Evolution of marine mammals

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.

Pakicetus 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.

Ambulocetus ("walking whale"), which lived between 48 and 41 million years ago, while still four-limbed, was probably fully aquatic: it spent all of its time in the water. The size of a modern male sea lion, Its legs were short with large webbed feet, perfect for swimming rather than walking. Ambulocetus was more whale-like than Pakicetus in other ways: its body was largely hairless; it lacked external ears; its eyes faced sideways (rather than forwards); its nostrils pointed upwards.

Basilosaurus

Toothed whales, represented by today’s dolphins and sperm whale, appeared in late Eocene times. Baleen whales, among them ancestors of modern species such as the blue whale, evolved later, during the Oligocene Epoch.

By the mid-Eocene, huge whales like the 20-metre-long (66-foot-long) Basilosaurus had evolved. As the skeleton shows, it still had tiny hind limbs and may even have been capable of still coming ashore as seals, sea lions and walruses do today.

Basilosaurus swam by wriggling its long, serpent-like body in the water and using its paddle-like front limbs for steering. It had triangular slicing teeth in its small head. The “melon”, a fatty organ modern whales have above their eyes and which they use to echolocate, was yet to develop. Basilosaurus probably still hunted by eye.

Seals and sea lions can move around on land, but they spend most of their time in the water. Their paddle-shaped, webbed feet propel them along. Like whales, they are coated with blubber and can dive to considerable depths for long periods. They became a separate group from the carnivores in the late Oligocene, about 30 million years ago. Scientists think that seals and sea lions shared a common ancestor, and walruses are descended from early seals.

Allodesmus

Allodesmus was a prehistoric sea lion from the Miocene, descended from the early sea lion Enaliarctos, that lived around 30–24 million years ago, and spent more time on land than in the sea. From its skeleton, it is possible to tell that Allodesmus had large eyes and dog-like ears. It probably relied on good eyesight to hunt for its prey. Its powerful front limbs were used for forward thrust.

Manatees

The sirenians, or sea cows (manatees and dugong), are the only herbivorous sea-going mammals. Along with the hyraxes, they may have descended from the same ancestor as the elephants, a browser in swamps and shallows during Palaeocene times. By the Eocene, sirenians were grazing the seabed in shallow waters, feeding on seagrasses. Their heavy rib bones enabled them to stay under water without difficulty.