Systematics, diversity dynamics, and paleobiogeography of early Paleocene mammals from northeastern Montana and the Western Interior of North America

Abstract

The early Paleocene was a critical interval in the evolution of mammals during which the group underwent a remarkable evolutionary radiation following the Cretaceous/Paleogene (K/Pg) mass extinction ca. 66 Ma. Although the broad patterns of this mammalian radiation are well established, the precise details remain more poorly understood and ongoing work continues to refine our understanding of this important episode in mammal evolution. The Hell Creek Formation and overlying Tullock Member of the Fort Union Formation in northeastern Montana are well known as an excellent study system for examining the evolution of mammals leading up to and across the K/Pg extinction event. Substantial work in this region has focused on the record of mammals immediately before and after the K/Pg boundary, whereas the younger mammal-bearing horizons have received less attention and until recently have produced fewer fossils. Consequently, this has hindered a more complete understanding of the recovery and subsequent diversification of mammals in this area. This dissertation seeks to add to our knowledge of early Paleocene mammal systematics and diversity dynamics by way of three studies that vary in spatiotemporal scale. In Chapter 2, a coauthor and I describe a new assemblage of mammalian dental fossils from the stratigraphically highest mammal-bearing localities from the upper part of the Tullock Member in Garfield County, Montana—the Farrand Channel and Horsethief Canyon local faunas. Both local faunas have been correlated to the early Torrejonian (To1) North American Land Mammal ‘age’ and currently represent the oldest and most northerly occurrences of To1 mammals. These new fossils substantially increase the previously known sample size from the Farrand Channel and Horsethief Canyon local faunas and document as many as 40 distinct taxa, several new occurrences, and likely more than one new species. Further, these fossils help better characterize the age and composition of both local faunas, which are temporally intermediate between the youngest Puercan and oldest Torrejonian faunas known elsewhere. More broadly, we contribute to the limited record of To1 mammals and demonstrate that within less than 1 Ma after the K/Pg boundary mammals were considerably more taxonomically diverse in the Hell Creek region than previously appreciated. In Chapter 3, coauthors and I report new plesiadapiform (putative stem primates) dental fossils from the Farrand Channel and Horsethief Canyon local faunas that record several poorly known taxa and represent the largest and most diverse assemblage of To1 plesiadapiforms known. We describe a new species of purgatoriid plesiadapiform (Ursolestes blissorum, sp. nov.) that represents the largest plesiadapiform known from the early Paleocene. We also document intraspecific variability and one undescribed tooth locus of the oldest known member of the Paromomyidae, Paromomys farrandi. Further, we evaluate plesiadapiform species richness, mean body mass, and body-mass disparity through the Paleocene and reveal unrecognized levels of richness in To1 and a general trend of stable body mass and body-mass disparity, thereby providing new insights into the early evolutionary history of Primates. In Chapter 4, I provide a quantitative assessment of the taxonomic composition of the Farrand Channel and Horsethief Canyon local faunas and the biogeography of early Paleocene mammals from the Western Interior using a newly assembled dataset of North American mammalian occurrences. Further, I compare geographic patterns of mammalian diet and body mass distributions to test for regional differences in community structure that may be indicative of differences in habitat and a driver of compositional differentiation. I find that the Farrand Channel and Horsethief Canyon local faunas are compositionally most similar to other To1 faunas, supporting their previous correlation. I also find evidence of temporal and latitudinal differentiation among early Paleocene faunas. My ecological analyses reveal some geographic patterning but ultimately cannot detect meaningful differences in community structure, likely due to the lack of certain ecological data currently available and sampling gaps in the fossil record. Collectively, these studies add to our knowledge of early Paleocene mammal systematics and help emphasize the importance of continued collecting efforts and specimen-based work.