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A walk along the lakeshore, dinosaur-style.

A walk along the lakeshore, dinosaur-style

Quarry workers have uncovered what may be the most extensive group of dinosaur tracks known in eastern North America. Paleontologists say this set of more than 1,000 fossilized footprints, found in Culpeper, Va., offers new insight into the behavior of some of the first dinosaurs.

"I'm overwhelmed by the magnitude of the information that's available on the floor of this one quarry," says Robert E. Weems of the U.S. Geological Survey in Reston, Va. "This is the earliest extensive look at dinosaur behavior that we've got."

In a 6-acre space at the bottom of the quarry, Weems and others have identified tracks from two types of carnivorous dinosaurs and a puzzling third animal, which may have looked like a large horned crocodile. The roving reptiles left their marks in muddy ground near a lakeshore during the later stage of the Triassic period, some 210 million years ago, Weems said last week at a press conference announcing the finds. This geologic time sits at the very beginning of the dinosaurs' long and successful history on Earth.

Fourteen years earlier in the same quarry, workers with the Culpeper Stone Co. unearthed tracks at a level about 150 feet higher and 300,00 years later than the rock at the current quarry bottom. Weems, who also studied this earlier find, says those tracks recorded a wider variety of animals but showed less detail and were not as well preserved as the newly discovered imprints.

In recent years, the study of preserved footprints has undergone a renaissance among paleontologists. A long, continuous set of prints can reveal more information than bones can about how extinct animals moved. "It doesn't tell you where the animal died or was buried; it tells you where the animal was actually living and walking around. That's a very important aspect and one that tends to get neglected," says Nicholas Hotton, a vertebrate paleontologist with the Smithsonian Institution in Washington, D.C., who recently examined the quarry prints.

Two-thirds of the quarry tracks were made by a three-toed dinosaur that walked on two feet and stood about 11 feet tall. From the shape and size of the print, Weems has identified this beast as a member of the carnosaur group, which included distant relatives of Tyrannosaurus rex.

Weems says the tracks indicate this animal may have hunted with considerable stealth. One set of prints shows how the creature stopped abruptly and stood still without shuffling its feet, the restarted by rocking back on one heel, creating a double heelprint.

A smaller, three-toed dinosaur made about 10 percent of the tracks in the quarry. This was most likely a coelurosaur, a bird-like biped that reached about 8 feet in length, Weems says.

The third animal remains a mystery for now. With right and left legs spread about 4 feet apart in an extremely wide stance, this lumbering quadruped left hoff-shaped prints in the mud. At first, Weems thought a large sauropod dinosaur might have created this path, but the strides are too short. He now believes the tracks reflect a reptile that looked like a flattened crocodile with horns.

The trackways on the quarry floor represent a "true slice in time," he says. Geologists often use this phrase to mean a few hundred or even hundred thousand years. But Weems says all the tracks were made within a week.

The quarry company has redirected its work elsewhere and is aiding Weems as he measures and photographs the tracks and makes plaster and latex molds. But Weems says the tracks will soon disappear as explosives blast deeper, perhaps uncovering another layer of buried reptile prints.
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Author:Monastersky, R.
Publication:Science News
Date:Jul 8, 1989
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