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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.
Archelon is considered the ancestor of all modern sea turtles, and was an example of the move of some turtles back to fully aquatic conditions.
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.
Living at the same time as, but not closely related to, dinosaurs, the mosasaurs and pliosaurs resembled crocodiles but were more strongly adapted to marine life. They became extinct within a few million years of the dinosaurs. Modern diapsids which have adapted to marine life include marine iguanas and marine crocodiles.
Euryapsids These marine reptiles had ancestors who moved back into the oceans, In the case of Ichthyosaurs adapting as fully as the dolphins they superficially resemble, even giving birth to live offspring instead of laying eggs, in other cases more to the extent of the seal, as with plesiosaurs and placodonts.
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 theory.
Lutrins Otters have existed along the coast of California for about 5 million years.
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.
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.
The ancestors of the dugong and manatees first appeared in the fossil record about 45 to 50 million years ago in the Eocene.
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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