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This 100 million-year-old snake had hind legs and a lost bone that changes evolution

A legged, big-mouthed ancient snake fossil flipped the origin story of snakes on its head.

Date:

April 24, 2026

Source:

University of Alberta

Summary:

Nearly 100 million years ago, snakes weren’t the sleek, limbless creatures we know today—they still had hind legs and even a cheekbone that has almost vanished in modern species. A remarkably preserved fossil of Najash rionegrina from Argentina has reshaped how scientists think about snake origins, suggesting early snakes were large, wide-mouthed predators rather than tiny burrowers.

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Research from a collaboration between Argentinian and University of Alberta paleontologists adds a new piece to the puzzle of snake evolution. Credit: Raúl Gómez

A remarkably preserved fossil from Argentina is helping scientists sharpen the picture of how snakes evolved. The specimen belongs to Najash rionegrina, an ancient rear-limbed snake that lived nearly 100 million years ago. Its skull shows that these early snakes still had a cheekbone, also called the jugal bone, a feature that has almost completely disappeared in living snakes. The 2019 study added an important piece to a fossil record that had long been too sparse to clearly explain the earliest stages of snake evolution.

The findings also challenged a popular older idea about snake origins. Instead of beginning as small burrowers, the evidence from Najash pointed to ancestors of modern snakes that were larger-bodied animals with wide mouths. The fossils also showed that early snakes held onto their hindlimbs for a long time before the rise of the mostly limbless snakes alive today.

"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," explained Fernando Garberoglio, from the Fundación Azara at Universidad Maimónides, in Buenos Aires, Argentina and lead author on the study. "The study also reveals that early snakes retained their hindlimbs for an extended period of time before the origin of modern snakes which are for the most part, completely limbless."

Hidden skull details inside a 100 million year old fossil

The fossil snakes described in the study came from Northern Patagonia and are closely tied to an ancient southern lineage that lived across the continents of Gondwana. Researchers say that group appears to be related to only a small number of unusual snakes still living today. To see inside the specimen without damaging it, the team used micro-computed tomography (micro-CT) scanning. That let them reconstruct the skull in exceptional detail, including the paths of nerves and blood vessels as well as bones buried inside the rock.

That level of detail helped resolve a long-running anatomical debate. Scientists had misunderstood the jugal bone in snakes and snake relatives for generations, and the Najash fossils gave them direct evidence to correct the record. The study's authors argued that these new skulls and skeletons clarified the sequence of bone loss that eventually produced the highly specialized skulls of modern snakes.

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

"This research is critical to understanding the evolution of the skulls of modern and ancient snakes," added Caldwell.

The paper, "New Skulls and Skeletons of the Cretaceous Legged Snake Najash, and the Evolution of the Modern Snake Body Plan," was published in Science Advances in 2019.

Later studies added more twists to the snake origin story

Research published after the 2019 Najash paper has made the story even more interesting. In 2020, paleontologists described Boipeba tayasuensis, a Late Cretaceous blind snake from Brazil. That fossil pushed the record of blind snakes deeper into the age of dinosaurs and suggested that some early blind snakes were much larger than their living relatives, topping 1 meter in length. The finding supported the idea that parts of early snake evolution in Gondwana were more diverse, and often bigger-bodied, than once assumed.

Then, in 2023, another Science Advances study approached snake origins from an entirely different angle by reconstructing the brains of living squamates and fossil snakes. That work suggested the ancestor of crown snakes, meaning the group that gave rise to living snakes, may have been adapted for burrowing while also behaving opportunistically. Rather than neatly settling the debate, the result showed that snake origins were likely complex