The New York Times, January 26, 2006
NEW YORK — 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 similar to a tyrannosaur or a velociraptor. But it is actually an ancient relative of modern alligators and crocodiles.
The discovery is a striking example of how completely different categories of animals can evolve the same kind of body over and over again.
For almost 60 years, the 210-million-year-old fossil had 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 Dr. Edward Colbert, a paleontologist at the natural history museum.
The site, called Ghost Ranch Quarry, is famous for hundreds of fossils of a very early dinosaur, Coelophysis. Coelophysis kept Colbert busy for decades, and he left several slabs sitting unopened at the museum.
“We always collect more than we can study,” said Mark Norell, the museum’s current chairman of paleontology.
In 2005 one of Norell’s graduate students, Sterling Nesbitt, began to open the slabs. One rock contained a pelvis and ankle. The bones clearly did not belong to a dinosaur. They showed distinctive features found only in modern 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 ever found.
Using Colbert’s notebooks, Nesbitt figured 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,” 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 a group of dinosaurs that evolved 80 million years later, known as ornithomimids, or ostrich-mimics. The similarity extends to subtle details, like air sacs in the vertebrae of both animals.
Nesbitt and 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 not far from the fossil site.
A paper describing their results will be published in The Proceedings of the Royal Society.
Effigia is a striking example of what biologists call convergence, when two lineages evolve the same body plan. Other examples of convergence include marsupials 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 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.”
Brochu also said Effigia offered evidence that ancient relatives of crocodiles were much more diverse and dominant than previously thought.
Nesbitt and Norell have re-examined isolated bones thought to be from dinosaurs of the same age and have concluded that they were actually relatives of crocodiles. “These crocodile-like animals are dominating the late Triassic,” Nesbitt said.
The extinction of Effigia and other crocodile relatives about 200 million years ago may have allowed dinosaurs to usurp 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,” Brochu said. “That’s nonsense.”
Copyright 2006 The New York Times Company. Reprinted with permission.