Even without arms or legs, snakes are some of the world’s most fearsome predators. Their unique anatomy suggests that modern snakes originated as habitat specialists, but researchers can’t agree if they lost their limbs as an adaptation for burrowing or for swimming. Now, researchers comparing the ears of dozens of snakes – including an extinct Cretaceous species – reveal that today’s snakes evolved from terrestrial ancestors who burrowed, not marine ones. The findings are published in Science Advances this week.

Most basal snakes we see today are burrowers; these are members of the first branches of the snake family tree after they diverged from lizards. There are many similarities between modern snakes and lizards that burrow, ranging from a modified braincase and an acute sense of smell to reduced limbs or no limbs at all (except vestigial ones). "But there is a complication. Limb reduction also happened in marine lizards, not the marine iguanas, but extinct lizards called mosasaurs," University of Edinburgh’s Hongyu Yi tells IFLScience. (You can see these sea monsters in Jurassic World.) "Many of these arguments concern the habitat of fossil lizards and snakes, but we are yet to have a reliable way to predict the habitat of these fossils, until today."

The inner ear is the hearing and balance organ of all snakes. Recent studies found that modern snakes are able to perceive vibrations in the ground thanks to the bony canals and cavities of their inner ear – which helps them detect prey or other predators. Like the trunk-to-tail ratio, inner ear shape is a good indicator for snake habitats – but complete vertebral series are rarely preserved.

So, Yi and colleagues created virtual, 3D models of the inner ear of 44 species of snakes and lizards based on X-ray CT scans. One of the extinct snakes they examined was the 90-million-year-old Dinilysia patagonica.



(A) photo of the braincase of Dinilysia patagonica. (B) X-ray CT model with the inner ear highlighted in blue. Hongyu Yi

The inner ear of Dinilysia patagonica seems to be typical of terrestrial burrowing reptiles. Slender semicircular canals and a large, spherical chamber (or vestibule) in the inner ear helps to enhance sensitivity to low-frequency vibrations. Modern snakes who hunt in rodent burrows and those who actively dig into the substrate using modified snouts all have these. But no aquatic species has a spherical vestibule that touches the semicircular canals, and while some terrestrial generalists do burrow when they’re disturbed, their vestibule doesn’t have a spherical shape.

"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," Yi adds 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."

Additionally, the team also built models to predict snake habitats based on vestibular shape. They found a high probability that Dinilysia patagonica – as well as the hypothetical ancestor of today’s snakes – were belowground dwellers. In fact, at 1.8 meters (5.9 feet) long, Dinilysia patagonica was the largest burrowing snake ever known. Adults of other species in their analysis ranged from 38 centimeters (15 inches) to 1.6 meters (5.2 feet). Like today’s Xenopeltis unicolor and Loxocemus bicolor, the extinct giant likely hunted for buried eggs of other reptiles. The ear of a mosasaur, on the other hand, looked much like modern marine snakes.