New Fossils Unlock Evolutionary Secrets of Two-Legged Snakes

Scientists have known about the ancient snake group Najash for years, but were not able to gain a thorough understanding of its skull until now

Artist's interpretation of a two-legged snake
Why did this cheeky snake have two hind legs? Raúl Gómez

In February 2013, Fernando Garberoglio was searching for fossils in the La Buitrera Paleontological Area, a vast region in Argentina’s Río Negro province. Then an undergraduate paleontology student at the Universidad de Buenos Aires, Garberoglio picked up an inconspicuous pebble, which, he was shocked to find, was in fact the well-preserved fossil of an ancient snake skull.

In the wake of this discovery, researchers uncovered multiple other snake fossils, including a total of eight skulls, reports CNN’s Ashley Strickland. The remains are around 95 million years old and belong to a prehistoric snake group known as Najash, after “nahash,” the Hebrew word for snake. It’s a fitting moniker, because like the crafty biblical creature that instigated the fall of man, Najash had legs—hind ones, at any rate.

Najash specimens were first described in 2006, based on a skull and partial skeleton fossils. The creature clearly had “robust hindlimbs,” something that had already been observed in ancient marine snake fossils, but Najash was unique because it was a terrestrial animal. But it was difficult for scientists to get a clear sense of what Najash’s head looked like, since the skull had been found in fragmentary condition.

There are, in fact, many gaps in experts’ understanding of snakes’ evolutionary history, because the fossil record for these slithering creatures is limited. But scientists are keen to learn more about how snakes came to look the way they do today. As Garberoglio and his fellow researchers write in a recent paper in Science Advances, snakes represent “one of the most dramatic examples of the evolutionary versatility of the vertebrate body”—not least because they lost their limbs over time.

The new finds from La Buitrera Paleontological Area are significant because, according to the researchers, they are “three-dimensional [and] largely uncrushed,” offering a remarkably clear glimpse into snakes’ ancient past. Garberoglio, who is now pursuing a Ph.D. at the Fundación Azara at Universidad Maimónides in Buenos Aires, tells Becky Ferreira of the New York Times that the skull specimen he discovered is “the most complete Mesozoic snake skull known and preserves key data on ancient snake anatomy.” That skull belongs to a species known as Najash rionegrina.

After studying the fossil specimens using a technology known as micro-computed tomography scanning, the researchers were able to glean a number of key insights into the anatomy and evolution of prehistoric snakes. Experts have long theorized that snakes arose from “a blind, burrowing lizard ancestor,” explain study co-authors Michael Caldwell and Alessandro Palci. It has been suggested that scolecophidians, an order of small, worm-like burrowing snakes, are the most primitive ones alive today.

But Najash’s skull didn’t look like those of the scolecophidians; while these little snakes have small mouths, Najash’s mouth was big, lined with sharp teeth, and equipped with some of the mobile joints that are seen in snakes today. But unlike modern snakes, Najash had a jugal bone, which is akin to a cheekbone. Experts previously thought that the absence of the jugal was a shared feature of all snakes, living and fossilized.

“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,” Garberoglio says. In fact, according to Caldwell and Palci, ancestral snakes were more similar to their big-headed lizard relatives, like the Komodo dragon.

Like the jugal bone, snakes’ legs disappeared over millennia. There are no surviving fossils of four-legged snakes, but the study authors write that the animals likely had forelimbs at some point—and lost them around 170 million years ago. Some 70 million years after that, Najash was still toting hind legs, suggesting that the appendages were not just a transitory phase in evolutionary history.

“[S]nakes retained their hindlimbs for an extended period of time,” says Garberoglio, “before the origin of modern snakes which are for the most part, completely limbless."

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