Parker, Irmis and their colleagues - graduate student Sterling J. Nesbitt of the Lamont-Doherty Earth Observatory at Columbia University in New York, graduate student Jeffrey W. Martz of Texas Tech University in Lubbock, and Lori S. Browne of the South Dakota School of Mines and Technology in Rapid City - reported their find in the May issue of the journal Proceedings of the Royal Society, Biological Sciences.
Dinosaurs are divided into two main groups, the ornithischians and saurischians, both of which, based on fossil finds, appeared to arise in the Late Triassic, between 225 and 200 million years ago. The early ornithischians were thought to have evolved into animals such as the stegosaurs of the Jurassic and the frilled and duck-billed dinosaurs of the Cretaceous, while the saurischians split into the sauropodomorphs - the long-necked browsers like Brachiosaurus - and the carnivorous theropods, like T. rex. Only the theropods survive to the present as birds.
If, as the team concludes, the first ornithischians outside South America did not appear until 25 million years later than people thought, the picture of dinosaur evolution radically shifts.
"Basically, you have two groups that were thought to appear at the same time and co-evolve, and now you have one completely wiped away," Parker said. "So where did the ornithischian dinosaurs come from? We don't know right now. That's something we are going to address in the future."
Irmis acknowledges that early ornithischians may have been around, yet were rare and living in places not conducive to the preservation of fossils. But they definitely were not as common and diverse in the Late Triassic as some paleontologists have claimed.
"Sauropodomorphs and theropods are geograph