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Odobenidae (walruses)
Otariidae (fur seals and sea lions)
Phocidae (true seals)

Recent molecular evidence suggests that pinnipeds evolved from a bearlike ancestor about 23 million years ago during the late Oligocene or early Miocene epochs, a transitional period between the warmer Paleogene and cooler Neogene period.^[1] The earliest fossil pinniped that has been found is Enaliarctos, which lived 24 – 22 million years ago. It is believed to have been a good swimmer, but to have been able to move on land as well as in water, more like an otter than like modern pinnipeds. There has been longstanding debate as to whether walruses diverged from a common otariid-phocid ancestor, or whether the phocids diverged before a common otariid-odobenid ancestor. The most recent evidence suggest that the latter hypothesis is more likely.^[2]

Pinnipeds are typically sleek-bodied, barrel-shaped and can be rather large. Their bodies are well adapted to the aquatic habitat where they spend most of their lives. Their limbs have evolved into short, wide, flat flippers. The smallest pinniped, the Galapagos fur seal, weighs about 30 kg (65 lb) when full-grown and is 1.2 m (4 ft) long; the largest, the male southern elephant seal, is over 4 m (13 ft) long and weighs up to 2,200 kg (4,850 lb, more than 2 tons). In the Phocidae the two back flippers have evolved into a tail-like structure which can no longer be used for walking on land.

The walrus is an exclusively Arctic species - the sole surviving member of the once diverse and widespread Odobenidae family. They are easily recognized by their long tusks and great bulk (up to 2000 kg). While they share with otariids the ability to turn their rear flippers forward, their swimming is more reminiscent of that of true seals, relying more on sinuous whole body movements. They also lack external ears. Unlike eared seals and true seals, which feed primarily by hunting fish and squid in the water column, walrus generally prefer benthic invertebrates, in particular clams. It is the development of the unique squirt and suck method of feeding on mollusks that differentiated the original walrus ancestor from the other pinniped lineages. There remains debate as to whether the walrus diverged from the eared seals before or after the true seals.

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