​Washington, Dec 9 (IANS) US researchers said on Thursday they have discovered a tumour in a 255-million-year-old mammalian ancestor called a gorgonopsian.

The tumour, found in the extinct species's fossilised jaw, is a benign one made up of miniature, tooth-like structures, they reported in the latest issue of Journal of the American Medical Association Oncology.

Known as a compound odontoma, this type of tumour is common to mammals today. Before this discovery, the earliest known evidence of odontomas came from Ice Age-era fossils, Xinhua reported.

"We think this is by far the oldest known instance of a compound odontoma," said senior author Christian Sidor, a University of Washington (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 tumour."

According to the study, gorgonopsians were distant mammal relatives and the top predators the apex predator during its pre-dinosaur era about 255 million years ago. 

These animals are part of a larger group of animals called synapsids, which includes modern mammals as its only living member.

Sidor and colleagues spotted the tumour when they examined wafer-thin slices of the fossilised gorgonopsian jaw, collected in southern Tanzani, to see how the tooth was nestled within its socket.

They immediately noticed irregular clusters of up to eight tiny, round objects embedded next to the root of a canine tooth. The objects within each cluster resembled small, poorly differentiated teeth, or toothlets, that harboured distinct layers of dentin and enamel.

"At first we did not know what to make of it," said Megan Whitney, lead author and UW biology graduate student. "But after some investigation we realized this gorgonopsian had what looks like a textbook compound odontoma."

In humans and other mammals, the tumour's toothlets grow within the gums or other soft tissues of the jaw and can cause pain and swelling, as well as disrupt the position of teeth and other tissues, the researchers said.

Odontomas are considered benign tumours because they do not metastasize and spread throughout the body. But given the disruptions they cause, surgeons often opt to remove them.

"Until now, the earliest known occurrence of this tumour was about one million years ago, in fossil mammals," said Judy Skog, programme director in the National Science Foundation's Division of Earth Sciences, which funded the research.

"These researchers have found an example in the ancestors of mammals that lived 255 million years ago. The discovery suggests that the suspected cause of an odontoma is not tied solely to traits in modern species, as had been thought," Skog added.