Coming of age in the Cretaceous period was no easy feat for young tyrannosaurs, as evidenced by a new study published today in the open access journal PeerJ. The new research provides an in-depth analysis of the remains of an adolescent Daspletosaurus—a smaller tyrannosaurid cousin to the T-rex.
The skull specimen is riddled with both premortem and postmortem injuries that appear to have been inflicted by other tyrannosaurs. This novel combination of healed injuries earned during life and scavenging marks acquired after death distinguishes the individual as a rare "twofer" victim of tyrannosaur combat and cannibalism.
Though this Daspletosaurus died young, it apparently saw a lot of action in its short life, some of which even spilled over into its afterlife.
"It's definitely pretty beaten up for its age," said paleontologist David Hone, lead author of the new study, in a Skype interview with Motherboard. "It certainly shows that for this animal at least, it is not that old, and it is not that big, and clearly it's got into at least one pretty major dust-up at some point."
The most obvious injury sustained by this young carnivorous rabblerouser was a giant bite in the back of the head, which left a distinctive puncture conspicuously shaped and sized like a tyrannosaur tooth.
"It's not impossible it was a triceratops horn," or some other blunt force trauma, Hone told me. "But I know what I've got my money on. Far and away, the most likely thing to knock a hole in the side of the head of a tyrannosaur is another tyrannosaur."
Perhaps even more surprising than the severity of the wound, however, is that this young buck lived through it. Hone and co-author Darren Tanke were able to deduce this was not the death blow for this resilient Daspletosaurus by observing the changes in the fossil's texture surrounding the injury. Differences in depth and structure are clear tip-offs that bone healing has occurred, which indicates that the victim of the attack lived long enough to patch itself up a bit.
Indeed, according to Hone, these skeletal fixups are signs of healing in many different animal genera. "Those types of injuries and healing are pretty uniform across most vertebrate—or at least most tetrapod—animals," he said. "You see them in frogs, you see them in crocodiles, you see them in birds, and you see them in dinosaurs."
Similarly, marks inflicted by tyrannosaur scavengers looking to cannibalize their dead brethren leave consistent signatures. For example, the animal that Hone and Tanke studied was missing part of its jaw. "It looks like something came in, bit, pulled in, left the marks, and snapped through some of the bone," Hone said.
"It's not a smoking gun," he clarified. "It's a gun that's been fired a few feet away. But in the absence of any other weaponry, it fits."
It's easy enough to understand why a tyrannosaur would go all Donner Party on the corpses of its own genus. The Cretaceous was, after all, a tough epoch in which to eke out a living, and wasting food over shared ancestry wouldn't make sense. The more elusive question is what prompted these spectacular predators to violently engage each other while they were still alive.
"That's where it gets tough," Hone told me. "My guess is the obvious things: some of them are fighting over food, some of them are fighting over mates, some of them are fighting over territory. Some of them may have been living in groups and it's a 'who's the top dog?' kind of thing."
"Any or all of those are perfectly reasonable explanations for any or all of the bites," he said. "But until you start putting the data together, you can only get so far."
Moreover, the fact that this animal was a subadult—meaning that it was the equivalent of an older teenager—obfuscates the reason for the behavior even further. "It's slightly frustrating because if it was a real juvenile, if it was a real young one, and it had these kinds of injuries, then you can rule a couple of things out," Hone said.
"I'd be very surprised if an animal that young owned a territory," he added. "An animal that young probably wouldn't be competing for mates."
We may never know the story behind this animal's impressive record of bodily harm, or whether it gave as good as it got when it comes to the proverbial "other guy" in the battle. But even without the specifics, it's clear that this Daspletosaurus deserves to be inducted into the honorary club of Mesozoic badasses.