Adaptive Evolution of Sea Lions and Walruses

Abstract

Repenning, Charles A., (U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025) 1976. Adaptive evolution of sea lions and walruses. Syst. Zool. 25:375-390.The order Otarioidea includes two living and two extinct families. Of these four, the extinct Enaliarctidae were ancestral to the others and were derived from canoid fissiped carnivores. This family, which was most diverse from 22 to 16 million years ago, rapidly developed the locomotor, cerebral, and olfactory adaptations to marine life. The extinct Desmatophocidae and the extant Odobenidae (walruses) and Otariidae (sea lions) evolved from the Enaliarctidae at different times and in the order in which they have been mentioned. Each of these three went through their period of greatest diversity in the same order; the Otariidae of today represent the period of greatest diversity of this family. Each of these three independently developed similar dental, vascular, and social adaptations to marine existence but in different ways. All families were native to the North Pacific. The odobenids alone entered the North Atlantic through the Central American Seaway about 8 million years ago, and the otariids alone invaded the southern hemisphere by 5 million years ago. There are at least eight types of marine adaptations recognizable to -some degree in the skeletal morphology of the marine carnivores of the North Pacific Ocean, the sea lions and the walruses. These adaptations involve hearing, seeing, smelling, eating, oxygen conservation, body heat conservation, locomotion, and behavior in both social and environmental contexts. As noted by Bartholomew (1970) in his analytical hypothesis of the evolution of of pinniped polygyny, each adaptive specialization has an effect on others and develops in harmony with others so that, viewed in