Odd as it may seem, a four-footed land mammal named Pakicetus, living some 50 million years ago in what we know as Pakistan today, bears the title of “first whale.”

Artist’s impression of Pakicetus attocki

© Carl Buell, 2006

Straddling the two worlds of land and sea, the wolf-sized animal was a meat eater that sometimes ate fish, according to chemical evidence. Pakicetus also exhibited characteristics of its anatomy that link it to modern cetaceans, a group made up of whales, porpoises, and dolphins.

A resin cast of Pakicetus, based on fossils found in Pakistan, was displayed in the special exhibition Whales: Giants of the Deep.

See skeletons of early whales cast from fossils in Whales: Giants of the Deep.

© Museum of New Zealand Te Papa Tongarewa, 2008

First discovered by paleontologists in 1983, Pakicetus lived along the margins of a large shallow ocean, the Tethys Sea. Although it had the body of a land animal, its head had the distinctive long skull shape of a whale’s.

Over time, fossils also revealed that Pakicetus had an ear bone with a feature unique to whales and an ankle bone that linked it to artiodactyls, a large order of even-toed hoofed mammals that includes hippos, pigs, sheep, cows, deer, giraffes, antelopes, and even cetaceans, the only aquatic artiodactyls.

A type of baleen whale, humpback whales reach up to 50 feet long.

Talia Romito and Sophie Webb/Courtesy of NOAA

Though rare, mammal species adapting to life in the sea has happened at least seven times in different major groups of mammals.

Still, this reverse pattern accounts for some 100 living mammal species that inhabit the oceans today, from three major groups.

Cetaceans aren't the only mammals in the sea. About 100 mammal species—including sea otters—live in the oceans today.

Wikimedia Commons/Mike Baird

The groups are cetaceans within Artiodactyla, as noted above; Carnivora, specifically seals, sea lions, and walruses (the “pinnipeds”) and an independent invasion of the oceans by sea otters; and Sirenia, which includes several species of aquatic manatees and dugongs—which live in rivers and shallow coastal waters and eat mainly seagrasses.