Description
Pinnipeds (seals, sea lions and walruses) are a small but diverse clade of marine mammals that reentered the water in the late Oligocene (27-25 Ma). Allodesmus is a genus of extinct pinniped in the family Desmatophocidae, whose phylogenetic position has long been debated as being more closely related to phocids (seals) or otariids (sea lions). Previous research has allied desmatophocids with otariids in the clade Otarioidea (otariids, desmatophocids and walruses), or desmatophocids with phocids in the clade Phocoidea (phocids, desmatophocids and walruses). The first objective of this study was to resolve interfamilial relationships within Pinnipedia and determine the placement of desmatophocids. Both molecular and morphological data were used. Molecular data were compiled for all extant species in the analysis and included 3 nuclear (IRBP, RAG1, SRY) and 2 mitochondrial (CTYB, ND2) genes totaling 5,196 base pairs, and partitioned by codon position. Morphological data were collected for all species included in the study and consisted of 75 characters (49 cranial, 26 postcranial). Both parsimony and Bayesian analyses were conducted, first individually on the separate molecular and morphological datasets, then on the combined dataset. Both analyses supported the validity of Otarioidea, containing otariids, odobenids (walruses), and desmatophocids with high support (BS = 71, PP = 0.72), and within Otarioidea, otariids and odobenids are sister taxa. The second objective of this study was to evaluate the evolutionary pattern of aquatic locomotion in pinnipeds. Phocids and otariids show such differing and distinctive aquatic locomotor styles, and the swimming method employed by extinct pinnipeds has been debated. Ancestral state reconstruction was performed for all postcranial characters showing a correlation with either hind or forelimb swimming. Percentage association with forelimb swimming was calculated for fossil pinnipeds with postcranial material and hypothesized for the ancestral pinniped and otarioid ancestor. Based on these calculations, the otarioid ancestor used a relatively equal combination of hind and forelimb propulsion, desmatophocids were combination swimmers that relied more heavily on their forelimbs for propulsion, and the ancestral pinniped was a combination swimmer that relied more heavily on hind limb propulsion. Therefore, it is likely that pinnipeds entered the water as hind limb dominated swimmers, with phocids and otarioids each specializing toward a different kind of locomotion (hind or forelimb). While the transition was almost completely polarized in phocids and otariids, odobenids maintained the ancestral condition, a combination of hind and forelimb swimming with hind limb dominating, while desmatophocids developed a combination style with forelimb dominating.
