THE MOST COMPLETE FOSSIL WALRUS FROM SOUTHERN CALIFORNIA PROVIDES INSIGHTS ON PATTERNS OF WALRUS DIVERSITY THROUGH TIME

Walruses (Odobenidae) diverged from other lineages of pinnipeds in the early to middle Miocene (>16 Ma). The single extant species (Odobenus rosmarus) is restricted to the Arctic, but a diversity of fossil odobenids (20 species,16 genera) is known throughout the North Pacific, especially in California. Although many fossil walruses have been described, our understanding of the evolutionary history of the Odobenidae is hindered by a lack of described postcranial material, despite the fact that many such specimens exist. Here we report on the most complete fossil odobenid known to date, a ~90% complete skeleton from the Late Miocene Oso Sand Member of the Capistrano Formation of Southern California. The specimen represents a new taxon that includes a combination of plesiomorphic and derived characters previously restricted to different groups of extinct walruses. As such, the Capistrano specimen provides new insights into the morphology and diversity of extinct walrus lineages. Our reassessment of walrus phylogeny reveals two distinct Miocene radiations: a middle Miocene radiation and a late Miocene radiation. Our analysis shows that during the middle Miocene radiation, as many as nine lineages of walruses are known to exist at one time. During the late Miocene there are as many as seven coeval species (including the lineage represented by the Capistrano specimen). Despite the high level of diversity found in the late Miocene, the maximum known number of coeval lineages drops to five by the late Pliocene.