MAMMAL POSTCRANIA SUPPORT TAXONOMIC RICHNESS AND BODY MASS INCREASES ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY IN NORTHEASTERN MONTANA

Mammal postcrania from latest Cretaceous and earliest Paleocene (K-Pg) assemblages of the Western Interior of North America are relatively rare and usually found as isolated elements, however this material can provide powerful and novel insight for body size and locomotor ecologies and can supplement taxonomic patterns derived from the well-sampled local dental record. This study describes proximal limb elements (femora and humeri) from K-Pg deposits in northeastern Montana from the Hell Creek (Upper Cretaceous) and Tullock formations (lower Paleogene), representing Lancian and Puercan North American Land Mammal Ages. This area contains some of the best sampled and stratigraphically constrained K-Pg terrestrial deposits in the world and is ideal for studying mass extinction and subsequent recovery dynamics.

We assigned 61 humeri and 64 femora to morphotypes and quantitatively and qualitatively assessed taxonomic and body-size diversity through time. Taxonomically, Lancian and Puercan samples record each of the major groups of mammals alive at the time (multituberculates, metatherians, and eutherians), and multituberculate postcrania predominate the samples. The Lancian sample has the most diverse multituberculate assemblage, and, among other taxa, includes a humerus of the largest Cretaceous metatherian, Didelphodon vorax. Eutherians are largely absent from our samples until the Puercan; among these we identified archaic ungulates, much larger mammals (possibly representing taeniodonts, triisodontids, or pantodonts), and a possible “plesiadapiform” primate.

Size of femur and humerus specimens implies a decrease in mean body size following the K-Pg mass extinction, and a subsequent increase in the mid-late Puercan, a trend that is also supported by the dental record. Maximum size for both elements is relatively small for Lancian and early Puercan samples, but greatly increases in the mid-late Puercan, with multiple specimens suggestive of body sizes an order of magnitude larger than the next largest specimens in the study. Future work will continue to analyze these and other elements to compare taxonomic and body-size diversity patterns to those derived from the dental record, and to assess locomotor inferences for these mammal communities before and after the K-Pg mass extinction.