For years, scientists have been trying to pinpoint why snakes eschewed their legs in favor of a long, slinky body. Was it because ancient snakes became expert, underground burrowers? Or was it because they wanted to glide through water as marine swimmers? New research just published in Science Advances suggests the former may have been the reason that ancestral snakes lost their legs.

Researchers Hongyu Yi of the University of Edinburgh’s School of GeoSciences and Mark Norell of the American Museum of Natural History answered this question with the help of the 90-million-year-old skull of Dinilysia patagonica. This ancient stem snake specimen, while millions of years old, is closely related to many modern day snakes. D. patagonica, a South American snake, was an adept burrower (and may have been the biggest burrower ever, measuring up to two meters long).

Yi and Norell examined the inner ear of the D. patagonica skull with CT scans and compared virtual 3D models of that prehistoric snake’s inner ear along with the inner ears of contemporary reptiles. Turns out that they’re remarkably similar.

“This discovery would not have been possible a decade ago – CT scanning has revolutionized how we can study ancient animals,” said Norell.

Both sets of snakes use their inner ear to control hearing and balance, but there was a distinctive structure that the scientists found there that’s found only in the inner ears of animals that actively burrow. That structure may help animals detect both predators and prey underground… and it holds the key to the leg question.

Modern snake skull shown with the inner ear in orange.
Dinilysia patagonica's skull, with the inner ear shown in purple

Scientists previously thought snakes may have lost their limbs so they could more easily live at sea, but it seems that’s not the case. When looking at modern-day snakes that live in water or above ground, researchers found that the inner ear structure common in active burrowers (including D. patagonica) was not present.

These findings suggest that the ancestors to modern snakes ditched their legs in order to become habitat specialists (i.e. skilled tunnelers).

“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 Yi. “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.”

Yi and Norell’s findings may also provide the foundation for future research about a potential ancient burrower from which modern day snakes are descended and have since evolved.

Photos via Hongyu Yi, Dan Kitwood/Getty Images