Fossil law tells tale of whale evolution.

Paleontologists have unveiled the fossilized lower jaw of an ancient whale from India that, they claim, pushes the origin of these marine mammals back before 53.5 million years ago, earlier than previously thought. This primitive species provides new information on the profound evolutionary transformation that turned a group of four-legged land mammals into modern cetaceans--the whales, dolphins, and porpoises inhabiting the ocean today--say the researchers.

Other paleontologists, however, question the dating of the Indian fossil and whether it adds much to the story of whale origins.

Indian scientists found the controversial jaw bone more than 12 years ago in the Subathu rock formation of northern India, but they failed to finish an analysis of the animal. Sunil Bajpai of the University of Roorkee in India and Philip D. Gingerich of the University of Michigan in Ann Arbor collaborated on the current study. They described the animal, unofficially named Himalayacetus, at a meeting of the Society of Vertebrate Paleontology in Snowbird, Utah, last week.

The oldest accepted fossil whale, called Pakicetus, comes from rocks in the Indian subcontinent dated to 50 million years ago. Gingerich had previously calculated that there were scant odds of finding any other species prior to 52 or 53 million years ago.

Bajpai and Gingerich dated Himalayacetus using the shells of one species of tiny marine organisms called foraminifera, found in the same rock formation. The species of foraminifera suggests that the whale lived during the early Eocene epoch, whereas Pakicetus fossils have come from middle Eocene rocks.

Other researchers contend the dating rests on thin evidence. The foraminifera in the Subathu formation lived near the seafloor and provide less accurate age information than do surface foraminifera, says paleontologist Hans Thewissen of the Northeastern Ohio Universities College of Medicine in Rootstown. "The dating question is rather critical," he says.

Nonetheless, the find extends the geographic range of known whale fossils. Thewissen, Gingerich, and others have found whale remains in Pakistan, central India, and Kashmir but not in Himachal Pradesh, the home of Himalayacetus.

If Bajpai and Gingerich are correct about Himalayacetus, the Indian fossil would shift ideas about when whales invaded the oceans, says paleontologist Mark D. Uhen of the Cranbrook Institute of Science in Bloomfield Hills, Mich.

Paleontologists believe that Pakicetus and other early cetaceans were furry, four-legged creatures that lived mostly on land, venturing into the water to feed on fish. According to current thinking, the earliest whales hunted in rivers and were unable to feed in salt water. Indeed, Pakicetus is found in river sediments, and the mixture of oxygen isotopes in its bones suggests it swam in fresh water.

The Indian fossil, however, came from marine sediments containing oysters and other ocean species, indicating that Himalayacetus swam in salt water. The ratio of oxygen isotopes in its bones supports this interpretation.

"Before, we might have thought that [these early whales] were restricted to fresh water, but here is a record from a marine environment," says Gingerich.

Despite its marine predilection, Himalayacetus apparently lacked key adaptations to aquatic life. Later whales developed enlarged canals in their lower jaws that improved their hearing underwater. Himalayacetus, though, had the small jaw canals of a land mammal, reports Gingerich. It appears that the Indian whale had already gained adaptations for feeding in salt water even though it could not hear well in that environment, he says.