When did snakes lose their limbs?

Snakes haven’t always been slitherin’—but the reason why they lost their legs has long puzzled scientists. It’s generally been thought that the limbs disappeared in order for snakes to make the transition from land to sea, but now a 90 million-year-old fossil is changing the discussion.

Losing legs left and right

The fossil consists of a reptile skull which, as reported in Science Advances, was compared to the skulls of modern lizards and snakes using CT scans—and led researchers to conclude that snakes lost their legs when they evolved to live and hunt in burrows, not when they evolved for the sea.

The specific fossil they studied was of Dinilysia patagonica, a six-foot-long reptile ancestral relative of modern snakes. In particular, they studied the inner ears of D. patagonica and modern snakes and lizards, which was then and still is now responsible for hearing and balance.

After the CT scans, the team built virtual 3D models of all the inner ears in order to compare them, and made an interesting discovery: In D. patagonica and modern burrowing reptiles, the inner ears contained a distinctive structure, which might help in detecting predators and prey. Such a feature is absent in modern snakes that live in the water or aboveground.

This not only shows that D. patagonica was the largest burrowing snake ever known, but that this theoretical ancestral species of modern snakes was a burrower, not a swimmer.

“How snakes lost their legs has long been a mystery to scientists, but it seems that this happened when their ancestors became adept at burrowing,” said Dr. Hongyu Yi of the University of Edinburgh’s School of GeoSciences, who led the research, in a statement. “The inner ears of fossils can reveal a remarkable amount of information, and are very useful when the exterior of fossils are too damaged or fragile to examine.”

“This discovery would not have been possible a decade ago – CT scanning has revolutionized how we can study ancient animals,” added co-author Mark Norell, of the American Museum of Natural History. “We hope similar studies can shed light on the evolution of more species, including lizards, crocodiles and turtles.”

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Pictured is a rendering of the reptile skull used in the study

Image credit: Hongyu Yi/Eurekalert