Dec. 20, 2007 -- The mysterious missing link between marine mammals known as cetaceans -- a group that includes whales, dolphins and porpoises -- and their land-based mammal ancestors has been found.

And an assistant professor in the University of Wyoming's Department of Geology and Geophysics helped find it.

Bridging a 10-million-year gap in the fossil record, UW's Mark T. Clementz and a team of researchers have found evidence that cetaceans evolved from Indohyus, a raccoon-sized raoellid from India. Their findings were published Dec. 20 in Nature, the world's foremost weekly scientific journal and flagship journal for Nature Publishing Group (NPG).

"There's two big things that make this a significant discovery," says Clementz. "The first is that, for a long time, there's been a debate over the closest living relative to whales.

"The general idea has been that it's hippos, based mainly on molecular evidence. The problem is that the hippo fossil record only goes back 10 million years and the fossil record for whales goes back 50 million years.

"But now we've finally found evidence of an artiodactyl group, and artiodactyls are the group that includes hippos and camels and cows, the even-toed hoofed mammals, that is a sister group to whales in the fossil record. That's pretty big. It helps to cinch the argument that whales are within artiodactyla."

He adds, "The other big thing is that this helps refine our idea of whale evolution. For a long time, it was just assumed that whales were unique because they were a group of artiodactyls that went in the water and then evolved. But now we know that the sister group to whales were also in the water."

While studying Indohyus fossils discovered in Pakistan by Hans Thewissen, of the Northeastern Ohio Universities College of Medicine, researchers found a striking similarity to cetaceans in the middle-ear space, or involucrum, which had never been found in other raoellids or artiodactyls.

The fossils show a thick covering over the involucrum, a feature that had only ever been seen in cetaceans.

"That ear bone is key," says Clementz. "That makes it pretty clear that they're closely related to each other."

Also, Clementz determined that levels of carbon and oxygen isotopes in the tooth enamel of Indohyus suggested that the animal spent considerable time in the water.

There was further proof in the limb bones of Indohyus, which were thick and dense, a characteristic that suggests the animal was a wader.

"I feel very fortunate to have been invited to participate in this research, and I'm excited that there's so much interest in the results," Clementz says. "I think this discovery will open a lot of doors and raise a lot of questions that we can continue to explore."

To view Nature's video about whale evolution, go to the Web site www.nature.com/nature/videoarchive/index.html.

Photo:
Mark T. Clementz, an assistant professor in the University of Wyoming’s Department of Geology and Geophysics. (UW photo)

Posted on Thursday, December 20, 2007