A bucktoothed, rabbit-like dinosaur related to Tyrannosaurus rex and other predators lived in China 128 million years ago, researchers report.

The fossil of the unusual Incisivosaurus was found in the Yixian formation near Beipiao City in northeast China, an area that has already produced many unusual fossils, including dinosaurs with feathers.

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Incisivosaurus is part of a group of dinosaurs known as oviraptors, small two-legged dinosaurs that had parrot-like beaks. Incisivosaurus, however, is the oldest oviraptor found to date and lacks the bird-like features found in others of its group, the researchers report in Thursday's issue of the journal Nature.

Instead of having a beak, Incisivosaurus has a long skull and jaws filled with teeth for grinding. However, in its most unusual characteristic, it sports two large buck teeth at the front of its jaw similar to those used by rodents for gnawing.

The buck teeth suggest the dinosaur was an herbivore rather than a meat-eater like its relatives, reported Xing Xu and colleagues at the Chinese Academy of Sciences.

Other paleontologists said buck teeth alone do not mean Incisivosaurus was a plant-eater. But they said the discovery shakes up the traditional view of theropod dinosaurs, which are widely assumed to have long, sharp teeth.

"The classic view of predatory dinosaur [theropod] teeth is that they are all basically the same and are shaped more or less like serrated steak knives," said geologist Joshua Smith of Washington University in St. Louis. "However, it is becoming more and more obvious as we begin to look closely at theropod teeth that they are far more complex than we have been led to believe and that the steak-knife view isn't accurate.

"This is true of Tyrannosaurus, and with new discoveries like Masiakasaurus last year in Madagascar and now Incisivosaurus in China, it is becoming apparent that it is true of other theropods as well."

The size of the teeth in the fossil vary widely. The front teeth appear two to three times longer than teeth further back, which is almost unheard of, Smith said.