A legged snake fossil found in 2003 in Patagonia, Argentina, may be the most primitive snake ever found – the sacral region is shown here (Image: H Zaher) An articulated portion of the Najash fossil (Image: H Zaher)

Scientists have found fossils of a legged snake with “hips” – a specimen that could be the most primitive snake ever unearthed. The find suggests early snakes were not creatures of the sea and has reignited the debate over how snakes evolved.


Sebastián Apesteguía at the Argentine Museum of Natural History and his team found the snake fossil in a terrestrial deposit in the Río Negro province of north Patagonia, Argentina, in 2003. Unlike a handful of legged fossils found in marine deposits and identified as snakes over the past decade, the new fossil, named Najash rionegrina, has a well-defined sacrum supporting a pelvis and functional hind legs outside of its ribcage.

The creature’s skeletal structure suggests it was evolutionarily closer to its four-legged ancestor than previous fossils. And since the scientists found it in a terrestrial deposit, it is near certain that the animal lived on land.

“This snake is an important addition because it is the first snake with a sacrum. This represents an intermediate morphology that has never before been seen,” says Hussam Zaher, curator of herpetology at the University of São Paulo in Brazil, and part of the research team.

The burrowers

The fossil was found in a deposit from the late Cretaceous period and Zaher says the snake is at least 90 million years old. “This fills an important morphological gap of information regarding the early evolution of snakes,” he says.

Zaher and Apesteguía argue that the Najash fossil supports the hypothesis that snakes evolved on land, eventually losing their limbs as they became soil burrowers.

That idea was popular for most of the 20th century, but when legged fossils found in marine sediments in and around Israel were identified as snakes around the turn of the millennium, a group of scientists resurrected an older, alternate theory. They say snakes lost their limbs in the oceans and seas rather than on land, and that they evolved from now extinct marine lizards called mosasaurs.

Fins and paddles

Zaher told New Scientist: “We can now reject the hypothesis of marine origin. This Najash snake fossil suggests that mosasaur lizards were not the most closely related group of lizards to snakes.” He says the marine legged snakes are of a more recent lineage and probably represent the first invasion of the sea by snakes.

Blair Hedges, an evolutionary biologist at Pennsylvania State University, US, says: “In one fell swoop, this new fossil kind of casts doubt on the aquatic hypothesis.”

His DNA sequencing studies suggest a terrestrial origin for snakes. And he says that, looking at evolutionary history, it is difficult to find examples of limb loss in an aquatic environment. “We see many cases where animals that walked on land eventually evolved lineages that invaded the oceans. Almost all of them kept their limbs and turned them into fins or paddles,” he says.

Missing ancestor

Zaher admits that even if the new fossil does prove snakes did not lose their legs in the seas, there are many questions about snake evolution left unsolved. At the top of that list is the question of what lizard group is most closely related to snakes. “We do not have an undisputed hypothesis on that question,” he admits.

Michael Caldwell, from the University of Alberta, Canada, and one of the researchers who reintroduced the marine hypothesis, told New Scientist: “These specimens provide important new information on the anatomy of Cretaceous snakes.” But he is also critical of the interpretations of the new fossil’s anatomy.

He argues that without identifying a closest ancestor, there is no robust way of gaining insight into the origin of snakes and says the new study’s assessment of the Najash snake only takes into account snakes rather than all the snakes and lizards in the squamate order, which includes mosasaurs.

Furthermore, he argues, calling the specimen the most primitive may be incorrect because the fossils identified in marine environments are at least 8 million years older.