The evidence comes from "Jane," who died when she was just a T. rex teen. Her fossils, found at Montana's Hell Creek Formation in 2001, reveal that another T. rex teenager severely bit her in the head, breaking her snout to the point of disfigurement.
"Juvenile dinosaurs are rare," lead author Joseph Peterson told Discovery News. "Juvenile meat-eating dinosaurs are even more rare. Juvenile meat-eating dinosaurs that exhibit evidence of aggressive interactions with their own species ... very rare!"
Peterson, a Northern Illinois University geologist, added that well-preserved Jane is the first known juvenile T. rex with bite marks. Her injuries are described in a paper published in the journal Palaios.
For the study, Peterson and his team analyzed Jane's fossils, now housed at the Burpee Museum of Natural History. Her skull shows she sustained a serious bite that punctured through the bone of her left upper jaw and snout in four places.
The injury eventually healed over, but Jane was never the same afterward.
"The front of Jane's muzzle is bowed slightly to the left, likely resulting from the healing of the lesions, though this deformation is not severe enough to have interfered with eating or biting," Peterson explained.
He said Jane also suffered from a toe infection that appears to have been active at the time of her death at the young age of 12 or 13. The 22 foot-long, 1,500 pound dinosaur must have been in pain and may have limped.
Eight great extinct speciesOther fossil finds indicate T. rex lived fast and died young, but many reached their 30s.
Based on CT scans showing her skull's oblong-shaped bite marks, the scientists believe that only another T. rex teen could have inflicted her injuries. Jane's jaw and teeth reveal she had the capacity to produce a similar bite.
Since the juvenile dinosaur had not yet reached sexual maturity, the fight was not likely due to sexual conflict or competition. The scientists instead think the combat might have been a learning behavior for young dinosaurs prompted by a show of dominance or territorial dispute.
Peterson said crocodilians and modern birds "all start to exhibit aggressive behavior towards members of the same species prior to reaching sexual maturity."
As Jane's deformed skull proves, these battles were more dangerous than the playful mock fights staged by other animals, such as wolf pups, dogs, kittens and lion cubs.
"Crocs and birds are less forgiving," he said. "Any sizeable population of alligators or crocodiles is very likely to have members missing fingers, toes, limbs, parts of tail or even parts of the face due to aggressive interaction."
Just a few weeks ago, yet another famous T. rex skeleton, "Sue," made headlines. An international team co-led by Ewan D.S. Wolff of the University of Wisconsin-Madison determined the 30-something Sue also sustained bite wounds that pierced through her skull.
The holes in Sue's jaw, however, are consistent with a parasitic disease called trichomonosis, which may have been spread through salivary contact or cannibalism and probably caused the 42 foot-long, seven-ton Sue to starve to death.
"This leads us to suspect that tyrannosaurs might have been the source of the disease and its transmission in its environment," Wolff said.
Peterson does not believe Jane suffered a similar fate, since the parasite, which also affects modern birds, only infests the lower jaw. Jane's injuries orient beyond that part of her head.
Together, however, the two papers support that T. rex was an aggressive dinosaur throughout its life, and that aggression extended towards its own species.
© 2012 Discovery Channel