Pinniped
Taxonomy

The German naturalist Johann Karl Wilhelm Illiger was the first to recognize the pinnipeds as a distinct taxonomic unit; in 1811 he gave the name Pinnipedia to both a family and an order. American zoologist Joel Asaph Allen reviewed the world's pinnipeds in an 1880 monograph, History of North American pinnipeds, a monograph of the walruses, sea-lions, sea-bears and seals of North America. In this publication, he traced the history of names, gave keys to families and genera, described North American species and provided synopses of species in other parts of the world. In 1989, Annalisa Berta and colleagues proposed the unranked clade Pinnipedimorpha to contain the fossil genus Enaliarctos and modern seals as a sister group. Pinnipeds belong to the order Carnivora and the suborder Caniformia (known as dog-like carnivorans). Of the three extant families, the Otariidae and Odobenidae are grouped in the superfamily Otarioidea, while the Phocidae belong to the superfamily Phocoidea. There are 34 extant species of pinnipeds, and more than 50 fossil species of pinnipedimorphs.

Otariids are also known as eared seals due to their pinnae. These animals swim mainly using their well-developed fore-flippers. They can also "walk" on land by shifting their hind-flippers forward under the body. The front end of an otariid's frontal bone protrudes between the nasal bones, with a large and flattened supraorbital foramen. An extra spine splits the supraspinatous fossa and bronchi that are divided in the front. Otariids consist of two types: sea lions and fur seals; the latter typically being smaller in size with pointier snouts, longer fore-flippers and heavier fur coats. Five genera and seven species (one now extinct) of sea lion are known to exist, while two genera and nine species of fur seal exist. While sea lions and fur seals have historically been considered separate subfamilies (Otariinae and Arctocephalinae respectively), genetic and molecular evidence has refuted this, indicating that the northern fur seal is basal to other otariids and the Australian sea lion and New Zealand sea lion are more closely related to Arctocephalus than to other sea lions.

Odobenidae has only one living member: the walrus. This animal is noticeable from its larger size (exceeded only by the elephant seals), nearly hairless skin, flattened snout and long upper canines, known as tusks. Like otariids, walruses can walk on land with their hind limbs. When moving in water, the walrus relies on its hind limbs for locomotion, while its forelimbs are used for steering. Also, it has no outer ears. The epipterygoid of the jaw is well developed and the back of the nasal bones are horizontal. In the feet, the calcaneuses protrude in the middle.

Phocids are known as true or "earless" seals. These animals lack outer ears and cannot position their hind-flippers to move on land, making them more cumbersome. This is because of their massive ankle bones and flatter heels. In water, true seals rely on the side-to-side motion of their hind-flippers and lower body to move forward. The phocid's skull has thickened mastoids, puffed up entotympanic bones, nasal bones with a pointed tip in the back and a non-existent supraorbital foramen. The hip has a more converse Ilium. A 2006 molecular study supports the division of phocids into two monophyletic subfamilies: Monachinae, which consists of elephant seals, monk seals and Antarctic seals; and Phocinae, which consists of all the rest.
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