affected by these changes is without question; however, the extent to which these factors helped shape the evolutionary patterns of this group is less obvious. To better comprehend the evolutionary dynamics between mammals and their environments across this interval, this dissertation focuses on the paleobiogeography of latest Cretaceous through the early Paleogene mammals from the Western Interior of North America. A number of statistical analyses (e.g., ordination, clustering, linear regression) were employed to assess mammalian faunal provinciality within North America during this interval, differences in the relative abundances of fossil mammals during the latest Cretaceous assemblages, and, finally, the latitudinal diversity gradient in latest Cretaceous and early Paleogene mammals. Notable discoveries include the absence of faunal provinciality and little variation in mammalian taxonomic richness across latitude, but considerable differences in the relative abundance of mammals within assemblages from the latest Cretaceous, with eutherians more diverse and abundant in some of the northerly assemblages. These findings suggest that the paleogeographic and climatic changes that characterized the latest Cretaceous and early Paleogene resulted in intricate biogeographic patterns among mammals from this interval. These studies additionally emphasize the importance of quantitatively assessing these patterns to understand the interactions between mammals and their environments.