Whale evolution: a sexual footnote?

Whale evolution: A sexual footnote?

Paleontologists working in Egypt have unearthed fossils of a 40-million-year-old whale with feet, documenting an important step in the whale's evolutionary journey from land to sea.

The bones belonged to a ancient species of whale known as Basilosaurus isis, a 16-meter-long giant with a thin body, a small "pinhead" and flipper-like forelimbs. The new finds indicate the species also had a functional pair of hindlimbs, each composed of a femur, tibia, fibula and three-toed foot, report Philip D. Gingerich and B. Holly Smith of the University of Michigan in Ann Arbor and Elwyn L. Simons of Duke University in Durham, N.C. These represent the first functional hindlimbs found on any whale, they say.

But lanky and leggy this whale was not. Its rear limbs were tiny compared with its overall body size, the researchers note in the July 13 SCIENCE. From the shape of the knee, Gingerich and his colleagues speculate that the leg had an extremely limited range of motion. They propose that the legs were too small to assist in swimming but could have come into play during copulation, serving as guides to help orient the reproductive organs during the tricky act of cetacean sex.

On the flip side, Lawrence G. Barnes, a paleontologist with the Natural History Museum of Los Angeles County, thinks B. isis used its hindlimbs to free itself when mired in the muddy bottom of the shallow coastal sea that once covered northern Egypt.

Future finds may clarify the function of the limbs. Among mammals, body parts related to reproduction are often larger on males than on females. If scientists find some early whale specimens with large legs and others with smaller legs, that would suggest a reproductive role. On the other hand, uniform limb sizes would back the locomotion hypothesis, says Barnes.

Whales evolved from four-limbed amphibious mammals that in turn developed from four-legged land mammals. The oldest known whale fossils date back about 50 million years but reveal little about the legs of these animals.

The Egyptian fossils resolve a standing debate among paleontologists, says Barnes. Most researchers believe that whales lost their hindlimbs long before B. isis arrived on the scene -- implying the creatures adapted relatively quickly to their watery environment. Barnes has proposed that ancient cetaceans retained the limbs much longer. But until now, his theory lacked a leg to stand on. The feet of B. isis, he says, suggest its predecessors possessed even better-developed limbs.