Several animal 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 generations 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.

Anapsids

Archelon, was a genus giant Cretaceous sea turtle.

Mesosaurus (and other mesosaurids) were one of the first reptiles to secondarily return to the sea.

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.

Diapsids

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.

Humans

Proponents of the Aquatic Ape Hypothesis believe that part of human evolution includes some aquatic adaptation, which has been said to explain human hairlessness, bipedalism, increased subcutaneous fat, descended larynx, vernix caseosa, a hooded nose and various other physiological and anatomical changes. In addition, there have been numerous inventions of devices by humans to adapt to the aquatic environments.

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.

Polar Bears

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

Sirenians

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

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