Pinniped phylogeny and a new hypothesis for their origin and dispersal

Abstract

The relationships and the zoogeography of the three extant pinniped families, Otariidae (sea lions and fur seals), Odobenidae (one extant species, the walrus), and Phocidae (true seals), have been contentious. Here, we address these topics in a molecular study that includes all extant species of true seals and sea lions, four fur seals and the walrus. Contrary to prevailing morphological views the analyses conclusively showed monophyletic Pinnipedia with a basal split between Otarioidea (Otariidae+Odobenidae) and Phocidae. The northern fur seal was the sister to all remaining otariids and neither sea lions nor arctocephaline fur seals were recognized as monophyletic entities. The basal Phocidae split between Monachinae (monk seals and southern true seals) and Phocinae (northern true seals) was strongly supported. The phylogeny of the Phocinae suggests that the ancestors of Cystophora (hooded seal) and the Phocini (e.g. harp seal, ringed seal) adapted to Arctic conditions and ice-breeding before 12 MYA (million years ago) as supported by the white natal coat of these lineages. The origin of the endemic Caspian and Baikal seals was dated well before the onset of major Pleistocene glaciations. The current findings, together with recent advances in pinniped paleontology, allow the proposal of a new hypothesis for pinniped origin and early dispersal. The hypothesis posits that pinnipeds originated on the North American continent with early otarioid and otariid divergences taking place in the northeast Pacific and those of the phocids in coastal areas of southeast N America for later dispersal to colder environments in the N Atlantic and the Arctic Basin, and in Antarctic waters.