Pinniped Pinniped Pinnipeds, commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic marine mammals. They comprise the extant families Odobenidae (whose only living member is the walrus), Otariidae (the eared seals: sea lions and fur seals), and Phocidae (the earless seals, or true seals). There are 33 extant species of pinnipeds, and more than 50 extinct species have been described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage (descended from one ancestral line). Pinnipeds belong to the order Carnivora and their
Mammal dogs, seals, and others). In cladistics, which reflect evolution, mammals are classified as endothermic amniotes. They are the only living Synapsida, which together with the Sauropsida (reptiles and birds) form the Amniota clade. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian "Dimetrodon". At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals (Therapsida) in the early Mesozoic era. The