Associated Press

New fossil discoveries add weight to the conclusion that whales are related to land-based plant-eaters such as cows and hippopotamuses rather than to an extinct group of carnivores, two groups of researchers report.

Scientists have known that whales evolved from four-legged land animals millions of years ago. However, which branch of the animal kingdom whales split from has been a matter of debate.

Immunological tests in the 1950s and recent DNA tests have shown a relationship to plant-eating artiodactyls — hoofed mammals having an even number of toes, such as pigs, cows and hippopotamuses.

Earlier, those test findings had not been supported by fossil evidence, which pointed more to a link to carnivores. Now, authors of two new studies say their fossil finds, in separate areas of Pakistan, have convinced them that the tests are correct.

"With this find, it’s clear that I and all my colleague were barking up the wrong tree," said Hans Thewissen of the Northeastern Ohio Universities College of Medicine in Rootstown, Ohio.

Thewissen, who found remains of two species of four-legged, 50 million-year-old whale ancestors, is the lead author of a paper appearing in today’s issue of the journal Nature.

Thewissen compiled the skeletons from bones found in a bed of fossils in the Punjab area of northeast Pakistan.

Philip Gingerich, a professor of geology and paleontology at the University of Michigan and the lead author of a paper appearing in the journal Science, said his group found two skeletons of two other separate species, about 47 million years old, in the Balochistan area of southwest Pakistan. One skeleton was almost complete, he said.

"Our molecular colleagues might be right that hippos are related," Gingerich said.

Gingerich’s paper appears in Friday’s issue of Science, but it was released at the same time as the Nature article.

The key factor in both papers is that the fossil animals’ ear cavities have specific formations that link them to whales, while they also have legs and a distinctive ankle structure similar to other artiodactyls.

Despite the DNA and immunological evidence, some researchers had believed whales are related to extinct carnivores called Mesonychians, which had teeth suited for eating fish.

"I have to say when I look at this new evidence, I was initially reluctant to believe it, but I have convinced myself," said Kenneth D. Rose, a researcher at the Johns Hopkins University School of Medicine who wrote a commentary accompanying the Science article.

With whales’ ancestry being linked to plant-eaters, the mystery that remains is the evolution of modern whales’ eating habits. Toothed whales such as killer whales eat fish and other marine mammals, while others use a mouth structure called baleen to filter tiny plankton from the water.

Gingerich noted that he has read descriptions of modern hippos killing and eating gazelles that stray too close to them at watering holes.

"We may have slightly exaggerated the plant-eating characteristics of artiodactyls, though they are certainly predominantly plant eating," Gingerich said.

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