Pinniped Evolution

Abstract

The name Pinnipedia was first proposed for fin-footed carnivores more than a century ago. Pinnipeds—fur seals and sea lions, walruses and seals—are one of three major clades of modern marine mammals, having a fossil record going back at least to the late Oligocene. There has long been a debate about the relationship of pinnipeds to one another and to other mammals. The traditional view, also referred to as diphyly, proposes that pinnipeds originated from two carnivore lineages, an odobenid (walrus) plus otariids (fur seals and sea lions) grouping affiliated with ursids (bears) and phocids (seals) being related to mustelids. The fur seals and sea lions (eared seals), the Otariidae, are diagnosed by frontals that extend anteriorly between the nasals, large and shelf-like supraorbital process of the frontal, secondary spine dividing the supraspinous fossa of the scapula, uniformly spaced pelage units, and by the presence of a trachea with an anterior bifurcation of the bronchi. The walruses or Odobenidae are diagnosed as a monophyletic group by the presence of a broad, thick pterygoid strut, fourth upper premolar with a strong posterolingually placed protocone shelf, lower first molar with the talonid heel absent, and a calcaneum with a prominent medial tuberosity. The purported first appearance of phocids in the North Atlantic suggests that the common ancestor of phocids had migrated to the North Atlantic, either northward through the Arctic Basin or southward through the Central American Seaway. The biogeographic pattern for phocine seals is no less complicated given the different phylogenetic hypotheses proposed. An extinct family of archaic pinnipeds, the desmatophocids is characterized by elongate skulls, relatively large eyes, mortised contact between two cheekbones, and bulbous cheek teeth.