Origins

Elephant seals are the largest in size of the 34 extant species of pinnipeds (King 1983). There are two species in the genus Mirounga , the northern elephant seal, M. angustirostris , and the southern elephant seal, M. leonina . The genus is in the family Phocidae, the true seals, as distinct from the other two families in the suborder Pinnipedia: Otariidae, the fur seals and sea lions, and Odobenidae, the walrus.

Northern elephant seals are distributed along the west coast of North America from mid-Baja California, Mexico, to the eastern Aleutian Islands in Alaska (Stewart et al., this volume). The distribution of southern elephant seals is circumpolar on island and mainland sites, from the Antarctic continent to Patagonia, but concentrated on subantarctic islands (Laws, this volume).

The origins of elephant seals, like all pinnipeds, are obscure, but there is general agreement that phocids originated in the Tethyan-Mediterranean part of what is now Asia during the middle Miocene, about 15 to 20 million years ago (King 1964; Hendey 1972; Hendey and Repenning 1972; Repenning 1980). J. L. Davies (1958), drawing heavily on J. E. King (1956), argues that Antarctic seals derived from ancestral monachinoid seals that spread southward to the Caribbean at the time of seawater cooling in the Miocene and thus took advantage of the absence of the Central American isthmian barrier and entered the East Pacific cold-water route to the south. C. A. Repenning (1980) also thinks that phocid seals first invaded the south during the Miocene, approximately 10 million years ago. The earliest fossil records of seals in Argentina and South Africa date from this time.

J. L. Davies (1958) and Q. B. Hendey (1972) argue that the elephant seal genus developed in the Antarctic and the present species, M. leonina , colonized most of the anti-Boreal zone. They reason that elephant seals reinvaded the Northern Hemisphere by retracing the ancestral route along the west coast of South America during a Pleistocene glacial age, a time when passage to the Caribbean was closed. When the rewarming of the seas occurred, the group in the lower California region, which we now know as M. angustirostris , was cut off from the main elephant seal population to the south. According to Hendey (1972:108), "Mirounga angustirostris can thus be regarded as a relict species, surviving in isolation far from the origins of the genus."

K. T. Briggs and G. V. Morejohn (1976) present a different interpretation. They argue that the presence of fossil elephant seals in California and relative primitiveness of the northern species (more complicated teeth and reduced sexual dimorphism) are inconsistent with a putative southern

origin. They propose that the genus originated in the subtropics from unspecialized ancestors of modern monk seals. Ancestors of Mirounga entered the Pacific by way of the Central American Seaway. Uplift of the Tertiary Central American Seaway and poleward retreat of elephant seal populations, due to climatic conditions, led to geographic isolation, speciation, and the present distribution of the two species. According to this interpretation, the northern species is the older of the two. The southern species either evolved directly from the northern congener or they shared a common ancestor.

The poor fossil record does not permit an adequate test of these competing hypotheses. It is also difficult to say how long the two species have been separated. Perhaps the separation was as recent as the last major glaciation, which ended about 5 to 10 thousand years ago, or possibly it began as long ago as the early Pleistocene. Elephant seals were in California 100 to 130 thousand years ago as revealed by fossils found in the San Diego formation (Miller 1971), about the time of the beginning of the last glaciation. Other fossil material from California may be from 1 to 4 million years old, but whether the material is definitely Mirounga is in question (C. A. Repenning, pers. comm.). That the two species are considered distinct and differ in many ways argues for a separation long enough to have permitted the evolution of different structures and behaviors. The distance separating the breeding ranges of the two species today is more than 8,000 km, and there is no evidence of intermingling.