Lower jaw of the Early Paleocene mammalAlveugenaand its interpretation as a transitional fossil

Abstract

The Paleogene Order Taeniodonta Cope, 1876—peculiar heavy-bodied mammals, some with evergrowing cheek teeth—are grouped with the Late Cretaceous eutherianCimolestesMarsh, 1889, along with a host of other taxa in a superordinal group, the Cimolesta. Taeniodonts were thought to have arisen fromCimolestesindirectly, through PaleoceneProcerberusSloan and Van Valen, 1965. The recently described PaleoceneAlveugenaEberle, 1999, until now known only from the upper dentition, has been put forth as a transitional form between cimolestids and taeniodonts on phylogenetic and biostratigraphic grounds. An older taeniodont, the Late CretaceousSchowalteriaFox and Naylor, 2003, has since been described, complicating taeniodont origins. We describe here a lower jaw that we refer toAlveugenafrom the lower part of the Ludlow Member of the Fort Union Formation in North Dakota. The lower jaw comes from strata of early Early Paleocene age (Puercan 1 North American Land Mammal Age) ~8.5 m above a Cretaceous-Paleogene boundary, identified using palynological criteria. A cladistic analysis is here presented using new data onSchowalteriaandAlveugena, added to that ofCimolestes, Procerberus formicarumSloan and Van Valen, 1965,P. grandisMiddleton and Dewar, 2004, andOnychodectes.This analysis revealedAlveugenaas the sister taxon of the taeniodonts but with a closer relationship toCimolestesthanProcerberus, suggesting that taeniodonts evolved from aCimolestes-like ancestor. We discuss the age relations of early taeniodonts and related taxa and propose a scenario of ancestor-descendent relations that minimizes, but does not eliminate, implied stratigraphic gaps.