When snakes walked the Earth . . . sort of
The ‘amazing’ discovery of four-limbed creature helps fill a gap in evolution
“I can tell you firsthand that this is the type of fossil where you see it and you’re like, ‘Whoa — this is cool. This is special.’ ” LUKE MAHLER UNIVERSITY OF TORONTO
Adam, Eve, Harry Potter, and haters everywhere can think whatever they want. The fact is that snakes are one of the most successful animal groups on the planet, with more than 3,000 species slithering their way across every continent but Antarctica.
Yet the evolutionary path that enabled snakes’ reptilian dominance has been a matter of debate. Did they emerge from the sea, or did early snakes burrow their way to success on land?
The stunning discovery of a fourlegged, 100-plus-million-year-old fossilized snake, described Thursday in the journal Science, is awing and agitating scientists and reigniting that debate.
“First and foremost, this is an amazing, absolutely fascinating discovery,” said Luke Mahler, an evolutionary biologist at the University of Toronto who specializes in reptile diversification and was not involved in the discovery. “I can tell you firsthand that this is the type of fossil where you see it and you’re like, ‘Whoa — this is cool. This is special.’ ” The ‘huggy’ one Scientists have long argued over whether snakes evolved from marine ancestors — like the massive, aquatic Mosasaur — or from burrowing lizards. Both environments tend to promote elongated bodies and the loss of limbs. Yet until recently, Mahler said, the prospects for resolving this debate were “grim”: snakes’ small bones preserve badly.
In the last decade, the discovery of several ancient fossilized snakes with two burrowing-adapted hind limbs tipped the debate in favour of terrestrial origins.
The new four-limbed snake, named Tetrapodophis amplectus, fills in the picture more clearly, said author Nick Longrich, a vertebrate paleontologist at the University of Bath.
“It shows nothing in the way of aquatic adaptations: it doesn’t have a big oar-like tail, it doesn’t have fins,” he said. He and his co-authors argue that the snake’s digits were for adapted for grasping prey or mating partners: “amplectus” means embrace in Latin. “Basically it’s ‘the huggy snake,’ ” said Longrich.
Yet not everyone is convinced about the purpose of the animal’s strange limbs — or even that the animal is a snake.
“A snake-like body does not a snake make,” said Robert Reisz, a vertebrate paleontologist at the University of Toronto Mississauga. Some ancient and modern lizards lack limbs and have long, slender bodies. Reisz also said the animal’s limbs seem paddle-like. “It’s going to be a highly controversial paper.” Gondwanese or Laurasian? The four-legged specimen was discovered in a private collection, but analyses traced its origins to a lime- stone basin in Brazil that holds incredibly well-preserved specimens from the early Cretaceous period. The animal has clearly visible scales and soft tissue, and even the remains of another animal inside it.
When the snake was alive, anywhere from 100 to 146 million years ago, Brazil was part of a southern supercontinent known as Gondwana. Gondwanaland, as it is also called, produced an incredible diversity of strange animal groups, and the au- thors of the Science paper argue snakes were one.
“Most of the weird things that evolved in Gondwanaland don’t make it to present day. But snakes are one success story from Gondwanaland that survived to present day and have really taken off,” said Longrich.
But Mahler points out that older snakes about which far less is known were discovered in the northern hemisphere, what was once part of the northern supercontinent known as Laurasia.
“I think it’s still an open question as to whether that (those) key changes that we think of when we think of snake evolution happened globally or happened on Gondwana,” he said. The better to eat you The animal remains that are visible inside the fossilized snake are also raising eyebrows. Longrich and his co-authors argue that they are the bones of a small vertebrate that the snake consumed. That’s significant because scientists are also still arguing over whether early snakes actually ate insects: this one apparently didn’t.
Yet other scientists aren’t so sure. “There’s nothing to say that this specimen was a direct line to modern snakes. It could have been an offshoot that went extinct,” said Bob Murphy, senior curator of herpetology at the Royal Ontario Museum, who agrees that the fossil is “fantastic.”
Mahler wonders if the smaller animal might even be unhatched offspring, rather than prey.
Only more fossils will answer these questions.
“I don’t think you can have the last word on anything right now,” said Mahler. “And that’s what makes it so fun.”