The surprising evolutionary heritage of the Atlantic walrus as chronicled by the fossil record

The walrus (Odobenus rosmarus) is the last surviving representative of a diverse and successful family of pinnipeds. Walrus fossils are common and are represented by material ranging from complete skeletons to isolated skeletal and tusk fragments. They are typically preserved in inner and middle-shelf depositional environments. Walruses evolved from ‘enaliarctine’ ancestors, although whether they are more closely related to otariids (fur seals and sea lions) or phocids (earless seals) has been a matter of debate. The first walruses belong to a paraphyletic assemblage known as the ‘Imagotariinae’. These walruses were initially small in body size, but reached truly enormous sizes before giving rise to the Dusignathinae (double-tusked walruses) and Odobeninae (true walruses). Odobeninae are remarkable for the development of prominent tusks, specialisation in suction feeding and were the only clade to disperse into the North Atlantic. Several major trends can be observed in walrus evolution including: tooth simplification, acquisition of large body size, increased baculum size, development of tusks, and the development of an intermediate style of aquatic and terrestrial locomotion (ascompared to phocids and otariids). At least two separate dispersals into the North Atlantic from the North Pacific occurred via the Arctic Ocean, with Odobenus likely evolving in the North Pacific. Walruses, including Odobenus, showed a much greater tolerance for warmer climates in the past, which may imply some ability to adapt towards anthropogenic climate change.