Snakes, with their sleek, slithering shape, are unmistakable amongst the reptiles. Yet for decades, scientists have been debating just how these limbless lizard relatives ended up with their distinctive, elongated body.
On one side are scientists who argue that the serpentine shape was an aquatic adaptation. Many snake traits, including an elongated body and reduced limbs, are also features of swimming animals (think whales and dolphins, for example, which have lost their hind limbs). Early evidence also suggested that snakes were closely related to mosasaurs, the terrifying and extinct group of lizards that were woven into pop culture the moment one was fed a great white shark in Jurassic World. Non-theatrically, these marine reptiles ruled the seas during the Cretaceous, and possessed many snake-y features, including a jaw which stretches for large prey. The discovery of extinct marine snakes with hindlimbs, including Pachyrhachis, Haasiophis, and Eupodophis, seemed further proof of a marine origin.
But later analyses have suggested that Pachyrhachis and others are secondarily marine, the offshoots of a more derived snake group, and the connection between snakes and mosasaurs has come under suspicion. The prevailing hypothesis is now that snakes evolved on land — or, even more specifically, in it. A burrowing or ‘fossorial’ lifestyle could also produce long, skinny bodies and reduced limbs. More recent finds like Najash, Dinilysia, and Coniophis, which date back further than Pachyrhachis, all lived on land. But the evidence for a largely underground existence isn’t conclusive, either, and some hold to the idea that snakes were born in the sea.
The debate has continued so long because there is a dearth of snake fossils to rely upon. Snake bodies are by and large small and fragile, with thin bones that do not lend easily to fossilization. So scientists have had little material to work with when trying to determine changes over time.
A new fossil hopes to end the debate once and for all. A paper published this week in Science describes what appears to be a four-legged burrowing snake from Brazil. “Here it is, an animal that is almost a snake” says David Martill, a paleobiologist from the University of Portsmouth, “and it doesn’t show any adaptations to being in an aquatic environment.” But is it really that cut-and-dry? While the latest fossil find is making a splash in the news, it’s one of four noteworthy papers this year examining snake evolution, and placing the new study in context helps explain what makes the fossil so exciting, if controversial.
