By Sean Henahan, Access Excellence

WEST LAFAYETTE, Ind. - Sixty-five million years ago, a giant 12-foot long mollusk lived at the bottom of the sea near Antarctica. The discovery of a complete fossil of the creature, an ammonite known as Diplomoceras maximum, is causing researchers to rethink their ideas on mollusk evolution.

Ammonites, which are related to the pearly nautilus, are among a group of invertebrates that date back more than 400 million years. The animals vanished approximately 65 million years ago, the same time that dinosaurs became extinct.

The Diplomoceras takes it name from its tube-like shell that doubled back upon itself. The pleated shell resembled a clothes dryer vent hose curled in the shape of a paper clip. The fossil shell is about six inches in diameter, tapering slightly from the opening where the animal lived to the tip of the last chamber.

Uncoiled, the fossil would measure more than 12 feet. The soft-bodied animal, which was about six feet long, lived in the extended chamber at the front half of the shell. The other half consisted of chambers that were filled with gas and fluid.

"Previously, we were at a loss as to what these creatures looked like. Our guesses were based on a series of fragments," said William J. Zinsmeister, professor of geosciences at Purdue University. Zinsmeister had published a composite drawing of the animal in the Journal of Paleontology in 1989 after finding a number of fossil fragments in the same region of Antarctica.

"We now see that our previous picture was not totally correct," he said, noting that the new fossil shows that the large part of the shell where the animal lived, and the animal itself, were much longer than predicted. "It would have made a nice morsel - a real Cretaceous-era hot dog - for a mososaur," he added.

Mososaurs were huge marine lizards that preyed on ammonites. A mososaur skull that measured 3 feet long with teeth 2 to 3 inches long was found by the Purdue group during its stay in Antarctica.

The find also has given the scientists new respect for the lowly Diplomoceras, long considered to be an evolutionary dead-end because it is the most awkward-looking member of the ammonite family.

"Hydrodynamically, this was the Forrest Gump of ammonites. It was slow and deliberate, but judging from the size of this fossil, it did very well for itself and was able to compete with its more agile relatives," noted Zinsmeister.

The completeness of the fossilized ammonite shell also provides enough information to indicate that the animal kept its shell and added to it throughout its life, Zinsmeister says.

The sheer size of the fossil also has made the scientists rethink their position on the Diplomoceras' place in evolution. Such uncoiled ammonites, which tend to appear sporadically in evolutionary records, once were viewed as evolutionary dead-ends, serving only as a fountainhead for their more symmetrical descendants.

"For years, it was assumed that the spiral-shaped ammonite was ideal. It was thought that the uncoiled ammonites, like this one, couldn't compete with their more mobile, agile cousins, and so quickly died out. We now see that appearance is deceptive. The Diplomoceras represents one of the last members of its species to become extinct. It may have looked ungainly, but it wouldn't have reached this size, or lasted as long, unless it was a good competitor," noted Zinsmeister.