Mammalian faunal recovery following the Cretaceous-Paleogene mass extinction: a multifaceted investigation
Smith, Stephanie M
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The Cretaceous-Paleogene (K-Pg) mass extinction and subsequent recovery were a turning point for terrestrial ecosystems, signaling a shift from communities dominated by dinosaurs to those dominated by mammals. Mammalian body size and taxonomic diversity increased dramatically after the K-Pg mass extinction, but studies of these trends have generally been on broad spatiotemporal scales, or failed to capture the period of earliest biotic recovery directly following the extinction event. This dissertation seeks to deepen scientific knowledge of the mammalian recovery via high-resolution study of taxonomy, faunal succession, and dietary ecology in earliest Pg mammals, on a restricted spatiotemporal scale in the Hell Creek and Tullock formations of northeastern Montana. My co-authored quantitative study of the morphological differentiation among species in the common K-Pg mammalian genus Mesodma reveals that at least one species within this historically problematic genus is invalid, suggesting that the incorporation of quantitative information in taxonomic diagnoses is important for objective identification of taxonomic affinity in isolated teeth. Further, taxonomic identifications constitute the raw data for faunal analyses, meaning that the quality of faunal analyses is directly related to the accuracy and repeatability of such identifications. In our faunal analyses of mammals from the McGuire Creek area of northeastern Montana, which incorporated the findings of my study of Mesodma, my co-authors and I find that mammalian recovery began slowly after the K-Pg mass extinction, with McGuire Creek local faunas showing only the earliest signs of recovery within the first 320 thousand years of the Paleogene. We further find that within the Western Interior of North America, McGuire Creek local faunas and other faunas from the Puercan 1 (Pu1) North American Land Mammal Age (NALMA) interval zone were quite similar, whereas in Pu2 and Pu3, North American faunas began to differentiate into northern and southern types. This suggests the possibility of a vicariance event in the early Pg that likely influenced the course of mammalian faunal recovery. My study of dietary ecomorphology in the early Pg of northeastern Montana further enriches our understanding of the mammalian recovery, with results indicating that following the K-Pg mass extinction, local immigrants to the area fueled replacement of dietary ecologies that were culled in the mass extinction. Mammals in the early Pg also showed a shift away from insectivory, which was the most common diet in the K, to increasing omnivory and frugivory in the early Pg, coincident with increasing fruit size in angiosperms. The methods used here for inferring dietary ecology in extinct mammals require some refinement before they can be confidently applied to carnivory and folivory. Overall, this work pushes forward the resolution of our knowledge of mammalian recovery following the K-Pg mass extinction. My results also add to our understanding of the evolutionary process of biotic recovery, as well as how mammals react to ecological perturbation, which is relevant for understanding and managing modern biodiversity crises.
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