Scientists have made a major discovery after examining an animal discovered deep in Tanzania 255 million years ago: the oldest known tumor. Researchers from the University of Washington were examining a saber-toothed beast and found a tiny, benign tumor embedded in the root of the canine tooth while they were looking at something else on its jaw.

This creature was part of a group of carnivores called gorgonopsians, which have long since gone extinct. They are like mammals but they have some reptile-like traits, stretching up to 10 feet long and looking like an early evolutionary link to mammals. They were one of the top predators of their era, living between about 270 million to 252 million years ago, when a mass extinction event wiped them and most other creatures out. They predate dinosaurs by about 20 million years.

Researchers were looking at a madible fossil from one of these animals that had been pulled from the Ruhuhu Valley in Tanzania when they found the tumor. Scientists said it was pure luck that they found it.

This type of tumor had only been known to mammals, but this discovery shows that it existed in the ancestors of mammals as well.

“We think this is by far the oldest known instance of a compound odontoma,” said senior author Christian Sidor, a UW professor of biology and curator of vertebrate paleontology at the Burke Museum of Natural History and Culture. “It would indicate that this is an ancient type of tumor.”

“Most synapsids are extinct, and we — that is, mammals — are their only living descendants,” said Megan Whitney, lead author and UW biology graduate student. “To understand when and how our mammalian features evolved, we have to study fossils of synapsids like the gorgonopsians.”