The fossil, of an ammonite known as Diplomoceras maximum, is the largest of its type ever found, and it has allowed scientists for the first time to reconstruct the animal from a single specimen.
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," says William J. Zinsmeister, professor of geosciences. "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."
The fossil, one of the largest ammonites ever found, was discovered by Anton Oleinik, a doctoral student working with Zinsmeister on Seymour Island, an area of Antarctica known for its abundance of fossils.
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 species changed rapidly throughout time, taking various shapes and forms, and for that reason is considered a "leading" group in the evolutionary record.
Like the nautilus, ammonites had external shells that were divided into a number of chambers. Though the shells were generally sculptured or coiled, ammonites also came in a variety of odd shapes, sometimes resembling a twisted rope or knot.
The Diplomoceras , named for its tube-like shell that doubled back upon itself, was perhaps the most ungainly member of its family, Zinsmeister says. 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 Purdue 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," says Zinsmeister, who 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 says, 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 says. 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 overall completeness of the fossilized ammonites shell also indicates that the animal, at least in most cases, kept its shell and added to it throughout its life, Zinsmeister says.
"We formerly thought that the Diplomoceras would shed the early part of its shell as it grew," he explains. "That idea was based on our early fossil findings, which consisted only of small sections of the shell."
Though the most recent fossil has a few sections missing -- they are being reconstructed by Oleinik at Purdue -- the animal was preserved almost in its entirety.
"Finding a fossil this well preserved is extremely rare," Zinsmeister says. "It's a remarkable specimen."
The new knowledge of the animals' size and shape raises questions about how the Diplomoceras moved about and gathered food 65 million years ago. Oleinik believes that the creatures were capable of swimming horizontally. Zinsmeister disagrees, noting the combination of its huge shell and bizarre configuration would make the animal almost immobile, forcing it to drift or creep along the bottom of the ocean floor.
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," Zinsmeister says. "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."
Source: William J. Zinsmeister, (765) 494-0279; Internet,
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