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Gorgonopsid head
Sketch of a gorgonopsian head, in side view. (CCA 3.0 / Dmitry Bogdanov via UW)

A weird type of benign tumor has been discovered in an unlikely place: the fossilized jaw of a distant ancestor of present-day mammals that lived 255 million years ago.

The tumor, known as a compound odontoma, is made up of miniature toothlike structures. It’s not unusual to find such tumors in mammals, including us humans. But it’s unprecedented to find them in the kind of orgonopsid studied by researchers from the University of Washington.

“We think this is by far the oldest known instance of a compound odontoma,” Christian Sidor, a UW professor of biology and curator of vertebrate paleontology at the Burke Museum of Natural History and Culture, said today in a news release.

Sidor is the senior author of a report on the find, published in today’s edition of the Journal of the American Medical Association Oncology. The research could have implications for cancer research as well as for paleontology.

“Until now, the earliest known occurrence of this tumor was about 1 million years ago, in fossil mammals,” said Judy Skog, program director in the National Science Foundation‘s Division of Earth Sciences, which funded the research. In today’s news release, she said the discovery “suggests that the suspected cause of an odontoma isn’t tied solely to traits in modern species, as had been thought.”

Compound odontomas grow within the gums, like jumbled-up mini-teeth. Studies indicate that the condition crops up in roughly one out of a thousand people.

Odontomas don’t metastasize and are considered benign, but surgeons often opt to remove them because they can become painful and disrupt the arrangement of teeth in the jaw.

Sidor and his colleagues weren’t looking for tumors when they began slicing up a fossilized gorgonopsid lower jaw that he had collected in southern Tanzania. Rather, they were hoping to address a different scientific question about gorgonopsids, which are sometimes called mammal-like reptiles.

When gorgonopsids walked the earth, they were leggy creatures, measuring 2 to 10 feet in length from nose to tail, with fearsome-looking saber teeth. They’re part of a larger group of animals called synapsids, which are thought to have given rise to mammals more than 100 million years ago.

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

Whitney wondered whether the gorgonopsid’s teeth were connected to the jaw the way mammalian teeth are. “Most reptiles alive today fuse their teeth directly to the jawbone,” she explained. “But mammals do not. We use tough but flexible stringlike tissues to hold teeth in their sockets. And I wanted to know if the same was true for gorgonopsians.”

The researchers sliced the jawbone into thin sections and mounted them on slides to study the inner details of the jaw tissues.

“These small details act as storybooks, preserving a lot of information about the biology of these animals while they were alive,” Sidor and Whitney wrote in a Q&A about their research.

An undergraduate researcher, Larry Mose, noticed a strange cluster of small circles on one of the slides. The circles resembled miniature teeth, which are a tip-off for compound odontoma.

Compound odontoma
A histological thin section of the gorgonopsid lower jaw, taken near the top of the canine root. The dark area on the right is bone. The backward C-shaped structure on the left is the canine root. The cluster of small circles resemble miniature teeth, indicative of compound odontoma. (Megan Whitney / Christian Sidor Photo / University of Washington)

Researchers have previously found hints of tumors in older specimens, including fossilized fish that date back as far as 350 million years. But the gorgonopsid jaw revealed that a specific type of tumor associated with modern-day mammals was around even before mammals existed.

That could help shed light on the evolutionary and biological mechanisms behind odontomas in particular and tumors in general.

“This discovery demonstrates how the fossil record can tell us a lot about our present-day lives — even the diseases or pathologies that are part of our mammalian heritage,” Sidor said. “And you could never tell that this creature had it from the outside.”

The research that resulted in the JAMA Oncology article, “Odontoma in a 255-Million-Year-Old Mammalian Forebear,” was funded by the National Science Foundation and a University of Washington Mary Gates Research Fellowship. Sidor’s fieldwork was funded by the National Geographic Society.

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