Fossil body outline on a slab.
One side of a slab contains a fossil’s body and a mold of its skull; the other reveals the skull and a mold of the body.
H. Tischinger

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Update: Controversial ‘four-legged snake’ may be ancient lizard instead

UPDATE: The fossil Tetrapodophis amplectus will return to the Bürgermeister-Müller-Museum in Solnhofen, Germany, later this month, sources say. The fossil’s owner had temporarily removed it because of damage it had sustained during CT scanning at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. “Two very important bones of the holotype were partially damaged,” says Martin Röper, director of the museum. After investigating the extent of the damage, the owner agreed to return it to the museum—but scientists will now only be able to study it in-house, Röper says. The good news, says Paul Tafforeau of ESRF, is that the facility has improved its imaging protocols for flat fossils, so that “it can never happen again.”

Here is our original story:

SALT LAKE CITY—It is a tiny, fragile thing: a squashed skull barely a centimeter in length; a sinuous curving body about two fingers long; four delicate limbs with grasping hands. In a major paper last year, researchers called this rare fossil from more than 100 million years ago the first known four-legged snake. But at a meeting of the Society of Vertebrate Paleontology (SVP) here last week, another team suggested that it’s a marine lizard instead. Even as scientists debate the identity of this controversial specimen, the only one of its kind, it appears to be inaccessible for further study. And paleontologists are mad as hell.

“It’s horrifying,” says Jacques Gauthier, a paleontologist at Yale University. As far as he’s concerned, if the fossil can’t be studied, it doesn’t exist. “For me, the take-home message is that I don’t want to mention the name Tetrapodophis ever again.”

A year ago, researchers led by David Martill of the University of Portsmouth in the United Kingdom reported in Science that the fossil, which they named Tetrapodophis amplectus (for four-footed snake), was a missing link in the snake evolutionary tree. Researchers knew snakes had evolved from four-limbed reptiles, but few transitional forms had been discovered, and researchers continue to wrangle over whether the first lizards to lose their limbs and become snakes were terrestrial burrowers or aquatic swimmers.

Martill and colleagues reported that the fossil, which they described as a specimen in a German museum, originated from a Brazilian outcrop of the Crato Formation, a 108-million-year-old limestone layer rich in both marine and terrestrial species. They identified snakelike features in the fossil, including a long body consisting of more than 150 vertebrae, a relatively short tail of 112 vertebrae, hooked teeth, and scales on its belly. Those features, they say, support the hypothesis that snakes evolved from burrowing ancestors.

In virtually every single respect [it] looks like a snake, except for one little detail—it has arms and legs.

Nicholas Longrich, paleontologist at the University of Bath, U.K.

But many paleontologists weren’t convinced. Last week at the annual SVP meeting here, vertebrate paleontologist Michael Caldwell of the University of Alberta in Edmonton, Canada, and colleagues presented their own observations of the specimen, rebutting Martill’s paper point by point to a standing-room-only crowd.

The new analysis hinges on the “counterpart” to the original fossil, which was also housed in the Bürgermeister-Müller Museum in Solnhofen, Germany. When the slab of rock containing the fossil was cracked open, the body of the organism stayed mostly in one half of the slab, whereas the skull was mostly in the other half, paired with a mold or impression of the body. This counterpart slab, Caldwell says, preserved clearer details of the skull in particular. In his group’s analysis of the counterpart, he says, “every single character that was identified in the original manuscript as being diagnostic of a snake was either not the case or not observable.”

For example, in snake skulls, a bone called the quadrate is elongated, which allows snakes to open their jaws very wide. This fossil’s quadrate bone is more C-shaped, and it surrounds the animal’s hearing apparatus—a “characteristic feature” of a group of lizards called squamates, says co-author Robert Reisz, a vertebrate paleontologist at the University of Toronto in Mississauga, Canada. He and Caldwell add that although the fossil has more vertebrae in its body than in its tail, the tail isn’t short, but longer than that of many living lizards. They are working on a paper arguing that the fossil is probably a dolichosaur, an extinct genus of marine lizard.

Martill and co-author Nicholas Longrich of the University of Bath in the United Kingdom, neither of whom was at the meeting, stand strongly by their original analysis. Longrich cites all the snakelike features discussed in the original paper. “In virtually every single respect [it] looks like a snake, except for one little detail—it has arms and legs,” he told Science by email.

Many researchers who attended the talk, including Gauthier and paleontologist Jason Head of the University of Cambridge in the United Kingdom, are persuaded that Tetrapodophis is not a snake. But as for what it is, there may be as many opinions as there are paleontologists. Hong-yu Yi of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, says she’s reserving judgment on the specimen’s identity until further analysis. “I was always waiting for a longer description of the specimen. I’m still waiting,” she says.

That analysis may never happen. Caldwell says he went back to the Bürgermeister-Müller Museum several months ago to study the specimen again; separately, Head says he also attempted to study the fossil. Neither could get access to it. Caldwell says that the fossil wasn’t actually part of the museum’s collection, but was on loan from a private owner. Researchers who declined to be named because of ongoing discussions around the fossil say that it may have been damaged during study, prompting the collector to restrict access to it. “I don’t even know if a publication at this moment is appropriate because no one else will be able to access this specimen,” Yi says.

If any good can come out of Tetrapodophis, it’s the recognition that we have got to maintain scientific standards when it comes to fossils … they have to be accessible.

Jason Head, paleontologist, University of Cambridge

In fact, some researchers say the original paper should not have been published, because the fossil was not officially deposited in a museum or other repository, so the authors couldn’t guarantee that future researchers could access it. “I have nothing against” private fossil collecting, Gauthier says. But when a fossil enters the scientific literature, he says, “then it has to be available. Science requires repeatability.” In response, Science deputy editor Andrew Sugden says, “Our understanding at the time of publication and in subsequent correspondence was that the specimen was accessible at the museum, as stated at the end of the paper.”

Researchers had already raised other questions about the fossil’s transport out of Brazil. Brazil passed laws in the 1940s making all fossils property of the state rather than private owners. “Most of the exploration of the limestone quarries in that region of the country began in the second half of the 20th century,” says Tiago Simões, a paleontologist at the University of Alberta, who was also an author on the SVP talk. “So the vast majority” of fossils from those areas were collected after the law had passed. “That really touches on some very sensitive ethical boundaries.”

Head agrees. “The best way to move forward is to literally erase the specimen from our research program. Tetrapodophis is no longer science. … It’s not repeatable, it’s not testable. If any good can come out of Tetrapodophis, it’s the recognition that we have got to maintain scientific standards when it comes to fossils … they have to be accessible."