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.
“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.”
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 Worldbig 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.
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