In 1996, researchers in Canada's Dinosaur Provincial Park recovered the partial left jawbone of a large tyrannosaur. Numerous remains of tyrannosaurs had been found from the location before, but this one was special. Embedded in the bone was the tooth of a second tyrannosaur.
This specimen sat in a museum collection for a number of years, but now paleontologists Phil Bell and Philip Currie have described it in the journal Lethaia. It is one of the rare fossils that captures the actions of a predator. Figuring out what actually happened to the tyrannosaur that was the victim, though, was like reconstructing a crime scene.
The first task was to identify the victim. The researchers did not have a complete jawbone to work with, but by estimating its size and comparing it to measurements from other dinosaurs, they determined that it probably came from either Daspletosaurus or Gorgosaurus. (Though they preferred Gorgosaurus because of the abundance of its remains. Daspletosaurus fossils are more rare.) Precise identification is difficult, but the bone certainly came from a tyrannosaurid of about that size and both genera have been discovered before in the park.
So who was the killer? They left one of their weapons, the tip of a single tooth, behind. Again, the paleontologists could not be absolutely certain what species of dinosaur it came from, but the tooth matched the tyrannosaurid type, meaning that the victim had probably been bitten by a member of the same species or a closely related one.
From there Bell and Currie attempted to reconstruct what happened. There are several scenarios. Perhaps two tyrannosaurs were fighting and one bit the other on the face, losing a tooth in the process. If this was the case then the victim must have died very soon afterward as the bone around the tooth shows no sign of healing. If it did not die immediately, it would have died within a few weeks of the battle.
Another possibility is that the victim was already dead and was a meal for another tyrannosaur. This would explain the lack of bone healing, although why the scavenging dinosaur would bite the dead tyrannosaur on the face is unknown. There is some evidence that tyrannosaurs bit each other on the face during fights while alive, but it is impossible to know for sure whether this case represents a fight or scavenging. As sometimes happens with crime scenes the trail of evidence has gone cold, but such evidence might be helpful in understanding tyrannosaur behavior if similar evidence is found in the future.