Aquatic adaptation

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Several mammal groups have undergone aquatic adaptation, going from being purely terrestrial animals to living at least part of the time in water. The adaptations in early speciation tend to develop as the animal ventures into water in order to find available food. As successive genereations spend more time in the water, natural selection causes the acquisition of more adaptations. Animals of later generations may spend the majority of their life in the water, coming ashore for mating. Finally, fully adapted animals may take to mating and birthing in water.

Cetaceans

During the Paleocene Epoch (about 55-65 million years ago), a group of wolf-like artiodactyls related to Pakicetus began pursuing an amphibious lifestyle in rivers or shallow seas. They were the ancestors of modern whales. For more details, see Evolution of cetaceans.

Sirenians

The ancestors of the dugong and manatees first appeared in the fossil record about 45 to 50 million years ago in the Eocene.

Pinnipeds

The fossil records show that phocids existed 12 to 15 million years ago, and odobenids about 14 million years ago. Their common ancestor must have existed even earlier than that.

Lutrins

Otters have existed along the coast of California for about 5 million years.

Polar Bears

Although still primarily a terrestrial animal, the polar bear shows the beginnings of aquatic adaptation to swimming (body fat, closable nostrils), diving, and thermoregulation. Distinctly polar bear fossils can be dated to about 100,000 years ago.

Humans

Some people believe that part of human evolution includes some aquatic adaptation, which has been said to explain human hairlessness, webbed digits, bipedal locomotion, and various other physiological changes. See the Aquatic ape hypothesis.

References

Seaworld - Animal Infobooks (http://www.seaworld.org/animal-info/info-books/index.htm)
Extinct Animals (http://www.meta-religion.com/Paranormale/Cryptozoology/Extinct_animals/gallery_extinct_animals.htm)
The Otter Project (http://www.otterproject.org/site/pp.asp?c=8pIKIYMIG&b=33672)

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