July 13, 2005 — About half of the dinosaurs in the Late Triassic may have never existed.

Paleontologists have announced the discovery of an approximately 210-million-year-old skull belonging to a creature known previously only by scattered teeth in the American Southwest. The skull casts doubts on the early history of one of the two main branches of the dino family tree.

If the doubts prove true, it would make the Late Triassic sort of like the Hatfields without the McCoys or the Montagues without the Capulets.

Revueltosaurus callenderi was thought to be the only confirmed North American member of the early ornithischian (pronounced "OR-nith-ISH-ee-in," and meaning "bird-hipped") dino group. Its existence had implied that ornithischians were evolving right alongside the other big branch in the dino family: the saurischians ("SOAR-ish-ee-ins," meaning "lizard-hipped").

One was probably often prey to the other, in fact. The ornithischians gave rise to stegosaurs and triceratops, while the saurischians evolved into long-neck plant eaters, tyrannosaurs and modern birds.

But the newfound Revueltosaur skull has revealed that all those scattered teeth lied: Revueltosaur isn't even a dinosaur. It's a sort of crocodile.

"We're definitely going back to square one," said William Parker, paleontologist for Petrified Forest National Park's in Arizona. "We know this material can't be assigned to ornithischian dinos."

Parker is the co-author of a paper about the Revueltosaurus skull, found in the park in March 2004, published in a recent issue of the Proceedings of the Royal Society, Biological Sciences.

For more than 15 years, the isolated teeth of Revueltosaur have been taken as strong evidence of early ornithischians in North America, simply because the teeth look just like those of later plant-eating ornithischian.

"It was an okay assumption," said Randall Irmis, a member of Parker's team and now a paleontology graduate student at the University of California, Berkeley. But now that the same teeth have been found in a crocodilian skull, the assumption doesn't wash, he said.

Other paleontologists agreed that Reveultosaur could no longer be considered a dino, but are less sure about the broader implications.

"Revueltosaurus indeed is not an ornithischian dinosaur," said paleontologist Spencer Lucas, curator at the New Mexico Museum of Natural History and Science. On the other hand, he's not so certain that means all ornithischian dino teeth found from that time are also deceptive.

The sudden transformation of the Revueltosaur into a Revuelto-croc leaves just two specimens of Late Triassic ornithischians: both from Argentina. That could suggest, said Irmis and Parker, that ornithischians got their start in that part of the world.

It might also mean that early ornithischians were too few to provide fresh meat to their flesh-eating saurischian counterparts. So perhaps they ate the crocodiles instead, the researchers suggested.

The Revueltosaur was first identified in 1989 by its teeth and named after the site of the discovery: Revuelto Creek, N.M. It was found in Petrified Forest in 1990. The teeth have been considered as dinosaur teeth by collectors as early as the 1930s, when the first organized digs got underway in Petrified Forest National Park.