National Geographic : 2010 Aug
It wasn't until 1989, however, that the team found the link they were seeking to the whales' terrestrial ancestors, almost by accident. Near the end of the expedition Gingerich was work- ing on a Basilosaurus skeleton when he un- covered the rst known whale knee, on a leg positioned much farther down the animal's spinal column than he had expected. Now that the researchers knew where to look for legs, they revisited a number of previously mapped whales and rapidly uncovered a femur, a tibia and bula, and a lump of bone that formed a whale's foot and ankle. On the last day of the expedition Smith found a complete set of slen- der, inch-long toes. Seeing those tiny bones brought her to tears. "Knowing that such mas- sive, fully aquatic animals still had functional legs, feet, and toes, realizing what this meant for the evolution of whales---it was overwhelm- ing," she remembers. ough unable to support a Basilosaurus's weight on land, these legs weren't completely vestigial. ey had attachments for powerful muscles, as well as functional ankle joints and complex locking mechanisms in the knee. Gin- gerich speculates that they served as stimula- tors or guides during copulation. "It must have been hard to control what was going on down there on that long, snakelike body, so far from the brain," he says. Whatever Basilosaurus actually did with its little legs, nding them con rmed that the an- cestors of whales had once walked, trotted, and galloped on land. But the identity of these an- cestors remained unclear. Certain skeletal fea- tures of archaic whales, particularly their large, triangular cheek teeth, strongly resembled those of mesonychids, a group of hoofed Eocene car- nivores. ( e massive, hyena-like Andrewsar- chus, probably the largest carnivorous mammal that has ever lived on land, may have been a mesonychid.) In the 1950s immunologists had discovered characteristics in whale blood that suggested a descent from artiodactyls, the mammalian order that includes pigs, deer, cam- els, and other even-toed ungulates. By the 1990s "Fossil whales are one of God's miracles," says Mohammed Sameh (at left), Wadi Hitan's head ranger, reassembling a Dorudon skeleton with University of Michigan postdoc Iyad Zalmout. The site's rare prehistoric whales helped earn it UNESCO World Heritage status.