The scars on dozens of Tyrannosaurus rex skulls suggest these apex predators repeatedly bit one another’s faces once they hit reproductive age.
When paleontologists analyzed 202 T. rex fossils of various sizes and ages, they found evidence of tooth marks in 60 percent of the adult specimens.
Some 66 million years ago, T. rex were the largest predators in North America during the Late Cretaceous. So what on Earth could have been big enough to scrape their towering faces?
The height of the scars and the spacing of the teeth marks have scientists thinking the bites came from the powerful jaws of the very same species. All the other carnivores on the continent at this time were simply too small to make such an impression.
Even more telling, the patterns of bite marks found on the T. rex skulls were remarkably consistent from specimen to specimen, indicating a common behavior.
Most of the ancient lesions identified had healed extensively, which suggests the bites were non-fatal and were inflicted repeatedly across a dinosaur’s life once they hit sexual maturity.
Males were especially likely to show these lesions, but some females were found with bite marks, too. This suggests the biting behavior might have acted as a sort of territorial telling-off, to protect resources or to fight off sexual competitors, as well as a possible part of T. rex courtship.
“The reason for the instigation of the face-biting behavior is unknown, but could be related to contests for territory, resources, or mates; establishing a dominance hierarchy; or courtship rituals,” the authors write.
The idea has support from other research, too. In the past, T. rex skulls have actually been found embedded with the teeth of their own kind, which suggests the species sometimes fought one another.
Some descendants show similar behavior even today, although birds are not nearly as bloodthirsty as their dino ancestors. Instead, avians tend to rely on song and dance to show off their voice and feathers to aggressors or potential mates.
But perhaps that’s a luxury that comes with evolving plumage. The authors think dinosaurs, with their reptilian skin and sharp jaws, were simply using what they were born with to their advantage.
The location and orientation of the bite patterns identified in the study suggest that one T. rex would usually stand next to another one when it was ready to fight. From this position, they would swing their heads about from side to side, making attempts to grab hold of the skull or lower jaw of their victim with their teeth.
A successful bite might puncture the skin and leave a circular lesion on the jawbone, whereas a glancing blow would leave only a scratch-like scar.
Crocodiles do something remarkably similar, as do salamanders and some alligators. And sometimes, these living animals precede the attack with aggressive posturing or visual threats, which mean the T. rex might have done something similar before going in for a bite.
For now, that’s just conjecture, but paleontologist Paul Barrett, who was not involved in the study, tells The Natural History Museum the analysis of T. rex bite marks is a great new insight.
“The team shows convincingly that while earlier-evolving, meat-eating dinosaurs still had croc-like behavior when fighting each other, this disappears when feathers evolve, which allowed display to take over from biting, as occurs in living birds,” he says.
“It’s amazing that we can get these direct insights into fossilized behaviors, which have been frozen in time as marks on their bones.”
The study was published in Paleobiology.