Abstract
New crania, dentitions, and postcrania of the fossil otariid Thalassoleon mexicanus are described from the latest Miocene–early Pliocene Capistrano Formation of southern California. Previous morphological evidence for age variation and sexual dimorphism in this taxon is confirmed. Analysis of the dentition and postcrania of Thalassoleon mexicanus provides evidence of adaptations for pierce feeding, ambulatory terrestrial locomotion, and forelimb swimming in this basal otariid pinniped. Cladistic analysis supports recognition of Thalassoleon as monophyletic and distinct from other basal otariids (i.e., Pithanotaria, Hydrarctos, and Callorhinus). Re-evaluation of the status of Thalassoleon supports recognition of two species, Thalassoleon mexicanus and Thalassoleon macnallyae, distributed in the eastern North Pacific. Recognition of a third species, Thalassoleon inouei from the western North Pacific, is questioned.
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