Whales are so unlike other living mammals that it has been difficult for scientists to identify their closest relatives. According to new fossil evidence discovered in Pakistan and India, ancient whales could both walk on land and swim by paddling with large limbs. Two teams of researchers reported this fall on previously unknown species of primitive whales -- whales whose remains were preserved in sediments deposited during the Eocene age, some 50 million years ago.

Until the discoveries, scientists had thought that whales evolved from ancient, land-dwelling carnivores (the mesonychids). The newly discovered fossils suggest they may be more closely related to cloven-hoofed mammals -- pigs, hippopotamuses, camels, deer and their kin (called artiodactyls).

In addition, studies comparing molecular differences between living representatives of these two groups go further to suggest a close evolutionary relationship between whales and hippos.

As with many scientific breakthroughs, however, the finds were enough to make scientists question existing ideas without being sufficient to resolve questions completely. A clear picture of the family tree of whales has yet to emerge.

The recent fossil discoveries present significant challenges to the prevailing theory that whales evolved from the meat-eating mesonychids. Many mesonychids (meso = middle, onyx = nail or hoof) were the size of wolves, but they had relatively short legs, small hooves, and disproportionately large heads. The suggestion that mesonychids and early whales are related is based primarily on similarities in their notched, triangular teeth.

New data from fossils of four species show, however, that the structure of the ankles of the most primitive whales was highly specialized. The ankles of primitive whales closely resemble those characterizing all artiodactyls (artio = even, dactyl = toe). Moreover -- so far -- the ankles of early whales are unlike the ankles in the few mesonychid skeletons discovered.

But fossils are not the only source of evolutionary information. Other evidence for the link between ancient whales and cloven-hooved creatures comes from a wide variety of comparative molecular studies of modern whales and artiodactyls. These studies compare, for example, immunological relationships, amino acid sequences, and aspects of DNA structure -- with results that consistently point to hippos as the closest living relatives of whales.

On the other hand, similar studies present a confusing picture of the interrelationships between modern artiodactyls. The lack of consistency in the results of these molecular studies is vexing and suggests they might not be able to supply the detail necessary to unravel the history of evolutionary events that occurred some 50 million years ago.

Rock deposits of Eocene age in India, Pakistan, and other parts of Asia preserve records of the diversification of the early artiodactyls and mesonychids. There is, however, a frustrating gap in the available fossil record. The first records of whales are from the Eocene era, but the oldest records of primitive hippos -- which appear to have been land-living beasts -- are from some 25 million to 30 million years later.

The interrelationships of primitive members of these groups are poorly understood. This is unsurprising given that the fossil records mostly comprise specimens known only from their teeth -- the most durable part of the mammalian skeleton. Little, for example, is known of the structure of the hind legs of many of these species. What did the ankles of the Asian mesonychids look like?

The next steps for scientists are to return to the field to look for more fossils and to existing museum collections for more clues. Similarly, researchers studying the molecular interrelationships of modern mammals will want to expand the scope of their studies to include more species, with the goal of providing a stable hypothesis of the evolutionary relationships of the modern groups of artiodactyls.

(For UPI by William Clemens, professor of integrative biology and curator of the Museum of Paleontology at the University of California, Berkeley.)