If Ahab only knew: cetaceans (whales) descend from hoofed mammals
Rudyard Kipling delighted generations with his tale of how the whale got its throat. But he left a larger question: How did the ocean get its whale?
This week, two teams of paleontologists report finding fossils they say will help answer that question and settle a long debate over which animals today represent the whales' closest terrestrial relative.
Digging in Pakistan at sites some 500 miles apart, each team last year unearthed sets of fossils that they say bears on the kinship issue. One set, which researcher Johannes Thewissen's team uncovered and describes in today's issue of the journal Nature, is interpreted as a pair of hoofed land mammals roughly 50 million years old. The animals belonged to a group known as artiodactyls - hoofed mammals with an even number of toes. The specimens' ear structures match those of primitive whales and are unique to cetaceans, meat-eating marine mammals that include dolphins, porpoises, and narwhals.
The other fossil sets, discovered by a team led by University of Michigan paleontologist Philip Gingerich and described in tomorrow's edition of the journal Science, include two new species of primitive whales some 47 million years old. The whales' fore and aft flippers have ankle bones virtually identical to those of artiodactyls.
Moreover, Dr. Gingerich's team found the skulls and flippers assembled on their respective skeletons instead of in a jumble of possibly unrelated bones. For years, many paleontologists, including Gingerich, had held that whales' terrestrial relatives had fallen extinct long ago. These new fossils, however, appear to indicate that whales share a mammalian ancestor with today's sheep, goats, deer, and hippopotamuses.
So far, virtually all fossils of early whales have been found in Pakistan and northwestern India, researchers say. This suggests that whales emerged in the ancient Tethys Ocean, which separated Asia and India 50 million years ago. Once cetaceans began to make the shift from land to water, they appear to have diversified quickly. Sediment beds have yielded fossils from at least six types of whale.
Whales represent one of the most robust examples of evolution, says Dr. Thewissen, an associate professor at the Northeastern Universities College of Medicine in Rootstown, Ohio. Their fossil record, "really documents enormous changes in a group of animals over short time scales," he says. Whales appear to have transformed from land critter to leviathan in less than 8 million years.