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All fur seals, sea lions, walruses and true seals belong to the order of mammals called Pinnipedia (from the Latin meaning wing or feather footed). Within this order fur seals and sea lions belong to the Otariidae family, while true seals belong to the family Phocidae. These two families evolved quite separately and show several differences in their anatomy and adaptations to land and water movement. Walruses belong to family Odobenidae which stemmed from an early branch of the pinniped line. They are unique and show similarities to both the Otariid and Phocid groups.
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It is thought that members of this family probably had a bear-like ancestor. They are characterized by visible ear pinnae, long, winding necks, fore flippers which are long and almost hairless, with thin webbing and rudimentary claws. Hind flippers are long, webbed and almost hairless. They can be turned forward for movement on land. The two outer hind flipper claws are rudimentary while the middle three are well developed. Their tails are distinct and free from the body. They move themselves through the water by using penguin-like strokes of the fore flippers. Movement on land is achieved by supporting the body on all four limbs.
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Creatures belonging to this family are thought to have evolved from an otter-like ancestor. They have no ear pinnae and no clearly defined neck region. Fore flippers are short and haired with each toe joined in thick tissue and each toe terminating in a prominent claw. Hind flippers are of mid length, haired with extensive thin webbing and claws on all five toes. Their hind flippers are directed backward and cannot be turned forward. Tails are distinct and free from the body. They swim by flexing the sides of the hind part of the body and side to side sculling of the webbed hind flippers. Movement on land appears quite clumsy but can be quick. The seal crawls by hitching the front flippers forward then humping the body like an inchworm to advance the hindquarters.
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Walruses have no ear pinnae and the ear canal is covered by a fold of skin. They have a thick, robust, sparsely haired to nearly hairless body. Their fore flippers are similar to the sea lion, but are relatively shorter, sparsely haired, webbed and can be turned forward. Claws are rudimentary on outer two toes but well developed on the middle three. Their tail is enclosed in a web of skin. Walruses have highly specialized teeth and very long upper canines. Their locomotion in water involves both sea lion-like fore flipper strokes and seal-like sweeps of the hind flippers. On land they move supported on all four limbs.
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