Famous ‘Four-Legged Snake’ May Really Be Dino-Era Lizard

SALT LAKE CITY, UTAHThe strange-looking fossil was hailed as a once-in-a-lifetime find: a rare snapshot of the way snakes evolved from four-legged lizards. But barely more than a year after it was unveiled, a fresh look at the animal suggests that it is no snake.

Perhaps even more troubling, the fossil may have first reached scientists’ hands under unethical—and possibly illegal—circumstances.

Originally, the 110-million-year-old fossil known as Tetrapodophis was interpreted to be a burrowing snake with two pairs of small limbs, potentially showing that snakes started out on land. If true, the six-inch-long animal would bring clarity to the scientific debate over whether snakes lost their limbs on land—the current favorite hypothesis—or in water.

The new reanalysis instead suggests that Tetrapodophis could be the oldest known dolichosaur, an extinct type of aquatic lizard that lived during the Cretaceous period. If it’s not a snake, the evolutionary waters get that much muddier. (Also see an amazing "nesting doll" fossil that reveals a bug in a lizard in a snake.)

“It doesn’t have to be a snake to be cool—it’s a very interesting thing all by itself,” says Mike Caldwell of the University of Alberta, who co-authored the latest analysis. “A four-legged snake will be really cool, and we will find one. But this one isn’t.”

Slippery Specimen

The as-yet unpublished findings, announced on October 26 at the Society of Vertebrate Paleontology’s annual meeting in Utah, mark the latest development in a controversy that has swirled around the fossil since July 2015, when it was first described in the journal Science. In that paper, a team led by University of Portsmouth paleontologist David Martill claimed that the fossil was a pivotal intermediate stage in snake evolution.

“This little animal is the Archaeopteryx of the squamate world,” Martill said at the time to National Geographic. Squamates are snakes and lizards, and Archaeopteryx is the famous feathered fossil from Germany that represents one of the best known evolutionary links between dinosaurs and birds.

In August 2015, Caldwell and collaborator Robert Reisz of the University of Toronto Mississauga flew to Germany to examine the fossil, which was then housed at the Bürgermeister-Müller-Museum in Solnhofen—a small museum largely housing loaned specimens from private collections.

Much of what Caldwell and Reisz found directly contrasted with the original analysis. For instance, Martill claimed that Tetrapodophis had teeth that curved sharply toward the throat, a classic trait shared by modern and extinct snakes. But when the pair examined the fossil through compound microscopes, they found that the teeth didn’t curve much at all from their base.

Instead, the tooth bases were caked in cementum, a bony material that affixed the teeth to the jaw, and Caldwell says that the teeth appeared to have been dislodged during fossilization. Combined, Caldwell argues, the two factors ultimately created the illusion that the teeth were sharply curved—undercutting one of Martill’s lines of evidence.

“The teeth are wrong, the jaw is wrong, the skull is not right ... and you can see that it has a complete series of bones between the eye and where the jaw is suspended from the skull,” says Caldwell. “You don’t find that in snakes.”

A Matter of Interpretation?

In an emailed statement to National Geographic, Martill—who did not attend the conference and did not see Caldwell’s presentation—stands by his assertion that Tetrapodophis is a snake. (Also see "Why Snakes Don't Have Legs (For Now).")

“Caldwell is simply very wrong,” he writes. “We did, of course, consider a dolichosaur. It is not a dolichosaur. Just take look at it.”

Martill’s co-author, University of Bath paleontologist Nick Longrich, agrees: “Long story short, I stand behind the paper we wrote,” he says in an emailed statement. “Maybe we didn’t get every single detail right, but we got most of it right, and I’m proud of that.”

Longrich—who also did not attend the conference—writes that based on what he knows of Caldwell’s interpretation, he deems it “highly creative” and biased toward Caldwell’s long-running stance that snakes evolved in water as opposed to on land.

“It sucks that some people are so wedded to their ideas that they aren’t open to new facts—and actually need to reinterpret the facts to fit their argument—but this type of thing happens in science,” Longrich adds.

But paleontologist Jason Head of the University of Cambridge is now less sure of the first interpretation.

“[Caldwell] is more right than the original publication, for sure,” says Head, an expert on squamate evolution who wasn’t involved with the 2015 study or the new analysis. “There was a lot of anatomical interpretation [in the original] that I have a hard time seeing from the photos.”

Yale paleontologist Jacques Gauthier, a world-renowned squamate expert, agrees with Head. “I think [Caldwell] makes a pretty good argument that it’s a dolichosaur rather than a snake,” he says.

However, Gauthier notes that Caldwell’s argument—which emphasizes snakes’ distinctiveness from water-dwelling dolichosaurs—undercuts Caldwell’s own ideas that snakes have aquatic origins.

“The very reasons [Caldwell] argues it’s a dolichosaur are the very same reasons that reject his hypothesis,” says Gauthier. “There’s the rub.”

‘It’s Not Science’

In addition to the scientific sniping, the situation became even stickier in August 2015, when officials announced that they are actively investigating whether the Tetrapodophis fossil left Brazil illegally.

In 1942, the Brazilian government outlawed the sale or export of Brazilian fossils without government permission. Yet the Crato Formation, the fossil’s putative place of origin within Brazil, wasn’t excavated intensively until the period spanning the 1950s to 1970s, says University of Alberta paleontologist Tiago Simões, one of Caldwell’s students and colleagues on the new analysis. That suggests the fossil left Brazil after World War II, most likely without permission—one of perhaps hundreds, if not thousands, of others like it reported in the scientific literature.

Jean-Claude Rage, a paleontologist with France’s National Museum of Natural History, also has concerns. “From what I know, the specimen was really exported illegally,” he writes. “Obviously, this should be checked, but if the story [of illegal exportation] is true, this may be an important problem.”

For his part, Martill argues that laws such as the one in Brazil unnecessarily restrict access to specimens. In a 2015 interview with the Brazilian newspaper Estadão, Martill said that he was “critical of all laws that interfere with the science of paleontology,” adding that they can “lead to xenophobia—Brazil fossils for Brazilians, British fossils for Brits. It should be fossils for all.”

Many of Martill’s peers, including Head, take umbrage with his views.

“The study of the history of life from the fossil record is a global process, and people from just about every nation on Earth are involved,” he says. “If you can’t respect the laws, customs, and culture of the country—the people—whence the fossil came, you are doing damage to that global study.”

Making matters even more contentious, the specimen in question is the only known Tetrapodophis fossil, and it was on loan to the museum in Germany by its owner.

However, if a scientifically studied fossil is privately owned, the owner can deny access to the fossil on a whim, making it all but impossible for future scientists to reevaluate Martill and Caldwell’s contrasting results. That’s why studying privately owned fossils is frowned upon by the Society of Vertebrate Paleontology.

And earlier this year, the fossil’s owner removed the specimen from the Bürgermeister-Müller-Museum. (At press time, National Geographic was unable to confirm the owner's identity. In an email, Longrich writes that the owner wishes to remain anonymous.)

Prior to the Science publication, Gauthier had informally consulted Martill's team on Tetrapodophis, at the time believing—like Martill—that the fossil had been formally ceded to the museum. Martill and Gauthier both point out that the museum had given the fossil a specimen number, which is almost always a sign of a fossil's permanent place in a museum's collection.

Now Gauthier says that because the fossil cannot be reliably reexamined, it should be treated as scientifically out-of-bounds. “Tetrapodophis is an invalid name that doesn’t exist, as far as I’m concerned,” he says.

Head agrees: “It’s not science once you’ve gotten to that point. Part of the scientific process is repeatability and the possibility of falsification. The specimen is gone, and there’s nothing we can do about it.”