The animal is interesting because it closely resembles a completely unrelated dinosaur called an ostrich dinosaur that lived 80 million years later, they report in the Proceedings of the Royal Society B, a British science journal.

"A lot of people, from seeing (the film) Jurassic Park know what an ostrich dinosaur looked like," said museum curator Mark Norell. "This is a case of convergence with the ostrich dinosaur. It evolved more than once."

The six-foot-long (2-metre) fossil is an archosaur, an extinct type of animal that includes the ancestors of dinosaurs, crocodilians and birds. It lived in what is now New Mexico, in the US southwest.

It was discovered in blocks of rock from the Ghost Ranch Quarry that were excavated in 1947 and 1948.

Scientists thought that all the specimens were Coelophysis, a small, carnivorous dinosaur that lived at the same time.

"It was collected in this quarry that literally had hundreds of skeletons in it," Norell said in a telephone interview.

NAMED AFTER FAMOUS PAINTER

Norell and graduate student Sterling Nesbitt were looking for Coelophysis fossils when they opened a plaster cast containing the archosaur, which they have named Effigia okeeffeae. The name recalls both the ranch and painter Georgia O'Keefe, who had an interest in the quarry.

Effigia is closely related to an ancient group of reptiles called crocodilians, which includes today's crocodiles and alligators. It was not a dinosaur.

Like other crocodilians of the time, it had a large eye, the researchers said.

Its skull and skeleton were very similar to those of ostrich dinosaurs, with a beak, a long tail, and two-legged stance. Its ankle, however, shows its relationship to crocodilians.

"There are still a lot of big questions about what they would have eaten," Norell said.

But he and Nesbitt noted that Effigia also resembles early theropod dinosaurs -- the two-legged carnivores.

So they reexamined some isolated Triassic reptile specimens and found that Effigia-like animals were common in the samples from western North America.

It could be, they said, that animals like Effigia dominated what are now the Americas, and that dinosaur evolution only took off after Effigia went extinct, leaving a niche.

Searching the storage rooms of museums often turns up treasures such as these, Norell said.

"Something that people often don't realize is that after you collect, it sometimes takes thousands of hours to remove the stuff from the cast for analysis," he said.