What makes a snake a snake? Hint: When it comes to the really ancient ones, it's not a lack of legs.

'I'm rather convinced that these animals would have had four legs.' - Michael Caldwell, University of Alberta

Knowing that helped a University of Alberta paleontologist discover snakes far older than any ever found before — snakes so old that they may have crawled on four legs.

University of Alberta paleontologist Michael Caldwell and his team have found four new ancient snake species that lived among the dinosaurs 140 million to 167 million years ago, between the Middle Jurassic and early Cretaceous periods.

Jaw of Parviraptor estesi

The jaw of the ancient snake Parviraptor estesi was originally thought to belong to a lizard. Caldwell and his team showed it is actually a snake, based on its teeth and the way the jaw connects to the skull. (Courtesy Michael Caldwell/University of Alberta)

"It tells us for sure that snakes are a great deal older in geological time than we thought they were," Caldwell said.

Caldwell was the lead author of a new paper describing the new snakes, published Tuesday in the journal Nature Communications.

Previously, the oldest snake fossils ever found – some of which had hind legs – lived around 100 million years ago.

Where did Caldwell manage to find snakes so much older?

The answer illustrates that paleontology is not always glamorous work.

Digging through drawers

Caldwell and his collaborators spent an entire decade digging, not in some exotic fossil bed in the desert, but through the drawers and cabinets of museum collections around the world.

Portugalophis lignites snake fossil

The ancient snake Portugalophis lignites, which lived in a coal swamp in Portugal during the Upper Jurassic, climbs a gingko tree in this illustration.

He was inspired to look for them after accidentally finding some ancient snakes while looking for lizard fossils during previous studies.

The new snakes found by Caldwell and his colleagues had been excavated in the U.K., Portugal and the United States. They had all been mistaken for ancient lizards.

Caldwell says that's understandable.

For one thing, the fossils were not complete – they consisted of partial skulls and pieces of the animals' backbones, ribs and hips, but not enough to tell how long the animals were and how they were shaped.

For another thing, lizards and snakes look quite similar. They're both reptiles and they're both scaly. Some lizards are even long and legless.

And ancient snakes were even harder to tell apart from lizards than modern snakes.

"I'm rather convinced that these animals would have had four legs," Caldwell said. "If they in fact did, they would have looked remarkably like a lizard."

Jaws that open wide. Really wide.

It was the fossils' jaws that tipped Caldwell off to their true identities.

Michael Caldwell

University of Alberta paleontologist Michael Caldwell was inspired to go looking for snakes in museum cabinets around the world after he stumbled upon some mislabelled snakes by accident while looking for lizard fossils during previous research. (Courtesy Michael Caldwell)

It turns out what makes a snake a snake is its ability to take gluttony to a whole new level.

Human and lizard jaws are hinged pretty tightly, setting strict limits on how much we can bite off at once.

A snake's jaws, on the other hand, contact each other in a "slidey kind of plastic way," Caldwell said.

"That skull is a little bit overall able to deform, and get out of way when the snake is trying to swallow an egg that is three times the size of its head or whatever they're taking on."

The teeth of the fossils were also very snake-like.

Caldwell added that it's pretty typical for animal lineages to evolve specialized skulls for a certain type of feeding before evolving specialized body shapes.

He also noted that groups of animals always evolve long before they appear in the fossil record.

Diablophis gilmorei snake

The ancient snake Diablophis gilmorei, which lived during the Upper Jurassic, hides in a ceratosaur skull in Fruita, Colo., in this artist's representation. (Julius Csotonyi)

Now, Caldwell is interested in looking for even older snakes in order to figure out where snakes came from in the first place.

"If we want to find four-legged snakes and get better insights on the origin of the group," he said, "then we need to go back before the Middle Jurassic."

Caldwell co-authored the paper with Randall Nydam at Midwestern University in Glendale, Ariz., his former grad student Alessandro Palci of the South Australian Museum in Adelaide, Australia, and Sebastian Apeseguia of Universidad Maimonides in Buenos Aires, Argentina.

The research was funded by the Natural Science and Engineering Research Council of Canada, the University of Alberta, Midwestern University Alberta Innovates and Maimonides University.