A new proterotheriid (Mammalia, Litopterna) from the Salla Beds of Bolivia (upper Oligocene): phylogeny and litoptern patellar pit knee locks

ABSTRACT

The Salla Beds of Bolivia reveal a rich, late Oligocene (Deseadan, South American Land Mammal “Age”) fauna composed of at least 50 mammalian species. Many taxa (e.g., rodents and typotherian notoungulates) are represented by dozens to hundreds of specimens. Litopterns are much less common but are represented by one macraucheniid and two adianthid species. Until now, only fragmentary postcrania of proterotheriids have been described from Salla. The recent discovery of a maxillary fragment with two molars, examination of previously recovered mandibular fragments, and a phylogenetic analysis, allow us to recognize Promylophis cifellii gen. et sp. nov. It is characterized by small size, absence of a metaconule on M1–2, and lower molars with a short “paracristid” and reduced or absent “paraconid.” Postcranial elements referred to cf. P. cifellii resemble geologically younger (e.g., Miocene) proterotheriids by way of anatomically tridactyl, but functionally monodactyl pes, and distal femora with a supra-trochlear fossa that forms a “patellar pit” that appears to function as a passive stay apparatus for the knee joint. Such a mechanical means of diverting forces from muscle to bone and sinew could save biologically significant amounts of metabolic energy in long-standing animals.

ACKNOWLEDGMENTS

Fieldwork associated with this study was supported in part by the Facultad de Geología of the Universidad Autónoma “Tomás Frías,” Potosí, Bolivia for providing a vehicle for our work at Salla. Summer research grants and a sabbatical from Manhattan College to BJS facilitated museum studies related to this project. For these studies, BJS and AMc thank G. Billet and C. de Muizon of the Muséum national d’Histoire naturelle, Paris; D. Brinkman, C. Norris, M. Fox of the Yale Peabody Museum of Natural History, New Haven, Connecticut; R. Hulbert and B.J. MacFadden of the Florida Museum of Natural History, Gainesville, Florida; B. Simpson of the Field Museum of Natural History; as well as R. O’Leary and J.J. Flynn of the American Museum of Natural History, New York. We also thank G. Schmidt, M. Bond for their critical reviews of an earlier form of the manuscript. The rigor of the JVP review process also improved this study. Journal reviewers/editors that labored over our work included R. Anemone, P. Godoy, J. Hersant, J. Jacobs, L. Leuzinger, A. Lopez-Arbarello, and T. Martin.