Collaborative Research: A Phylogeny of Placental Mammals based on Paleocene Taxa: Determining the Impact of the K-Pg Extinction on Mammalian Evolutionary History

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Sixty-six million years ago, an asteroid impact in the Gulf of Mexico profoundly changed life on Earth. In the following mass extinction event, non-avian dinosaurs, which had been the dominant terrestrial vertebrates, did not survive. One surviving group was the mammals; yet, they did more than just survive, they in turn became the dominant vertebrates in most ecosystems. However, the scientific community still does not understand how mammals flourished in the post-extinction world. Numerous mammalian fossils from the first few million years after the extinction event have been discovered, including the first large mammalian herbivores. However, it is unclear how these unusual fossils relate to mammals alive today - are they direct ancestors or merely distant cousins? This project will focus on this neglected fossil record from the first 12 million years after the asteroid impact, a time characterized by high global temperatures and brief episodes of rapid global warming. By understanding what these extinct mammals were, this project will explore the true role of this extinction event in the evolution of mammals, examining which kinds of mammals survived, and how and why they recovered and evolved in a very rapidly changing climate. Today, Earth is experiencing a modern-day extinction event, as environmental change and human actions are threatening many species. Although the current circumstances differ from those 66 million years ago, understanding the underlying biological processes acting in a post-extinction world will guide us in the future.Controversy surrounds the origin of placental mammals and the role of the Cretaceous-Paleogene (K-Pg) extinction event on their evolution, with evidence from morphology on the one hand and DNA on the other at odds. DNA-based studies argue for a Cretaceous placental origin, with the K-Pg event having little impact, whereas morphology-based studies argue for a Paleogene origin driven by the K-Pg event. Previous phylogenetic analyses of placentals have sampled heavily from older (Cretaceous) and younger (Eocene, 56 to 34 million years ago or earlier) taxa, but the sizable mammalian diversity from the Paleocene (66 to 54 million years ago) has been largely ignored. This project will correct this shortcoming by including most Paleocene lineages in a comprehensive phylogeny designed to directly address the controversy regarding the impact of the K-Pg event. Over 1,500 morphological characters, including some studied with computer tomography (CT), from 262 extinct and extant taxa will be combined with over 35,000 base pairs from 26 nuclear genes from extant mammals, and analyzed together using maximum parsimony, maximum likelihood, and Bayesian methodology. The reconstructed phylogenetic trees will be used to resolve the origin of Placentalia and diversification of major subclades using divergence time analyses, and to estimate body size and morphological character evolution rates using model-based techniques. The evolutionary and diversity rates will be compared to paleoecological data to assess the impact of early Paleogene climate variation on mammalian history.