The ostrich-size dinosaur might have been on its way to get a drink from a small pond between two sand dunes in what is now western Colorado.

It was just another day in the life of the coelophysis, a fairly common meat-eating inhabitant of the Triassic period nearly 200 million years ago. But its well-preserved three-toed footprints have provoked uncommon interest by three young scientists who are prying secrets out of layers of sedimentary rock called the Chinle formation.The Chinle, a product of a wet, swampy environment, preceded the sandstone that preserves the coelophysis tracks. But over millions of years the mobile animal, which ran around on its hind legs, managed to survive from a very damp period to a very dry one. "The critters did persist through that climatic change," says Russell F. Dubiel, a sedimentologist with the U.S. Geological Survey in Denver.

This sets coelophysis apart from most animals, including marine species such as clams and snails, which perished in the transition.

Millions of years of fossil history are compressed in formations only a little taller than a three-story building. "You have this transition over about 70 vertical feet from what seems to be a river system to a complete desert," says J. Michael Parrish, a vertebrate paleontologist at the University of Colorado at Boulder.

Dubiel, Parrish and Steven C. Good, a graduate student in invertebrate paleontology at the university, are specialists in the Chinle formation. Their research is supported in part by the National Geographic Society.

"Any time that you're getting into an environment where you're preserving small terrestrial animals, it's going to tell us something about animals that we really don't know anything about," Parrish says.

"The period of time that we're looking at is the beginning of the development of dinosaurs," Dubiel says. "When most people think of dinosaurs, they think of the giant tyrannosaurus and stegosaurus, the things you see in the museums. But things usually start out small and then grow larger through evolution."

The Chinle sprawls across thousands of square miles in the Four Corners area of the southwestern United States. Multicolored Chinle outcroppings in Arizona give spectacular beauty to formations such as the Petrified Forest and the Painted Desert.

But the three-man team is concentrating on drabber parts of the Chinle in eastern Utah and western Colorado, hoping to cast more light on life in the murky millenia before large dinosaurs ruled the planet. "We're getting a pretty good idea of what the ecology at that time was like," says Dubiel.

Parrish doesn't subscribe to the theory that a meteorite collision toward the boundary of the Triassic and Jurassic periods caused the swift extinction of the animals whose fossils have been found in the Chinlee. "I'm not convinced," he says. "It probably took a few million years."