Tribosphenic mammals from the Lower Cretaceous Cloverly Formation of Montana and Wyoming

ABSTRACT

We report a diverse assemblage of tribosphenidan mammals from several localities in the Cloverly Formation (Lower Cretaceous, Albian) of Montana and Wyoming. This unit is of historical significance for yielding well-known dinosaurs (e.g., Deinonychus antirrhopus, Tenontosaurus tilletti) and early mammals (e.g., Gobiconodon ostromi, Montanalestes keeblerorum). We provisionally identify 13 taxa (five of which are formally recognized), including Pappotherium pattersoni, a new species of the deltatheroidan Oklatheridium (O. wiblei, sp. nov.), a eutherian (Montanalestes, previously named), and two new basal tribosphenidans (Argaliatherium robustum, gen. et sp. nov., and Carinalestes murensis, gen. et sp. nov.). An unnamed taxon, represented by associated but almost edentulous dentaries, is interpreted to have had four incisors, a single-rooted canine, three premolars, and four molars, indicating that the metatherian tooth formula was established by the Albian. In addition, an indeterminate lower molar fragment preserving twinned talonid cusps and a buccal postcingulid provides the earliest evidence for Marsupialiformes. We also provide a more detailed description of the associated dentaries (holotype) of Montanalestes keeblerorum. The mammalian fauna from the Cloverly Formation shares several taxa with the roughly contemporaneous Trinity Group of Oklahoma and Texas, an observation that also applies to the dinosaur fauna, suggesting some degree of latitudinal homogeneity among described terrestrial vertebrates in this part of the North American Early Cretaceous.

http://zoobank.org/urn:lsid:zoobank.org:pub:F62E498B-9E95-475B-B2E3-0CA6FC31FB2A

ACKNOWLEDGMENTS

We thank the many students, staff, and colleagues who helped in our field and laboratory investigations, among whom we are especially grateful to C. L. Gordon, S. K. Madsen, W. D. Maxwell, R. L. Nydam, C. R. Schaff, and J. R. Wible. The extraordinarily difficult and time-consuming task of preparing quarried specimens was accomplished by S. K. Madsen. N. J. Czaplewski kindly prepared Figure 6. We thank J. Maisano and M. Colbert, of the University of Texas High-Resolution X-ray CT Facility, for scanning and image processing; Maisano and Colbert also prepared the relevant text of the Methods and Conventions section and gave us permission to use it verbatim. We also thank C. Kulicki and J. Cohen for providing us with SEM images. We are grateful to L. Keebler, J. Taylor, and J. Owen for granting us access to private lands, and to the Montana State Bureau of Land Management for access to public lands. Partial support for this research was provided by the American Chemical Society Petroleum Research Fund (ACS-PRF 38572-AC8), the National Geographic Society (5918–97), and the National Science Foundation to R.L.C. (grants DEB 9401994 and 9870173).