Insights into the evolution and ecology of mammals from the Hell Creek region of northeastern Montana

Abstract

The Cretaceous-Paleogene (K-Pg) mass extinction event has historically been viewed as the catalyst for the radiation of mammals, with marked increases in body mass, taxonomic richness, and ecological disparity during the Paleocene biotic recovery. The majority of studies during this interval have focused on taxonomic proxies rather than ecological ones and applied the proxies over coarse spatiotemporal scales, potentially obscuring local patterns. Moreover, the taxonomic and ecological aspects of recovery often proceed at differing rates, thus interpretations based solely on taxonomic proxies may miss important details. In addition, recent analyses have demonstrated that several mammal groups began diversifying ecologically prior to the K-Pg mass extinction, changing how we view Mesozoic mammal communities. This dissertation seeks to add to our understanding of (i) the biotic recovery of mammals through analyses of the taxonomic and ecological aspects of recovery in a limited spatiotemporal window (ii) the ecological diversity of Late Cretaceous mammals through an investigation of the ecology of the metatherian Didelphodon. My first study details the mammalian faunal assemblages from three vertebrate microfossil assemblages, the deposition of which we constrained to the first 28 ka to 80 ka of the Paleocene using new stratigraphic observations. Through quantitative and qualitative comparisons with other local faunas from the Hell Creek region and Denver Basin, we proposed a new model of biotic recovery that subdivides the already established ‘disaster’ and ‘recovery’ phases into early and late sub-phases, each characterized by differences in taxonomic composition. My second study utilized the new model to compare the taxonomic and ecological aspects of biotic recovery. We investigated both aspects over a large temporal interval, ~1 million years following the Cretaceous-Paleogene boundary (KPB), utilizing 2-dimensional ecospace models with estimates for body mass and dietary ecology for each taxon. Our results showed a concordance in taxonomic and ecological recovery, both reaching pre-mass-extinction levels 500–900 ka following the KPB. Moreover, there was increased ecological redundancy during the early phases of recovery, potentially related to abiotic or biotic factors, such as the environmental instability related to Deccan volcanism and the recovery of angiosperms, respectively. The last study used stable isotope analyses to investigate the ecology of Didelphodon. Using isotopes of oxygen (δ18O), and carbon (δ13C) derived from enamel and the established relationship between these values and numerous ecological factors, we determined that Didelphodon was likely the earliest known semiaquatic therian mammal. This finding adds to our understanding of the ecological diversity of Mesozoic mammals and provides a framework for future isotopic analyses. Overall, my results add to our understanding of the ecology and evolution of mammals from the Hell Creek region and are potentially applicable to other studies of mammalian recovery patterns and ecological diversity