The discovery of the fossilized remains of an ancient primitive snake species suggests rear-legged snakes living during the Cretaceous Period still had a cheek bone.

The remains of the rear-limbed snake species Najash rionegrina, unearthed in Argentina, were remarkably well-preserved, revealing the presence of a type of cheek bone called a jugal bone that is absent in most modern snake species.

Researchers used micro-computed tomography to image the well-preserved fossil and identify its unique components. In addition to the novel jugal bone, scientists observed the pathways of nerves and blood vessels inside the snake.

Scientists described their discovery this week in the journal Science Advances.

"Our findings support the idea that the ancestors of modern snakes were big-bodied and big-mouthed -- instead of small burrowing forms as previously thought," Fernando Garberoglio, lead study author and a researcher with the Center for Natural, Environmental and Anthropological Sciences at Maimonides University in Buenos Aires, said in a news release. "The study also reveals that early snakes retained their hind limbs for an extended period of time before the origin of modern snakes which are for the most part, completely limbless."

Reconstructing the evolutionary history of early snakes has proven difficult as a result of the dearth of well-preserved snake fossils, but the latest discovery has allowed paleontologists to begin to piece together parts of the puzzle.

"This research revolutionizes our understanding of the jugal bone in snake and non-snake lizards," said study co-author Michael Caldwell, professor of biological Sciences at the University of Alberta in Canada. "After 160 years of getting it wrong, this paper corrects this very important feature based not on guesswork, but on empirical evidence."

The rear-legged snakes of the Cretaceous, including the species described in the newly published paper, are close relatives of the serpentine lineage that populated the Southern Hemisphere continents that made-up the supercontinent Gondwana. However, the rear-legged snakes are only related to a small group of relatively obscure modern snakes.

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