Comparative description of a female Enaliarctos emlongi (Carnivora, Pinnipedimorpha) from the mid-Miocene of Oregon and the evolution of sexual dimorphism within Pinnipedia

Sexual selection is an important evolutionary process effecting the behaviour and morphology of many animal groups. Sexual size dimorphism (SSD) often occurs within species as a result of differing mating systems. An important example of this can be observed within pinnipeds. Modern members of this group show two main mating systems: a land based harem system with extreme SSD, and a water based system with minor SSD. How this system evolved is poorly understood, and is analyzed here using a combination of fossil comparisons and geometric morphometrics on modern species. A previously undescribed specimen of Enaliarctos emlongi, a fossil relative of pinnipeds, was examined and found to most likely represent an adult female individual, with another known E. emlongi specimen representing an adult male. Geometric morphometric and discriminant function analysis showed that species with significant SSD could also be distinguished sexually through cranial shape. Otariids were found to show consistent patterns of cranial diversity across all sampled species, whereas phocids showed a much more variable pattern. E. emlongi was found to plot within otariids in cranial morphospace, suggesting a morphological stability between primitive pinnipeds and modern otariids. This, when added to previously noted fossil evidence, suggests that extreme SSD and the related mating systems are plesiomorphic in pinnipeds. This may also have implications for our understanding of the ecology and biogeography of this group, and our broader understanding of sexually selective evolutionary processes.