Eared Seal - Evolution and Taxonomy

Evolution and Taxonomy

Along with the Phocidae and Odobenidae, the two other members of Pinnipedia, Otаriidae are descended from a common ancestor most closely related to modern bears. Debate remains as to whether the phocids diverged from the otariids before or after the walrus.

Otariids arose in the late Miocene (10-12 million years ago) in the North Pacific, diversifying rapidly into the Southern Hemisphere, where most species now live. Callorhinus (northern fur seal) has the oldest fossil record of any extant otariid, extending to the middle Pliocene, and probably arose from the extinct fur seal Thalassoleon.

Traditionally, otariids had been subdivided into the fur seal (Arctocephalinae) and sea lion (Otariinae) subfamilies, with the major distinction between them being the presence of a thick underfur layer in the former. Under this categorization, the fur seals comprised two genera: Callorhinus in the North Pacific with a single representative, the northern fur seal (C. ursinus) and eight species in the Southern Hemisphere under the genus Arctocephalus, while the sea lions comprise five species under five genera. Recent analyses of the genetic evidence suggests that the Callorhinus ursinus is in fact more closely related to several sea lion species. Furthermore, many of the Otariinae appear to be more phylogenetically distinct than previously assumed; for example, the Japanese sea lion (Zalophus japonicus) is now considered a separate species, rather than a subspecies of the California sea lion (Zalophus californius). In light of this evidence, the subfamily separation has been removed entirely and the Otariidae family has been organized into seven genera with 16 species and two subspecies. Nonetheless, because of morphological and behavioral similarity among the "fur seals" and "sea lions", these remain useful categories when discussing differences between groups of species: all fur seals have certain features in common: the fur, generally smaller sizes, farther and longer foraging trips, smaller and more abundant prey items and greater sexual dimorphism. And all sea lions have certain features in common, in particular their coarse, short fur, greater bulk and larger prey than fur seals. For these reasons, the distinction remains useful.