Fossil whale feet: a step in evolution.

Paleontologists digging in Pakistan have discovered the 50-million-year-old remains of a whale with legs and feet - a missing link in the evolutionary chain connecting aquatic cetaceans with their landlubbing forebears.

The new-found fossil, called Ambu1ocetus natarts, is the first known ancient whale with large, functional hind limbs, says J.G.M. Thewissen of the Northeastern Ohio Universities College of Medicine in Rootstown. He and his colleagues report their find in the Jan. 14 SCIENCE.

Paleontologist Annalisa Berta of San Diego State University calls Ambulocetus "a very significant discovery. It shows us for the first time a whale that had welldeveloped hind limbs. It's very clear this animal was using its hind limbs in locomotion."

Researchers believe that modern whales descended from four-legged carnivorous mammals, somewhat like large wolves, that once roamed the continents. Sometime around the start of the Eocene period, 57 million years ago, these carnivores gave up their dry lifestyle for one under the waves, forcing their bodies to undergo a profound evolutionary transformation. Among the changes, ancient whales lost their legs and pelvises and developed the characteristic fluked tail that propels these modern leviathans through the seas.

Living whales have no visible hind limbs, but some have internal finger-size bones that are vestiges of hips and legs, an indication that they evolved from land creatures. Three years ago, Philip D. Gingerich of the University of Michigan in Ann Arbor reported the discovery of a 40-million-year-old whale in Egypt that had external legs, although they were too small to help propel the animal (SN: 7/14/90, p.21). Gingerich suggested that this animal, called Basilosaurus, used its tiny hind limbs to grasp its partner during copulation.

While Basilosaurus most likely spent all of its time in water, Thewissen suggests that the sea-lion-size Ambulocetus led an amphibious lifestyle. Ambulocetus could walk on land, but the shape of its bones suggests it had weak hind limbs. It may have walked by dragging its body as sea lions do, Thewissen says.

In the ocean, Ambulocetus probably swam by flexing its back up and down, using the surface area of its big feet to push against the water, Thewissen suggests. Modern whales swim with the same undulating motion, but their broad tails provide the propulsion surface.

Berta argues that paleontologists will need to find more fossils to decipher how Ambulocetus swam. Because Thewissen did not unearth a pelvis with the rest of the bones, he cannot determine how the whale's legs attached to the rest of its skeleton, a critical factor in understanding the animal's locomotion.

The Ambulocetus discovery is one of several recent fossil finds that has spurred interest in early whale evolution. While in Pakistan in 1991 and 1992, Thewissen unearthed a well-preserved skull of Pakicetus, the oldest known whale species, which apparently spent much of its life on land.

Researchers in Georgia are studying an as-yet-unnamed species of whale discovered in that state. The 40-million-year-old creature had a pelvis like that of a land mammal, suggesting that the whale had large hind limbs, says Richard C. Hulbert Jr. of Georgia Southern University in Statesboro.

Early cetaceans have captured the attention of paleontologists because the trek from land to sea provides a dramatic picture of evolution at work, Hulbert says. Throughout history, other animals have made the same voyage, but whales are the most recent group, giving scientists their best chance at documenting this transition.