Fifty million years ago, where an ocean washed the shores of a drifting continent, a primitive race of walking whales moved from the land to the sea to open a new act on the evolutionary stage.
Now scientists have found the first fossil skeleton of such a unique whale - a swimming mammal whose four legs give evidence of the long-sought missing link between land animals and the whales, porpoises and river dolphins of today's marine world.The scientists discovered their fossil in the sediment of an ancient warm sea that covered what is now a remote part of Pakistan, halfway between Islamabad, the modern nation's capital, and the storied Khyber Pass that marks the border with Afghanistan.
Hans Thewissen, an anatomist at a small Ohio medical school who led the team that uncovered the nearly complete skeleton in 1992, has formally named the fossil creature Ambulocetus natans, which means "walking and swimming whale." It is, he said, the ancestor of the entire order of marine mammals collectively called cetaceans, and his discovery is being reported in the journal Science.
"It's a very important missing link that's not missing anymore," commented Philip Gingerich, a paleontologist at the University of Michigan who developed theories about the creature 10 years ago.
Scientists have long known that the first land animals emerged from the oceans about 370 million years ago, when a few primitive fishlike creatures developed lungs and legs to become the ancestors of today's amphibians. But the origins of marine mammals have remained more obscure.
Although most four-legged animals have remained on land ever since they climbed ashore, a few returned to the sea millions of years ago - perhaps to escape swift-running predators - even while they retained their limbs, Thewissen said.
The Dutch-born researcher, who has hunted fossils in Pakistan since 1985, said in an interview that during earlier expeditions he had found frustrating scraps of jawbones, teeth, knee joints and skulls but never enough skeletal remains to prove the 100-year-old theory that the earliest whales must have originated on land.
The knee joints found in the sedimentary rocks of the ancient seabed showed clearly that the marine mammals could walk, and their fossil ear bones were clearly designed to hear both on land and underwater, Thewissen said.
From the new find, he estimated that his walking whale must have been about the size of a sea lion, perhaps 10 feet long, and weighed between 600 and 700 pounds.
Its rear feet were huge, and its legs were fully developed, he said, while its front legs were short, stubby and joined close to its shoulders. Its snout was long like a modern dolphin's, and its skull looked very much like those of other primitive whales, he said.
The nearly complete skeleton shows that the creature apparently walked on dry land with a clumsy humping gait, much like sea lions, while in the water it swam by undulating its back while swinging its huge webbed rear feet through the water.
"It's now clear how this critter got around in both worlds," Thewissen said. "It's clearly a missing link between land animals and modern whales."