THE RISE OF PLACENTAL MAMMALS AFTER THE END-CRETACEOUS MASS EXTINCTION

The end-Cretaceous mass extinction was a turning point in vertebrate evolution, as non-avian dinosaurs went extinct and were replaced in many niches in the ensuing Paleogene by placental mammals: species like us that give live birth to well-developed young. Our team is using a combination of fieldwork, computed tomography (CT) study of fossil anatomy, phylogenetics, and statistical analyses to better understand the tempo and mode of the early Paleogene placental radiation. A wealth of new specimens from the Paleocene Nacimiento Formation of the San Juan Basin, New Mexico, demonstrates high levels of turnover and considerable diversification within the first 3.5 million years after the extinction. Many of these mammals belong to so-called ‘archaic’ groups whose relationships to the modern placental orders are poorly understood. Our work demonstrates that these species generally had relatively small brains (but sometimes keen senses of smell) compared to extant mammals, had developed a diversity of locomotor behaviors and a unique robust skeletal morphology, and underwent body size changes that may have been related to temperature shifts (particularly the herbivorous ‘condylarths’ Tetraclaenodon and Periptychus). Some groups, such as taeniodonts, had skeletons adapted for burrowing, a lifestyle that may have promoted survival during the extinction, whereas others, like pantodonts, may have exhibited gregarious behaviors. We are building a comprehensive higher-level phylogeny based on anatomical and genetic data, to resolve the relationships of the Paleogene taxa. Analysis of a preliminary version of our dataset, with 140 taxa scored for 503 anatomical characters and subjected to both maximum parsimony and Bayesian techniques, finds that many early Paleogene taxa are stem members of major extant clades (e.g., Primates, Afrotheria, Laurasiatheria, Carnivoramorpha, Euungulata). In particular, some long-problematic ‘condylarths’ appear to be closely related to the modern hoofed mammals (perissodactyls and artiodactyls), whereas other groups like taeniodonts currently fall outside Placentalia. When coupled with a new high-resolution geochronological record from the Nacimiento Formation, these analyses show that many major placental groups had probably originated by the very early Paleogene.