Digging up the Past: Postcranial Perspectives on Mammals Across the Cretaceous-Paleogene Boundary
DeBey, Lauren Berg
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The extinction of non-avian dinosaurs at the Cretaceous-Paleogene (K-Pg, 66.04 Ma, million years ago) boundary is arguably the most seminal event in mammalian history. The K-Pg mass extinction and subsequent recovery marked the onset of rapid morphological, ecological, and phylogenetic diversification in mammals into the wide range of body sizes and ecological niches occupied throughout the Cenozoic. Most mammalian patterns across the K-Pg boundary are based on dental data; however, postcrania are uniquely suited to inform the locomotor ecologies and substrate preferences of mammals. As such, K-Pg mammalian postcrania can be used to test (1) locomotor-based patterns of extinction selectivity that are predicted in certain mass extinction scenarios, and (2) increases in early Paleogene locomotor diversity predicted by ecological release from dinosaur-imposed selection during the Cretaceous. This study represents the first comprehensive description and assessment of mammal postcrania across the K-Pg boundary. Femora, humeri, and astragali from the well-sampled and well-studied K-Pg deposits of eastern Montana were studied qualitatively and quantitatively to infer taxonomic affinities and locomotor modes, and to investigate changes in body size. This study encompasses three faunas: the latest Cretaceous pre-extinction fauna, earliest Paleocene “survival” fauna, and later Paleocene more “recovered” fauna. Results show significant changes in body size throughout these three faunas that are consistent with patterns derived from dental data. Among mammals in the “survival” fauna, the average body size of latest Cretaceous mammals that survived the K-Pg mass extinction was smaller than during the latest Cretaceous, which is consistent with the ‘Lilliput Effect’; however, early Paleocene immigrants actually had significantly increased body sizes. Mammals in the later “recovered” fauna have the largest body size of all mammals in this study. Results from analyses of locomotor mode show latest Cretaceous mammals had semifossorial and arboreal locomotor modes, and there was an expansion of scansorial and arboreal locomotor modes among the early Paleocene mammals. These ecomorphological findings are consistent with hypotheses concerning the transition from open Cretaceous landscapes to more closed, forested Paleocene habitats. Postcrania represent a valuable, independent dataset for examination of extinction and recovery dynamics that can deepen our ecological understanding of the Cretaceous-Paleogene mass extinction.
- Biology