Marine Mammals II: The Pinnipeds

In my last article, I wrote about cetaceans, the whales and porpoises (which includes the dolphins). Another group of mammals that we associate with ocean life are the seals, known as pinnipeds. The name comes from the fact that their feet have been modified into flippers, or pinnae, and means "flipper foot".

Unlike the cetaceans, which are completely adapted for life in the water, the pinnipeds regularly come out of the water. While cetaceans spend their entire lives in the water, and even give birth underwater, pinnipeds spend most of their lives out of the water, and really only enter the water to hunt for food. The exception is the Sirenians (in the Odobenidae family), or sea cows. The sea cows hae become completely adapted to aquatic life and are herbivores, they include the manatees and the dugong.

The pinnipeds are divided into three families: the Otariidae or eared seals, the Odobenidae or walruses, and the Phocidae or earless seals. The Otariidae and Odobenidae were long believed to have developed from bear-like ancestors, while the Phocidae were believed to have developed from members of the mustelid family, which includes weasels, badgers, skunks, and otters. The current opinion favors a single origin for all three families from a common ancestor of the mustelid family.

Early pinnipeds are relatively scarce in the fossil record. It is believed that they developed in the northern hemisphere during the late Oligocene Epoch (36 to 22.5 million years ago). They are not found in the southern hemisphere until the Miocene Epoch (22.5 to 5 million years ago), 10 million years later.

The Otariidae first appeared in the early Miocene Epoch (22.5 to 5 million years ago) along the Pacific coast of North America. The earliest known member of the Otariidae was Enaliarctos, a primitive-looking sea mammal that has characteristics of both its mustelid ancestor and a sealion. Its body was streamlined like an otter, with distinct legs and tail (while modern pinnipeds have greatly reduced or flipper-like tails), and its feet still resembled that of the mustelids, although they were becoming more flipper-like. Enaliarctos probably lived like a modern sea otter, spending time both on land and in the water, eating a variety of marine animals, including both fish and shellfish. Their ears had already become specialized for life underwater. The modern Otariidae include sealions and fur seals.

The Odobenidae, or walruses, differ from the sealions and eared seals in that they feed primariliy on shellfish rather than fish. Their upper canine teeth are greatly enlarged into a pair of heavy tusks that they use to probe mollusks from the seabed. About 5 million years ago, in the early Pliocene Epoch (5 million to about 10,000 years ago), five genera of walruses lived on the northern Pacific coast. They spread further north and crossed the seaway that separated North and South Amrica to American and European north Atlantic coastal waters. During the late Pliocene Epoch, walruses became extinct in the Pacific, where they had originated, but the populations in the north Atlantic continued to flourish and eventually made their way back to the north Pacific by way of the Arctic Ocean about 1 million years ago.

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