Jan 26 '6

Scientists at the American Museum of Natural History have discovered a fossil in New Mexico that looks like a six-foot-long, two-legged dinosaur along the lines of a tyrannosaur or a velociraptor. But it is actually an ancient relative of today's alligators and crocodiles. The discovery is a striking example of how different animals can evolve the same kind of body over and over again.

American alligators compete for prime position during a feeding frenzy at the Australian Reptile Park near Sydney. Their ancient ancestors stood on hind legs and looked similar to dinosaurs.

For almost 60 years, the 210-million-year-old fossil has been hiding in plain sight. It was lodged in a slab of rock dug up in 1947 in New Mexico by a team led by Edward Colbert, a paleontologist at the museum.

The site, called Ghost Ranch Quarry, is famous for hundreds of fossils of a very early dinosaur, Coelophysis. Coelophysis kept Dr. Colbert busy for decades, and he left several slabs unopened at the museum.

"We always collect more than we can study," said Mark Norell, the chairman of paleontology.

In 2005 one of Dr. Norell's graduate students, Sterling Nesbitt, began to open the slabs. One rock contained a pelvis and an ankle. The bones clearly did not belong to a dinosaur. They showed distinctive features found only in living crocodiles and alligators, as well as their extinct relatives. That alone made the discovery exciting, because it represented one of the oldest crocodile like fossils.

Mr. Nesbitt paged through Colbert's notebooks to figure out which slabs had been next to the one with the pelvis and ankle. When he opened them, he found almost all the remaining bones in the skeleton.

It quickly became clear that the fossil was unlike any crocodile like species ever found.

"Right away I knew it was something spectacular," Mr. Nesbitt said.

The reptile stood on its hind legs, keeping its tail erect. Its arms were tiny, its neck long, its eyes huge. It was toothless, and its jaws were covered in hard tissue, like a bird's beak. Although the fossil was more closely related to alligators and crocodiles, it bore an uncanny resemblance to dinosaurs that evolved 80 million years later, known as ornithomimids. The similarity extends down to subtle details, like air sacs in the vertebrae of both animals.

Mr. Nesbitt and Dr. Norell named the fossil Effigia okeeffeae. Effigia means "ghost," referring to the decades that the fossil remained invisible to scientists. The species name honors the artist Georgia O'Keeffe, who lived near the site. A paper on the results will be published in The Proceedings of the Royal Society.

Effigia is a striking example of what biologists called convergence, when two lineages evolve the same body plan. Other examples of convergence include marsupial mammals related to kangaroos and opossums that evolved into creatures resembling lions and wolves.

"When I first saw the skull, I thought this can't be related to crocs," said Dr. Christopher Brochu, an expert on crocodilian evolution at the University of Iowa. "But then I saw the ankle and said, 'Yep, it's a croc.' So ornithomimids were convergent on Effigia 80 million years later. There are only so many ways you can do something, and as a result you get this convergence."

Dr. Brochu also said Effigia offered evidence that ancient relatives of crocodiles were much more diverse and dominant than thought.

Mr. Nesbitt and Dr. Norell have re-examined isolated bones claimed to be from dinosaurs of the same age and have concluded that they were actually relatives of crocodiles. "These crocodilelike animals are dominating the late Triassic," Mr. Nesbitt said.

This diversity began to disappear about 200 million years ago. The extinction of Effigia and other crocodile relatives may have allowed dinosaurs to take over their ecological roles. Today's 23 species of alligators and crocodiles offer few hints of their former glory.

"Everyone thinks that crocodilians are living fossils that haven't changed since the Triassic," Dr. Brochu said. "That's nonsense."