New crania, dentitions, and postcrania of the fossil pinniped, Proneotherium repenningi are described from the early to middle Miocene-age Astoria Formation of Lincoln County, Oregon. This sample includes specimens of four adult individuals; all probably males. The skull of Proneotherium repenningi shares many generalized features with basal pinnipeds (e.g., species of Enaliarctos) 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. Proneotherium, however, differs from species of Enaliarctos in possession of a continuous and horizontal crest connecting the mastoid and paroccipital processes and in having a less secodont dentition. The molariform premolar dentition 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 capable of processing food (i.e., chewing) to a dentition that serves primarily to seize and hold prey. The hindlimb anatomy of Proneotherium provides important information concerning the evolution of aquatic adaptations and clearly shows that many pinniped innovations (short and broad femur, long and slender tibia, and elongated metatarsals I and V [relative to metatarsal III]) 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 genus Proneotherium and suggests a close sister group relationship with another basal walrus, Prototaria. The parsimony analysis also confirms the monophyly of the Odobenidae, which is here defined as the clade containing the most recent common ancestor of Proneotherium and Odobenus and all of its descendants including Prototaria, Neotherium, Imagotaria, the dusignathines, and the odobenines.