Scientists examining the jawbone of a saber-toothed mammal-like beast that prowled Tanzania 255 million years ago have come across a remarkable fossil rarity: one of the oldest-known tumors.

University of Washington researchers Thursday described a benign tumor composed of miniature tooth-like structures they found embedded next to the root of the creature's enlarged canine tooth while studying an unrelated aspect of the jaw.

The animal was a member of an extinct group of four-legged carnivores called gorgonopsians that mixed mammal-like and reptile-like traits. They reached up to 10 feet (3 meters) long and appeared early in the evolutionary lineage that led to mammals. The jawbone came from one of the smaller gorgonopsian species.