Cetacea
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, dolphins, and porpoises. The cetacea are extensively adapted to marine life and cannot survive on land at all. Their adaptation can be seen in many unique physiognomic characteristics such as the dorsal blowhole, baleen teeth, and the cranial 'melon' organ used for aquatic echolocation. The closest extant terrestrial relative to the whale is the hippopotamus, which spends much of its time in the water and whose name literally means "horse of the river."
Read more about this topic: Aquatic Adaptation