Swimming Mode Inferred from Skeletal Proportions in the Fossil Pinnipeds Enaliarctos and Allodesmus (Mammalia, Carnivora)

Swimming modes are crucial for understanding evolutionary transitions from land to sea, because locomotion affects many aspects of an animal's life. The modern pinniped families Otariidae (fur seals and sea lions), Phocidae (true seals), and Odobenidae (walruses) are thought to share a common origin, but each differs in its primary mode of aquatic locomotion. Previous studies of locomotor evolution in pinnipeds suggested: (1) forelimb swimming was ancestral; (2) hind limb swimming evolved once at the base of the clade including Phocidae, Odobenidae, and the extinct Desmatophocidae; and (3) reversal to forelimb swimming occurred in the odobenid subfamily Dusignathinae. The oldest and most basal pinnipedimorph Enaliarctos mealsi has been portrayed as a forelimb swimmer, and the desmatophocid Allodesmus kelloggi has been portrayed as a hind limb swimmer. These interpretations have been questioned by others and are tested here. Principal components analysis of trunk and limb measurements from 58 modern semiaquatic mammals demonstrates that Enaliarctos is most similar in skeletal proportions to hind limb-dominated swimmers, whereas Allodesmus is most similar to forelimb-dominated swimmers. Principal components and discriminant function analyses of trunk and limb measurements from 24 modern pinniped species demonstrate that Enaliarctos is most similar to hind limb-swimming phocids, while Allodesmus is most similar to forelimb-swimming otariids. These interpretations complicate previous portrayals of swimming evolution in pinnipeds and can paint a very different picture of how this behavior evolved when viewed in the context of alternative phylogenetic hypotheses.