Mammalian distal humerus fossils from eastern Montana, USA with implications for the cretaceous-paleogene mass extinction and the adaptive radiation of placentals

Abstract

Postcrania of Cretaceous-Paleogene (K-Pg) mammals offer insights into richness, body size, and locomotor ecology that supplement patterns from well-sampled dental assemblages. Here, we describe and morphotype 50 distal humeri from Lancian–Puercan assemblages of eastern Montana. Using geometric morphometric analysis of a taxonomically broad sample of humeri from extant small-bodied therians of diverse locomotor modes, we constrain locomotor inferences of some morphotypes. We use this database to preliminarily assess body-size and locomotor diversity across the KPg boundary. The seven Lancian humerus morphotypes include the multituberculates ?Mesodma sp., ?Cimolodon nitidus, and ?Meniscoessus robustus and the metatherian ?Didelphodon vorax. Morphotype richness decreased to four or five across the K-Pg boundary and rebounded in the late Puercan to six, mostly eutherian, morphotypes. Puercan morphotypes include the multituberculate ?Stygimys kuszmauli, the “plesiadapiform” primate ?Purgatorius, small and large archaic ungulates, a possible palaeoryctid, and a very large eutherian. Humerus size data imply a decrease in body size across the K-Pg boundary, followed by an increase by the late Puercan, a trend consistent with the dental fossil record. Geometric morphometrics analysis and functional morphology imply greater locomotor diversity among K-Pg mammals than previously recognized: we infer that most Lancian and Puercan multituberculates were arboreal; the Lancian eutherian was arboreal or semifossorial; the early Puercan palaeoryctid was semifossorial and the small archaic ungulate was terrestrial; and the late Puercan “plesiadapiform” primate was arboreal and the large archaic ungulate was scansorial. Taken together, these preliminary results expand our understanding of K-Pg mammals and our basis for testing ecological hypotheses of the K-Pg mass extinction and recovery.