Better traces of whale pedigree discovered.

Many paleontologists believe that Pakicetus, a carnivorous mammal that flourished 50 million years ago, helped bridge the evolutionary gap between whales and their land-dwelling ancestors.

Now, fossils uncovered in Pakistan provide the best evidence to date that Pakicetus teetered on the midpoint of this radical evolutionary change, pursuing its meals in the water but spending significant time on dry land.

Arguments for this theory hinge on whether Pakicetus had the hearing of a land-dwelling or a marine mammal. Newly recovered jaw and middle-ear bones strongly indicate that Pakicetus was not well adapted for underwater hearing, says paleontologist Hans Thewissen of Duke University School of Medicine in Durham, N.C. Thewissen discussed the new Pakicetus fossils and their implications at last week's meeting of the Society of Vertebrate Paleontology in Toronto, Canada.

"I think for the first time there is what you could call a missing link -- if there is such a thing as a missing link -- between the hearing mechanism of the marine mammal and the terrestrial mammal," he says.

Thewissen and paleontologist S. Taseer Hussain of Howard University College of Medicine in Washington, D.C., unearthed the fossils in the Kala Chitta Hills of the Punjab region of Pakistan. Researchers found the first remains of such creatures at the same site more than a decade ago. The deposits, called the Kuldana Formation, have also yielded fossilized ancestors of sea cows.

According to a widely accepted theory, whales have large fat pads in their jaws that channel sound vibrations to each ear. These fat pads and other adaptations give cetaceans -- members of an order that includes whales, dolphins, and porpoises -- their directional hearing.

A decade ago, paleontologist Phillip D. Gingerich at the University of Michigan in Ann Arbor first described Pakicetus. Based on a reconstruction of the creature's skull, Gingerich determined that Pakicetus did not seem to have the necessary equipment for underwater hearing. Also, the whale ancestor's remains were found with those of land mammals. This evidence suggested that Pakicetus had an amphibious life-style.

The new fossils strongly confirm Gingerich's theory. They show that Pakicetus had very narrow channels in the back of its jaw, making it quite unable to accommodate the large fat pads characteristic of cetaceans, explains Thewissen. The structure of the middle-ear bones -- the first recovered for Pakicetus -- are also decidedly uncetanean, Thewissen notes.

Gingerich says the ear bones provide especially strong evidence for Pakicetus' transitional status in cetacean evolution.

"It's another important characteristic that shows this thing is really intermediate," he explains.

Although the new fossils clarify Pakicetus' place in the evolution of whales from land-dwelling mammals, they don't tell the whole story. As Thewissen notes, "Every missing link makes two more -- one above it and one below it."