A reevaluation of Proneotherium repenningi from the Miocene Astoria Formation of Oregon and its position as a basal odobenid (Pinnipedia : Mammalia)

New crania, dentitions, and postcrania of the fossil pinniped, Proneotheriu m repenningi are described from the early to middle Miocene-age Astoria For mation of Lincoln County, Oregon. This sample includes specimens of four ad ult individuals; all probably males. The skull of Proneotherium repenningi shares many generalized features with basal pinnipeds (e.g., species of Ena liarctos) including its overall elongation, low profile, low sagittal crest , well developed lambdoidal crests, slender zygomatic arches, small orbits, slightly arched palate, dental formulae, and distinct embrasure pit. Prone otherium, however, differs from species of Enaliarctos in possession of a c ontinuous and horizontal crest connecting the mastoid and paroccipital proc esses and in having a less secodont dentition. The molariform premolar dent ition of Proneotherium is noteworthy and can be interpreted as representing the initial stage in a morphologic series that extends through Neotherium to Imagotaria. This morphologic series reflects a functional change from a shearing dentition to more of a piercing dentition and from a dentition cap able of processing food (i.e., chewing) to a dentition that serves primaril y to seize and hold prey. The hindlimb anatomy of Proneotherium provides im portant information concerning the evolution of aquatic adaptations and cle arly shows that many pinniped innovations (short and broad femur, long and slender tibia, and elongated metatarsals I and V [relative to metatarsal II I]) evolved very early. Evaluation of available specimens of Proneotherium supports recognition of a single species, Proneotherium repenningi. A cladistic analysis utilizing 24 cranial, dental and postcranial characters supports monophyly of the gen us Proneotherium and suggests a close sister group relationship with anothe r basal walrus, Prototaria. The parsimony analysis also confirms the monoph yly of the Odobenidae, which is here defined as the clade containing the mo st recent common ancestor of Proneotherium and Odobenus and all of its desc endants including Prototaria, Neotherium, Imagotaria, the dusignathines, an d the odobenines.