Tumors existed in living creatures long before mammals evolved. Scientists at the University of Washington have found the earliest evidence of benign tumor – a compound odontoma – that grows in gums and other jaw soft tissues. The dental tumor, which can be surgically removed in humans, is well known among mammals. According to a study published in the Journal of the American Medical Association Oncologythe earliest evidence was found in the jaw fossil of a mammal predecessor that roamed in modern Tanzania about 255 million years ago.

Tumor University of Washington
Image source: Megan Whitney/Christian Sidor/University of Washington / Facebook Video

No sign of tumor before the jaw was cut open

Judy Skog, program director at the National Science Foundation’s Division of Earth Sciences, said in a statement that, until now, the earliest known evidence of compound odontoma was a mammal fossil about one million years old. The latest discovery indicates that the cause of odontoma wasn’t just linked to the traits in modern species.

The mammal-like beast was an extinct member of four-legged carnivores called gorgonopsians. The fossils were unearthed from Tanzania’s Ruhuhu Valley in 2007, but scientists were unaware of the tumor at the time. Gorgonopsians had both mammal-like and reptile-like traits. The tumor was discovered when the University of Washington biologist Megan Whitney and fellow graduate student Larry Mose were studying the similarities among the teeth of reptiles, mammals, and gorgonopsians.


Scientists cut the jaw and teeth into thin slices before examining them under a microscope. The jaw looked normal before they cut it open, with no sign of a tumor. Researchers then found strange clusters of small, round objects near the root of an enlarged canine tooth. At first, researchers didn’t know what to make of it. Upon further analysis, they realized that the creature had a “textbook compound odontoma.” The gorgonopsid reached up to ten feet in length.

 Ancient tumors are rarely preserved in fossils

According to Megan Whitney, the gorgonopsians were the apex predators of their time. They thrived from 270 million years to 252 million years ago before being wiped out during our planet’s worst mass extinction at the end of the Permian Period. It means they disappeared from the planet about 20 million years before the first dinosaurs appeared.

Tumors usually involve soft issues. They are rarely preserved in fossils. Paleobiologist Christian Sidor said ancient tumors could be preserved in fossil records only if they affected hard parts such as teeth and bones. The tumor in gorgonopsid involved hard enamel and dentin. Scientists have also found tumors in fossils of duck-billed dinosaurs, and a titanosaur. A tumor has also been found in a 350 million-year-old armored fish, which many researchers dispute.