A Blog by Ed Yong

A Fossil Snake With Four Legs

Snakes can famously disarticulate their jaws, and open their mouths to extreme widths. David Martill from the University of Portsmouth did his best impression of this trick while walking through the Bürgermeister Müller Museum in Solnhofen, Germany. He was pointing out the museum’s fossils to a group of students. “And then my jaw just dropped,” he recalls.

He saw a little specimen with a long sinuous body, packed with ribs and 15 centimetres from nose to tail. It looked like a snake. But it was stuck in unusual rock, with the distinctive characteristics of the Brazilian Crato Formation, a fossil site that dates to the early Cretaceous period. Snake fossils had been found in that period but never that location, and in South America but never that early. The combination of place and time was unusual.

Tetrapodophis specimen. Credit: Dave Martill.
Tetrapodophis specimen. Credit: Dave Martill.

“And then, if my jaw hadn’t already dropped enough, it dropped right to the floor,” says Martill. The little creature had a pair of hind legs. “I thought: bloody hell! And I looked closer and the little label said: Unknown fossil. Understatement!”

“I looked even closer—and my jaw was already on the floor by now—and I saw that it had tiny little front legs!” he says. Fossil-hunters have found several extinct snakes with stunted hind legs, and modern boas and pythons still have a pair of little spurs. “But no snake has ever been found with four legs. This is a once-in-a-lifetime discovery.”

Tetrapodophis forelimb. Credit: Dave Martill.
Tetrapodophis forelimb. Credit: Dave Martill.
Tetrapodophis hindlimb. Credit: Dave Martill.
Tetrapodophis hindlimb. Credit: Dave Martill.

 

Martill called the creature Tetrapodophis: four-legged snake. “This little animal is the Archaeopteryx of the squamate world,” he says. (Squamates are the snakes and lizards.) Archaeopteryx is the feathered fossil whose mish-mash of features hinted at the evolutionary transition from dinosaurs to birds. In the same way, Martill says, the new snake hints at how these legless, slithering serpents evolved from four-legged, striding lizards.

There are two competing and fiercely contested ideas about this transition. The first says that snakes evolved in the ocean, and only later recolonised the land. This hypothesis hinges on the close relationship between snakes and extinct marine reptiles called mosasaurs (yes, the big swimming one from Jurassic World). The second hypothesis says that snakes evolved from burrowing lizards, which stretched their bodies and lost their limbs to better wheedle their way through the ground. In this version, snakes and mosasaurs both independently evolved from a land-lubbing ancestor—probably something like a monitor lizard.

Tetrapodophis supports the latter idea. It has no adaptations for swimming, like a flattened tail, and plenty of adaptations for burrowing, like a short snout. It swam through earth, not water.

It hunted there, too. Its backward-pointing teeth suggest that it was an active predator. So does the joint in its jaws, which would have given it an extremely large gape and allowed it to swallow large prey. And tellingly, it still contains the remains of its last meal: there are little bones in its gut, probably belonging to some unfortunate frog or lizard. This animal was a bona fide meat eater, and suggests that the first snakes had a similar penchant for flesh.

Martill thinks that Tetrapodophis killed its prey by constriction, like many modern snakes do. “Why else have a really long body?” he says. In particular, why have a long body with an extreme number of vertebrae in your midsection? None of the other legless lizards have that, even burrowing ones. Martill thinks that this feature made early snakes incredibly flexible, allowing them to throw coils around their prey.

Their stumpy legs may even have helped. It’s unlikely that Tetrapodophis used these limbs to move about, and they don’t seem to have any adaptations for burrowing. With tiny “palms” and long “fingers”, they look a little like the prehensile feet of sloths or climbing birds. Martill thinks that the snake may have used these “strange, spoon-shaped feet” to restrain struggling prey—or maybe mates.

Tetrapodophis catching a lizard. Credit: James Brown, University of Portsmouth
Tetrapodophis catching a lizard. Credit: James Brown, University of Portsmouth

But is it even a snake? “I honestly do not think so,” says Michael Caldwell from the University of Alberta, who also studies ancient snakes. He says that Tetrapodophis lacks distinctive features in its spine and skull that would seal the case. “I think the specimen is important, but I do not know what it is,” he adds. “I might be wrong, but that will require me to see the specimen first hand. I’m looking forward to visiting Solnhofen.”

It’s certainly possible that Tetrapodophis could be something else. In the squamates alone, a snake-like body has independently evolved at least 26 times, producing a wide menagerie of legless lizards. These include the slow worm of Europe, and the bizarre worm-lizard Bipes, which has lost its hind legs but has kept the stubby front pair. True snakes represent just one of these many forays into leglessness.

Susan Evans from University College London, who studies reptile evolution, is on the fence. “This happens every time a possible early snake is described,” she says. “Opinions on snake evolution are highly polarised.”  She says that Tetrapodophis has some features you’d expect from an early snake, and doesn’t easily fit into any other known group of squamates. The specimen is also more complete than many other recently alleged snakes, some of which are known only from fragments of vertebrae or jaw. “Unfortunately, the skull is poorly preserved and this complicates interpretation,” says Evans. “The most important thing is that it is now brought to notice and it will be thoroughly scrutinised by other workers.” Above all, she hopes that someone finds one with a better skull.

Martill insists that Tetrapodophis has “got loads of little things that tell you it’s a snake.” There’s the backwards-pointing teeth, the single row of belly scales, the way the 150 or so vertebrae connect to each other, and the unusually short tail. (In lizards and crocodiles, the tail can be as long as the entire body, but a snake’s tail—everything after the hip—is relatively short.) Some of these features are found in other legless lizards, but only snakes have all of them. And Martill adds that you just wouldn’t expect an ancestral snake to have all the features that its descendants picked up over millions of years of evolution.

He also teamed up with Nick Longrich at the University of Bath to compare Tetrapodophis’s features to those of both modern and fossil snakes. Their analysis produced a family tree in which Tetrapodophis came after the earliest known snakes like Eophis, Parviraptor, and Diablophis, but is still very much a snake.

But how could that be? Eophis and the others only have two legs, so how could four-legged Tetrapodophis have come after them? The answer is that evolution doesn’t proceed along simple, straight lines. Even if four-legged lizards gave rise to four-legged snakes, then two-legged snakes, then legless ones, the later stages don’t displace the former ones. For a long time, they would all exist together, in the same way that birds co-existed with the feathered dinosaurs that gave rise to them. (This, incidentally, is also the answer to that tired question: “If we evolved from monkeys, why are there still monkeys?”)

“At any one time in the Cretaceous, chances are you’ve got ten, twenty, maybe thirty species [of early snakes], all going off on their own evolutionary paths,” says Martill. “There would be a whole bunch of very snake-like lizards, all with the potential to become today’s snakes. One of them does. Maybe one of them goes off and loses its front legs and retains its back legs for 20 million years. One maybe loses its back legs and keeps its front legs—and we haven’t found that one yet.”

Reference: Martill, Tischlinger & Longrich. 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science http://dx.doi.org/10.1126/science.aaa9208

29 thoughts on “A Fossil Snake With Four Legs

  1. That is one funny-looking bunch of bones. Too bad about the skull. Need to find more fossils….more fossils always good.

  2. How does this sort of thing just get put on display at a museum, and be labeled “Unknown Specimen?” Is this the German equivalent of the Creation “Museum?”

  3. Do not forget similar early snakes (“protosnakes”) occur all the way back into the Middle Jurassic.

  4. To Slurpy: I interpreted it as he was showing students (of the University rather than elementary school type) through the museum’s collection, rather than leading a tour through the museum’s public exhibits. I imagine there are lots of unknown things gathering dust in the museums of the world, waiting for someone to get around to analysing them!

  5. Just to remember fellow scientists that fossil trade is illegal in Brazil. How such a beautifully preserved specimen was sent abroad still remains an unsolved mistery. This way of conducting scientific discoveries makes Brazilian scientists very upset, regarding that Dave Martill is a well known unethical arrogant scientist who mysteriously hovers over big discoveries and amazes evebody with his capacity of passing unscathed through peer reviews. This kind of discovery could stimulate new generations of scientists in Brazil. Science may have won as Dave Martill has said in the link bellow, although Brazilian Science looses again. Shame on him. Shame on him…

    Please, I invite you all to check this enlightening link and see how challenger this illustrious Sir is:

    http://goo.gl/VeBn4G

  6. Hi Ed,

    The issue I mentioned on Twitter is that export of fossils from Brazil has been illegal since 1942. Sale of specimens from “old collections” is still illegal, but Martill himself has said that most of these “old collections” are poached fossils being illegally traded (see: https://www.geolsoc.org.uk/Geoscientist/Archive/November-2011/Protect-and-die). This is almost certainly a poached specimen acquired on the black market. The authors of this paper are certainly aware of that, by their own admission. One has to wonder whether the authors know more about the provenance of this specimen than they report but cannot say so because it would give the game away.

    This is by all accounts a spectacular specimen. But this specimen is part of the natural heritage of Brazil. The way this specimen has been handled by the authors (by “permanently loaning” the specimen to a museum in Germany rather than repatriating it to Brazil) shows contempt towards Brazilian laws, towards our Brazilian colleagues, and to the Brazilian people. The choice of Science to publish this paper knowing full well the ethics issues involved demonstrates a similar attitude.

  7. @Slurpy: Solnhofen is a small town, and the museum is equally tiny, albeit located in close proximity to some fossil-bearing limestone deposits.
    My guess is that whoever put up the fossil for display was simply an amateur curator and might not have realized its importance.

  8. Very interesting find indeed. I hope they can uncover similar fossils to get a better understanding of the specimen.

  9. i’m really astonished and like Martill said, my jae is equally dropping
    This is a really good job from Martill
    i think his elbows should be greased.

  10. So when do we get some DNA evidence…. Oops, it’s too old for that. Or is it? Fossilized dino bones recently yielded some DNA. With DNA, you can count the chromosome pairs, and that will tell you which animals it’s related to. Or not related to.

    [Nope, fossilised dino bones have not yielded DNA. You’re thinking of proteins. – Ed]

  11. Solnhofen is supposed to be one of the big fossil hotspots, though, isn’t it? Shouldn’t they have a pretty decent paleontologist collection onboard, a la Museum of the Rockies?

  12. “Why else have a really long body [if not constriction]?” – lots of reasons, such as: swimming like an eel, burrowing or sand-swimming or grass-swimming like most of those 26 ‘snake-like’ lizard lineages that don’t constrict, swallowing lots of prey items one after another, entering narrow crevices to catch prey or to hide. Far too many good reasons to pick one without any evidence that singles it out. The total number of trunk vertebrae is a snake-like feature and some snakes constrict, but the causal connection is missing: for example, the Australian elapid snake _Simoselaps littoralis_ has reduced trunk vertebra number to only just over 100 and uses constriction, while many long-bodied snakes don’t constrict at all. Calling _Tetrapodophis_ a burrower and claiming that it supports a burrowing over aquatic origin is also going well beyond the evidence: we won’t know that before a well-enough-preserved skull is described. Most aquatic snakes don’t have paddle-like tails and they swim just fine, but burrowing snakes not only have modified skulls (not simply and not always a short snout or long braincase) but almost always short or thick tails and narrow belly scales – unlike Tetrapodophis.

    Also this: “Eophis and the others only have two legs” – NO, nobody knows how many legs (or vertebrae) they had because the Jurassic ‘snakes’ are only known from disarticulated and extremely incomplete remains.

    I’m thrilled by this fossil, and find it more convincing as a snake than any of the older ones reported by Caldwell and colleagues, but I really want to see more detail on the skull and the vertebral column. We should be able to say how long a snake’s neck is now, but the paper doesn’t even attempt to locate the atlas or axis vertebra or the cervical-thoracic junction! (it looks to me like at least 10 cervicals, which is more than most lizards but not as many as I was expecting).

    The possibility that this is a recently collected, smuggled specimen is a concern, but absolutely should not have prevented it being studied and described. It is (so far) absolutely unique and irreplaceable, and nothing would be gained by suppressing the information it embodies.

  13. I feel modern science should throw everything non-destructive at the specimen, including intricate 3D imaging with all the internal scans considered. Then it should be repatriated, with best copies sent to current and recent repositories. Then specimen-destroying test samples can be requested of the rightful repository, to be provided at the choice of the rightful holder. I know all this isn’t inexpensive, but I feel it is appropriate.

  14. It should be noted that tetrapodophis may be, but most likely is not, a direct antecedent of modern snakes. It more likely would be a close or distant cousin to a forefather of our snakes. Indeed, their predecessors of tetrapo’s time might already have partly or wholly lost limbs. Given the expected diversity of the era, who kows how many parallel foot-losing evolutionary paths have been followed. But this proves the development path potential, and that’s really all we need to know. (Pardon my repeated inputs. Laid up with the flu and no better way to spend my time. This is the current most interesting paleo topic.)

  15. Amazing find! I hope that more like it are found, to build some concrete evidence of these creatures and to give us a deeper look of where they came from. Thank you!

  16. Tony S: yes, next stop should be Grenoble for 3D scanning by synchrotron, like DOI: 10.1080/02724634.2011.539650
    (regular X-rays can’t distinguish fossil bone from limestone)

  17. Snakes can famously disarticulate their jaws

    No. They have extra joints in the parts of the skull that suspend the jaw joint, and extra joints in the lower jaws themselves; furthermore, they lack a connection between the left and the right lower jaw which can therefore spread apart. But they do not disarticulate anything.

    Solnhofen is supposed to be one of the big fossil hotspots, though, isn’t it? Shouldn’t they have a pretty decent paleontologist collection onboard, a la Museum of the Rockies?

    No, why? It’s really a small town, and in western Europe nobody assumes that people would go very far to visit a museum.

    What is the definition of the term “snake” ?

    Today’s snakes and everything more closely related to them than to any other kind of lizard…

  18. According to Genesis 3, upon the fall of man God made life difficult for Adam, Eve and the serpent who deceived them. The serpent up to that point walked. At that point, he was made to slither.

    Is this evidence of evolution? There are no other transitional forms with which evolution occurred. It is however proof of the credibility of the scriptures. Walking serpents were recorded in the scriptures six thousand years ago, yet we are only now seeing the evidence of what snakes looked like to Adam and Eve.

    Just another perspective for those who are “tolerant” of “diverse” ideas. Let the assault on me and my non-conformist view commence.

  19. Did it really take this long for a “non-conformist” (who conforms to a literal interpretation of a garbled narrative much less than 6000 years old in any written form) to find this story?

    Of course there are plenty of transitional forms in the fossil record (Archaeopteryx, Tiktaalik, Australopithecus, Homo erectus and thousands more), and each time we find evidence for a pattern of genealogical relationship among known species, the evolutionary hypothesis predicts that more intermediates may be found. Because snakes are demonstrably members of more inclusive groups characterised by four legs (Squamata, Diapsida, Amniota, Tetrapoda etc.), fossils of four-limbed stem-snakes were predicted by science more than a century ago, and specifically predicted to have lived in the late Jurassic or early Cretaceous. The existence of fossils is never even mentioned in Genesis (or anywhere else in the bible), let alone any specific predictions of what combination of morphological features to expect in rocks of early Cretaceous age.

    Which idea is more informative about the actual history of the world? What else can Scott predict, and where should we dig for evidence for or against the ‘goddidit’ hypothesis?

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