(CNN) -- Was Tyrannosaurus rex a predator or scavenger? The question has been a point of controversy in the scientific community for more than a century.
"You see 'Jurassic Park,' and you see T. rex as this massive hunter and killer, as incredibly vicious. But scientists have argued for 100 years that he was too big and too slow to hunt prey and that he was probably a scavenger, an animal that feeds only on dead things," University of Kansas paleontologist David Burnham said.
Burnham and researcher Robert DePalma got what Burnham described as his "lucky break" when they found the fossil of a duckbill dinosaur's tail with a tooth in it.
"The features of the tooth are like fingerprints, and we were able to identify it as T. rex," he said.
They took the fossil to be analyzed at the University of Kansas and for a CT scan at the local hospital, where the doctor told them, "It's too late for your patient."
But Burnham was thrilled at what the fossilized bones told him about the life of the duckbill.
"We were giddy like schoolkids," he said. "This now returns T. rex as a predator. So the monsters that we see in dinosaurs are real. They did go chasing after things, kill them and eat them. They actively pursued live prey."
The mystery was solved.
"Our evidence is new because we have a T. rex tooth inside a duckbill dinosaur, that bit it so hard and so viciously that the tooth broke off, the duckbill dinosaur got away, and over the next few years, the wound healed," Burnham explained.
He determined that the duckbill lived long after the attack by comparing the time it took for the bone to heal around the wound with studies done on modern animals.
"Before our evidence, everybody was looking at bite marks and tooth punctures on dinosaur bones and using that evidence for both theories of predation and scavenging. They were using the same bit of evidence to argue both sides," Burnham said.
For Burnham, the discovery brought to life the behaviors of these legendary creatures and proved the T. rex's place in the prehistoric world.
"Here is T. rex, one of the biggest animals ever known, 5-foot-long skull, 40 feet long nose to tail and weighed 7 tons. What were these guys doing in the ecosystem? Now we know they were the top feeders."
The full study is published in the Proceedings of the National Academy of Sciences.