LOWER JAW OF THE EARLY PALEOCENE MAMMAL ALVEUGENA AND ITS INTERPRETATION AS A TRANSITIONAL FOSSIL

The Paleogene Order Taeniodonta Cope, 1876-peculiar heavy-bodied mammals, some with ever-growing cheek teeth are grouped with the Late Cretaceous eutherian Cimolestes Marsh, 1889, along with a host of other taxa in a superordinal group, the Cimolesta. Taeniodonts were thought to have arisen from Cimolestes indirectly, through Paleocene Procerberus Sloan and Van Valen, 1965. The recently described Paleocene Alveugena Eberle, 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 Cretaceous Schowalteria Fox and Naylor, 2003, has since been described, complicating taeniodont origins. We describe here a lower jaw that we refer to Alveugena from 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) similar to 8.5 m above a Cretaceous-Paleogene boundary, identified using palynological criteria. A cladistic analysis is here presented using new data on Schowalteria and Alveugena, added to that of Cimolestes, Procerberus formicarum Sloan and Van Valen, 1965, P. grandis Middleton and Dewar, 2004, and Onychodectes. This analysis revealed Alveugena as the sister taxon of the taeniodonts but with a closer relationship to Cimolestes than Procerberus, suggesting that taeniodonts evolved from a Cimolestes-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.