As described by paleontologists Nicholas Longrich, Jack Horner, Gregory Erickson and Philip Currie, at least four Tyrannosaurus rex bones bear toothmarks made by a large carnivorous dinosaur. They are several foot bones and an upper arm bone from four different individual animals. The bite traces they bear are not just punctures into the bone, but U- and V-shaped gouges which suggest that the feeding dinosaur was biting onto the body of the Tyrannosaurus and pulling the flesh off the bones. This is consistent with a set of 13 other bones bearing similar toothmarks, including parts of horned dinosaur and hadrosaur skeletons.
That Tyrannosaurus rex is the most likely culprit in each case rests upon the fact that there was no other creature capable of inflicting that kind of damage in each locality from the end of the Cretaceous. The toothmarks were inconsistent with damage done by crocodiles, the predatory lizards in the area were far too small, and the only predatory dinosaur of suitable size to make such bite marks was Tyrannosaurus rex itself.
The collection of bite marks most likely represents feeding rather than combat. The marks are in places and positions that appear to be impossible for fighting animals, and since the bite-marked bones show no evidence of healing it is most probable that the damage was done after the individual animals died. The fact that the bite marks were found primarily on limb and toe bones hints that the feeding Tyrannosaurus was a scavenger that came by after most of the soft tissues had been removed from the dead Tyrannosaurus. There would not have been very much meat on the upper arms and toes of Tyrannosaurus, and so the authors of the new study hypothesize:
Tyrannosaurus therefore seems to have been an indiscriminate and opportunistic feeder, feeding not only on herbivorous dinosaurs, but also on members of its own species. The traces described here likely result from opportunistic scavenging, and were probably made after most of the flesh and organs had been removed from the carcass.Furthermore, that four traces from different specimens have already been found hints that Tyrannosaurus may have regularly fed upon its own kind. Considering how rare fossils are to start with, and how much rarer carcasses destroyed by predators are, that scientists have found so many traces already suggests that Tyrannosaurus-on-Tyrannosaurus scavenging was relatively common. It is impossible to know whether these Tyrannosaurus were actually victims of predation or died from some other cause—such as wounds from a fight with another Tyrannosaurus—but the damaged bones show that a hungry Tyrannosaurus would not let a good carcass go to waste.
For more on tyrannosaur feeding, see these posts:
Did Giant Predatory Dinosaurs Eat Bones? Tarbosaurus: A Predator and a Scavenger With a Delicate Bite
Longrich, N., Horner, J., Erickson, G., & Currie, P. (2010). Cannibalism in Tyrannosaurus rex PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013419