New pantolestids from the Uinta Formation, Utah

ABSTRACT

New dental and postcranial specimens of Pantolestes from the Uinta Basin, Utah, are described, and species-level diversity of Pantolestes from the Bridgerian and Uintan North American Land Mammal Ages is assessed. Because few morphological features can be found to distinguish species, we use size as the primary criterion for species membership. We recognize two species from the Bridgerian, P. longicaudus and P. natans. We consider P. elegans a junior synonym of P. longicaudus based on essentially identical size and dental morphology and include P. phocipes within P. natans based on the same criteria. Pantolestes intermedius is problematic, because the only known specimens are postcranial and pantolestids as a group show significant postcranial growth even after dental eruption is complete. However, its extremely reduced size relative to P. natans suggests that it is distinct from this species, and, although larger, it is closer in size to a referred specimen of P. longicaudus. We place all specimens of Uintan Pantolestes within either P. longicaudus or P. natans based on overall similarity in size and morphology. The postcranial remains of Uintan Pantolestes are fragmentary but show little morphological difference from earlier pantolestids. This suggests that Uintan pantolestids maintained an aquatic lifestyle despite the recession of ancient lakes in the continental interior occurring during that time. It is possible that the extreme rarity of Uintan pantolestids compared with earlier relatives is linked to the shrinking of aquatic habitats.

ACKNOWLEDGMENTS

We would like to thank the field crews in the Uinta Basin from 1994 through 2017, the Bureau of Land Management for permitting, and the Vernal Field Office for logistical help. We thank Midwestern University for funding and support. QEP Resources has been gracious in assisting with logistical support during field work. We also thank J. Galkin (AMNH), S. Tomiya and W. Simpson (Field Museum, Chicago), P. Holroyd (Museum of Paleontology, University of California, Berkeley), and D. Brinkman (YPM) for access to museum collections. Two anonymous reviewers are thanked for thoughtful comments on the manuscript. We extend our deep appreciation and gratitude to G. Gunnell for his numerous insights on pantolestid morphology and evolution; his love of enigmatic taxa has left a lasting impact on the field.