A fossil tooth found buried inside the healed tailbones of a duckbill dinosaur suggests the animal survived a close encounter with a Tyrannosaurus rex about 65 million years ago, according to a new study.

Scientists say the embedded tooth, discovered in the Hell Creek Formation in Montana, is the first conclusive proof that T. rex was not just a scavenger, but also a predator that hunted and killed prey. (Related: "Scarred Duckbill Dinosaur Escaped T. Rex Attack.")

"It's not just a smoking gun—we've actually found the bullet," said study co-author Peter Larson, a paleontologist at the Black Hills Institute of Geological Research in South Dakota.

But not everyone agrees. Jack Horner, a paleontologist at Montana State University who has long argued that T. rex was incapable of hunting healthy adult prey, remains unconvinced.

"It's one data point, and that's the least amount of data you can have," said Horner, who was not involved in the research.

Scavenger or Killer?

A long-running debate about T. rex concerns whether it was an "obligate scavenger"—surviving solely on the flesh of dead animals—or whether it was an active predator that hunted prey.

The fossil evidence has been inconclusive on this matter. That's because while dinosaur bones and even fossilized skin have been found with tooth marks linked to T. rex, it's impossible to conclusively prove whether an animal was alive or dead when it was bitten.

"If a T. rex bit a bone of an animal that was already dead, that mark looks identical to what would be made on an animal that it just killed," said lead author Robert DePalma, a paleontologist at Florida's Palm Beach Museum of Natural History.

So instead of relying solely on bite marks, some scientists have tried to determine T. rex's feeding style by analyzing its physical and sensory attributes.

Paleontologists such as Horner have concluded that T. rex was not only too slow to chase down prey, its arms were too puny and its eyesight was too poor for it to successfully attack or chase down a healthy adult animal. At best, T. rex could have chased down lame, young, or weak animals, but otherwise it lived on carrion, Horner said.

"My argument has always been that T. rex is [an opportunist] like a hyena," he added.

End of Debate?

But DePalma and his team say the T. rex tooth, which measures about 1.5 inches (3.75 centimeters) long, solidly refutes that idea. "It puts the debate to rest," DePalma said.

The fact that the bitten duckbill dinosaur—also known as a hadrosaur—managed to escape suggests it was more than a match for its pursuer, he argued, and thus unlikely to have been young, weak, or handicapped.

"We can tell [the T. rex] bit a living animal because the two vertebrae were infected and then fused together," DePalma said. "So it's unmistakable that the tooth was in there before the animal died."

Also, the location of the injury—near the tip of the duckbill's tail—suggests T. rex was pursuing the animal when it was bitten. "The fact that the injury is on the hindquarters of the animal is actually not surprising," DePalma said. "If you examine Kalahari lions, for example, they will attack the hindquarters of their victims to immobilize them and then they'll go in for the kill."

To identify the animal the tooth belonged to, DePalma and his team extracted it from the tailbones and made detailed measurements. "The morphology is unmistakable for T. rex," DePalma said. "It could not be mistaken for any other carnivorous species."

The tooth's size and shape indicates it came from the T. rex's lower jaw, and its loss likely did not affect the creature in any significant way.

"Like sharks and lizards, T. rexes kept replacing their teeth throughout their entire lives," DePalma explained.

"That tooth would have been replaced in a matter of months, and it would have been sharper than ever." (Learn the latest on what scientists know about T. rex.)

How It Went Down

Using modern predators as a model, DePalma and his team have imagined a number of ways the failed hunt might have happened.

One possible scenario, DePalma said, is that the T. rex was prowling around a watering hole or food source where duckbills gathered, or perhaps it was hiding in nearby foliage and attempted to ambush the animals.

Hell Creek at the end of the Cretaceous era was not the dry, rocky desert it is today. It was a temperate forest environment crisscrossed by rivers filled with crocodiles and turtles.

The T. rex "would have probably gone after the closest hadrosaur it could get its mouth on, and in this case it proved too much of a match for it," DePalma said.

Larson, on the other hand, imagines a small pack of T. rexes joining forces to take down the duckbill. Multiple T. rex individuals have been found buried together, which suggests that they could have lived, and possibly hunted, together.

"A scenario might be a family of two or three related individuals that hunted together," he said. "One animal would distract the prey or perhaps send a herd of duckbills scattering, while the other Tyrannosaurs would lie in wait and eventually take up the chase for the weaker individual."

Had the T. rex managed to run down its prey, its next step would have been to use its weight to muscle the duckbill down to the ground.

It would have then "grabbed on with its arms in order to move its head forward to try to either clamp the muzzle to suffocate it or try to tear open the throat," Larson said.

No Evidence

Horner said such imagined scenarios are idle speculation not supported by the evidence.

This fossil only "shows that a T. rex bit a live duckbill dinosaur. There's no evidence that it was chasing it," he said.

Horner offered his own speculative scenario for what might have happened: "A T. rex could have walked up to a sleeping duckbill dinosaur and bit it, realized it was alive, and then backed away," he said. "That's just as plausible as saying that it's chasing it because there's no evidence for either one."

In addition to ending the debate about T. rex's predatory behavior, DePalma thinks the tailbones will help paleontologists reconstruct the ecology of the Hell Creek Formation 65 million years ago.

"When you have a giant carnivore that composes a significant portion of the ecosystem, identifying its role as either a predator or a scavenger is going to have significant implications for paleoecological reconstructions," DePalma said.