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Crunch! T. rex tooth found in dino tailbone 

A T. rex tooth was found lodged in the rear vertebrae of a duck-billed dinosaur.
A T. rex tooth was found lodged in the rear vertebrae of a duck-billed dinosaur.Fallon Cohen

A T. rex tooth was found lodged in the rear vertebrae of a duck-billed dinosaur.
A T. rex tooth was found lodged in the rear vertebrae of a duck-billed dinosaur.Fallon Cohen

Talk about battle scars. A broken tooth lodged in the fused tail vertebrae of a hadrosaur is evidence of a dinnertime face-off between Tyrannosaurus rex and the duck-billed dinosaur. But this time, the hunter went hungry, and the hadrosaur carried off a trophy.

"T. rex was the monster in Jurassic Park," said David Burnham, a paleontologist at the University of Kansas who was part of the team that found the specimen. "It wasn't some vulture or scavenger. It would have chased you down and ate you. It's bone-chilling if you think about it."

But when a T. rex went after this bit of hadrosaur dinner, about 80 million years ago, it lost its tooth in a scuffle. In the weeks and months following the attack, the hadrosaur's bones grew over the tooth and puncture wound, which tells researchers that the smaller dinosaur got away. Like lions today, T. rex would have lunged after its dinner, biting into their tails. This time, it was "a lucky break when that tooth snapped off," Burnham told NBC News.

Robert DePalma, Burnham's student, found the specimen in the Hell Creek Formation of South Dakota, a dig site that has thrown up all kinds of large predators from the Cretaceous period. DePalma, Burnham and their colleagues describe the specimen in the Proceedings of the National Academy of Sciences.

"It is the rare case of a bite mark where the tooth got stuck, and that tooth is definitely from Tyrannosaurus rex," Tom Holtz, a paleontologist at the University of Maryland, told NBC News in an email. The researchers identified the tooth by the length of the crown and the unique pattern of serrations on the tooth, which serve as a "fingerprint" for a species.

Paleontologists have known for a while that T. rex was built to munch on other dinosaurs, but some researchers — including Jack Horner, who advised the Jurassic Park movies — made the case that T. rex was not primarily a fearsome predator, but a skulker that scavenged off dead dinos.

Robert DePalma, David Burnham
A CT-scan of the hadrosaur vertebrae, taken at a local hospital, shows how the bone grew around the tooth.Robert DePalma, David Burnham

Most paleontologists today accept that T. rex did a bit of hunting and scavenged when it could. One study published in 2011 suggested that because T. rex was competing with hundreds of smaller, faster scavengers for food, it couldn't have lived off carcasses alone.

Though the new specimen does prove that T. rex went after living dinosaurs, Mary Schweitzer doesn't rule out the possibility that T. rex also ate carcasses that it chanced upon. "Probably like every other creature on the planet, T. rex took a free meal when he could," Schweitzer, a molecular paleontologist at North Carolina State University, told NBC News.

Without the bones of the animal, Burnham and his colleagues can't tell the age of the T. rex exactly, but the size of the tooth indicates that it was an adult. As for the hadrosaur, there's not enough evidence to suggest how it finally died.

Whether T. rex chased after its prey or was a stealth predator is an open question. "It could be that the hadrosaur was just a little bit careless, and the T. rex jumped out of the bushes and the hadrosaur got away," Schweitzer said.

In addition to David Burnham, the authors of "Physical Evidence of Predatory Behavior in Tyrannosaurus Rex," published in the Proceedings of the National Academy of Sciences, include Robert DePalma, Larry Martin, Bruce Rothschild and Peter Larson.

Nidhi Subbaraman writes about science and technology. Follow her on Facebook, Twitter and Google+.