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Age of Mammals

Evolution of marine mammals

RodhocetusRodhocetus Three mammal groups are water-dwelling animals: the cetaceans (whales, dolphins and porpoises), the sirenians (manatees and dugongs) and the pinnipeds (seals, sea lions and walruses). Quite independently, animals from each of these groups evolved bodies designed for survival in the water. Other mammals, for example otters, beavers or hippos, spend a great deal of time in the water, but they also live on land, so their bodies are not so fundamentally altered.

Whales and dolphins

PakicetusPakicetusCetaceans are the most specialized marine animals: they have fins and tails just like fish, but also flippers for steering. They would die if stranded out of water. It is difficult to imagine that the ancestors of whales once lived on land, but they did.

Pakicetus hunting for fishPakicetus hunting for fishPakicetus looked very different from modern cetaceans—more like a land-dwelling, hoofed mammal with its body covered in short fur. It lived in what is now Pakistan between 52 and 48 million years ago, during the early Eocene. Measuring up to 2 metres (6.6 feet) in length, it had four legs, a tail, a long and a dog-like snout. Pakicetus spent much of its time in shallow waters feeding on fish and small animals, so it probably had webbed feet to help it swim quickly. Widely accepted to be an ancestor to all cetaceans, it was probably an ancestor to hippos as well.

Whales are related to the even-toed ungulates, hoofed land animals. They share a common ancestor with the hippos.

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