A FORELIMB-DOMINATED SWIMMING MODE IS PLESIOMORPHIC FOR OTARIOID PINNIPEDS (MAMMALIA, CARNIVORA) AS REVEALED BY GEOMETRIC MORPHOMETRIC ANALYSIS OF HUMERAL MORPHOLOGY

Among modern marine mammals, pinnipeds are the only clade not to employ tail-propelled dorsoventral undulation as their primary swimming mode. Instead, phocids (true seals) utilize lateral pelvic oscillation of their hindlimbs, otariids (eared seals) pectoral oscillation of their forelimbs, and odobenids (walruses) an intermediate mode with forelimb propulsion at low speeds and lateral hindlimb oscillation at higher speeds. The evolutionary origin of these swimming modes remains indeterminate. Previous studies utilizing nearly complete specimens of stem pinnipeds Puijila darwini and Enaliarctos mealsi have supported the hypothesis that hindlimb-propelled swimming is plesiomorphic for pinnipeds. Our understanding of locomotor evolution at the base of crown Pinnipedia has been curtailed by the lack of complete specimens, particularly in the Otarioidea (odobenids, otariids, and the extinct desmatophocids).

Carnivoran forelimb morphology is influenced by locomotor mode, so geometric morphometric investigation of pinniped humeral morphology offers an opportunity to explore locomotor evolution in taxa represented only by isolated skeletal elements. Consequently, I performed two canonical variates analyses (CVA) on a preliminary dataset of 28 three-dimensional humeral landmarks taken from 58 modern marine/aquatic carnivorans (2 polar bears, 6 river otters, and 47 pinnipeds). The first CVA evaluated whether principal component variation could differentiate among the ursids, lutrines, and pinnipeds. The second CVA evaluated the pinniped data alone, testing whether I could distinguish between the primarily hindlimb swimmers (phocids and odobenids) and the forelimb swimmers (otariids). Both analyses yielded a cross-validated classification rate of 100%. When provided with landmark data from stem pinnipeds Potamotherium valletoni and Pinnarctidion rayi, each was classified as most similar to pinnipeds in the first CVA and as most similar to the hindlimb-propelled crown pinnipeds in the second. Notably, these analyses also categorized an unspecified desmatophocine specimen, derived desmatophocid Allodesmus kernensis, the early walrus Imagotaria downsi, and the more derived odobenid Valenictus imperialiensis as forelimb swimmers. These results suggest that forelimb-dominated locomotion is the plesiomorphic condition for Otarioidea.