A new genus of perissodactyl (Mammalia) from the Bridgerian of Wyoming, with comments on basal perissodactyl phylogeny

ABSTRACT

An unusual specimen of a perissodactyl from the Bridgerian (late early to early middle Eocene) of Wyoming provides the basis for a new genus and species, Mesolambdolophus setoni. The specimen (MCZ 19585) displays an unusual combination of features: first lower premolar absent; p2 and p3 not molariform; p4 submolariform; anterior end of metalophid lingually placed, contacting metaconid; metaconid not twinned; and large hypoconulid on m3. The orientation of the metalophid excludes this specimen from Ceratomorpha, and the size and characters of this specimen exclude it from any known genus of Bridgerian perissodactyl, as well as from any other Eocene perissodactyl taxon. Including this taxon in a phylogenetic analysis of basal perissodactyls places Mesolambdolophus as sister taxon to Tapiromorpha, but we consider any assessment of the position of Mesolambdolophus tentative at best, given the limitations of analyses of basal perissodactyls currently available. The presence of a small Bridgerian ‘hippomorph’ form that is larger than Orohippus reduces the reliability of assigning isolated postcrania to Bridgerian perissodactyl taxa on the basis of size.

ACKNOWLEDGMENTS

F. Jenkins, C. Schaff, and J. Cundiff of MCZ kindly allowed the authors access to MCZ 19585. R. Emry, C. Ray, and R. Purdy provided invaluable assistance at United States National Museum. This research was supported by the Theodore Roosevelt Memorial Fund of the American Museum of Natural History, a Sigma Xi Grant-in-Aid of Research, a Smithsonian Graduate Fellowship, and a grant from the National Science Foundation (DEB-0211976). E. Heck of the American Museum of Natural History provided photographic assistance. J. Hunter, M. Coombs, D. Froehlich, and two anonymous reviewers provided valuable comments on earlier versions of this paper.

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