A few other herbivore fossils had been found with traces of T. rex bite wounds, but the evidence wasn't conclusive.
Burnham and his colleagues were excavating in the Hell Creek formation in South Dakota. During the Cretaceous Period, the area was a vast network of forested rivers, and the formation now contains myriad fossils of dinosaurs and small mammals from the period.
'Beyond a reasonable doubt' Burnham's graduate student, Robert DePalma, uncovered two fused vertebrae from the tail of a hadrosaur, likely Edmontosaurus annectens, a plant eater that munched on pine needles using scissorlike teeth.
Lodged inside the vertebrae was part of a tooth, and the area around it showed signs of healing. When the team analyzed the serrations on the tooth, they confirmed it belonged to a T. rex.
"We were able to establish beyond a reasonable doubt that that is a T.rex tooth," Burnham told LiveScience.
The T. rex likely attacked the hadrosaur, but the herbivore was able to escape, Burnham said. The healing over the tooth indicates that the hadrosaur lived awhile after the attack.