Doctor Bruce Rothschild is an expert in ancient diseases. He's studied tuberculosis in mastodons and arthritis in a Tyrannosaurus rex. But even he'd never seen this particular type of tumour in a dinosaur before. No one had.

The small jaw of a young duck-billed dinosaur had a huge, strange growth on one side - a non cancerous facial tumour called a ameloblastoma. Humans get them, as do modern mammals and some reptiles. But they'd never been seen in fossilised form.

"The discovery of an ameloblastoma in a duck-billed dinosaur documents that we have more in common with dinosaurs than previously realised," Rothschild said in a news release.

In a study published this week in the journal Scientific Reports, Rothschild and his colleagues describe the disfigured jaw of an ailing 67-million-year-old dinosaur. It was a Telmatosaurus transsylvanicus - a species of dwarfed hadrosaur that lived in Romania's "Valley of the Dinosaurs" just before the asteroid impact that would bring about the ancient reptiles' demise.

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But before that catastrophic impact, dinosaur life was dictated by the same concerns that preoccupy animals today: getting food, not becoming food, avoiding injury and disease. Understanding those last two factors is crucial for "revealing a very particular facet of dinosaurian life and death," the researchers write, because they say a lot about a creature's behaviour. Healed-over wounds might indicate an escape from a predator or a fight with a peer; evidence of chronic disease can illuminate how long a creature lived, and whether its relatives helped it survive.

Past research suggests that hadrosaurs - the family of dinosaur whose members had flat duck-like bills, called "the cows of the Cretaceous" - were especially prone to tumours. That may have been due to their diet, which consisted largely of carcinogenic conifers, Rothschild pointed out to Nature in 2003. Or the animals may have been unusually long-lived, and developed cancer in their old age.

When the telmatosaur in the study was discovered a decade ago, "it was obvious that the fossil was deformed, Zoltan Csiki-Sava of the University of Bucharest in Romania, who led the field expedition that uncovered the fossil, said in a news release. "But what caused the outgrowth remained unclear until now."

Advanced micro-CT scanners allowed the team to "peek" inside the jawbone without cutting it open. They found a network of empty cavities separated by thin struts of bone - almost like soap bubbles made solid. It was a tell-tale sign of ameloblastoma. Though the tumour isn't cancerous, it can cause painful swelling that eventually makes it hard to breathe.

The researchers can tell from the size of the jaw that the dinosaur died before it reached adulthood. But without the rest of the creature's skeleton, it's impossible to know what killed it.

A reconstruction of the young, deformed Telmatosaurus individual, with the ameloblastoma just becoming visible on its lower left jaw. Photo / Mihai Dumbrav
A reconstruction of the young, deformed Telmatosaurus individual, with the ameloblastoma just becoming visible on its lower left jaw. Photo / Mihai Dumbrav

Was the tumour to blame? Csiki-Sava thinks that's unlikely - the tumour wasn't large enough to be disabling at the time of the dinosaur's death. But the palaeontologist pointed out that modern predators often go after members of a herd that look disfigured or different.

"The tumour in this dinosaur had not developed to its full extent at the moment it died, but it could have indirectly contributed to its early demise," Csiki-Sava said.