It lived in a place we now call Alberta, Canada, but this was in an ancient time, long before any names existed.
It was a young hadrosaur, a giant 'duck-billed' herbivore, and something was wrong with it. We don't know what killed this dinosaur, but at some point, it swished its long, heavy tail for the last time, lay down on the prairie, and died.
Today – at least 66 million years later – all that is known to remain of this magnificent creature are 11 fossilised bone segments from that ancient tail, and scientists have discovered something pretty incredible embedded inside them.
Now, many millions of years after the hadrosaur's death, the nature of its strange affliction has finally been diagnosed.
"There were large cavities in two of the vertebrae segments," explains evolutionary anatomist Hila May from Tel Aviv University in Israel.
"They were extremely similar to the cavities produced by tumours associated with the rare disease Langerhans cell histiocytosis (LCH) that still exists today in humans."
In a new study, an international team used micro-CT scanning to investigate these mysterious cavities in extremely fine detail.
The analysis confirmed the researchers' suspicions, indicating that this rare form of cancer has existed on Earth for at least 66 million years, when the Late Cretaceous epoch ended.
"We scanned the dinosaur vertebrae and created a computerised 3D reconstruction of the tumour and the blood vessels that fed it," May says.
"The micro and macro analyses confirmed that it was, in fact, LCH. This is the first time this disease has been identified in a dinosaur."
But we've never seen it in the fossil record before – nor had the means of identifying it like this.
Langerhans cell histiocytosis is a rare kind of cancer where excess immune system cells build up, forming tumours called granulomas. The disease usually affects young children, and while the vast majority of patients who experience LCH recover, the condition causes pain and swelling.
The exact causes of LCH remain a matter of debate, but with every new piece of evidence we find, we learn more the pathogenesis of this rare, and seemingly very ancient disease.
Even knowing a long-gone dinosaur was afflicted by it over 60 million years ago could be important for finding a treatment, researchers say.
"Ultimately, the goal of such studies is to understand the real cause of these illnesses and what evolutionary mechanisms allowed them to develop and survive," palaeopathologist Israel Hershkovitz from Tel Aviv University, who assisted in the study, told Haaretz.
"Perhaps if we understand a disease's underlying mechanisms we can treat its causes more effectively, instead of focusing on the symptoms, as modern medicine tends to do."
The findings are reported in Scientific Reports.