Back to the sea, Carl Zimmer

Discover, January 31, 1995

If evolution is a movie, it’s the job of paleontologists to look for the lost footage. This past year they came out of the vaults with an awesome director’s cut of one of the strangest films ever made: A Whale Is Born.

For decades researchers have claimed that whales are descended from an extinct hyenalike land mammal, called a mesonychid, that walked back into the sea between 50 and 60 million years ago. (Mesonychids and all other land mammals are themselves descended from a fish that crawled out of the sea much earlier.) By 40 million years ago the transition from four-legged land animal to fishlike ocean dweller was almost complete.

The evidence is a fossil whale called Prozeuglodon. It was perfectly adapted for life at sea–but as University of Michigan paleontologist Philip Gingerich discovered in 1989, it still carried, near the end of its 15-foot body, a pair of vestigial 6-inch legs.

Last year researchers announced the discovery of two new fossils in Pakistan that fill the void between these opening and closing scenes. In January, Hans Thewissen, a paleontologist and anatomist at Northeastern Ohio Universities College of Medicine in Rootstown, reported the first known whale with legs made for walking. Ambulocetus, as he dubbed it, lived 50 million years ago in coastal waters. The legs an this seven-foot-long creature were shorter than those of a mesonychid, but it still had feet with toes (and presumably webbing) that could carry its weight on land. It didn’t yet have the tail flukes that living whales use to swim; instead it flexed its spine up and down and kicked its legs, otter-style.

The year’s second missing-link whale was unveiled in April by Gingerich. Named Rodhocetus, this 46-million-year-old whale falls between the shore-hugging Ambulocetus and the water-bound Prozeuglodon. Rodhocetus’s legs were a third smaller than those of Ambulocetus, restricting it to a crocodile waddle on land. Its legs were shrinking because Rodhocetus no longer depended on them for swimming–massive tail vertebrae indicate that it had a powerful tail that allowed it to go where no whale had gone before. “Ambulocetus was pulling itself up on the shore every night, but Rodhocetus was probably out there for weeks at a time, more committed to the water,” says Gingerich. Within a few million years, whales like Prozeuglodon had given up land completely.

While there are many more primitive whales to be discovered, the evolutionary case is now closed. “We were making it up before,” says Gingerich. “Now we don’t have to.”