New work on a 90-million-year old reptile fossil is helping scientists determine how snakes lost their limbs. The research, conducted by Mark Norell, the Macaulay Curator and chair in the Museum’s Division of Paleontology, and Hongyu Yi, of the University of Edinburgh’s School of GeoSciences, indicates that snakes lost their legs when their ancestors evolved to live and hunt in burrows, which many snakes still do today. The findings, recently published in the journal Science Advances, show that snakes did not lose their limbs in order to live in the sea, as has been previously suggested.
Snake skulls, with inner ear in orange. Scale bar is 5 mm. © H. Yi
The scientists used computed tomography (CT) scans to examine the bony inner ear of Dinilysia patagonica, a 2-meter (6.5-feet)-long reptile closely linked to modern snakes. These bony canals and cavities, like those in the ears of modern burrowing snakes, controlled its hearing and balance.
Researchers built 3D virtual models to compare the inner ears of the fossils with those of modern lizards and snakes and discovered a distinctive structure within the inner ear of animals that actively burrow. This shape, which may help burrowers detect prey and predators alike, is not present in modern snakes that live in water or above ground.
“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 said. “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.”
Brain casing (A) and CT scan (B) of the skull of Dinilysia patagonica, with inner ear shown in blue. © H. Yi
The findings help scientists fill gaps in the story of snake evolution, and confirm Dinilysia patagonica as the largest burrowing snake ever known. They also provide clues about a hypothetical ancestral species from which all modern snakes descended, which was likely a burrower.
“This discovery would not have been possible a decade ago,” Norell said. “CT scanning has revolutionized how we can study ancient animals. We hope similar studies can shed light on the evolution of more species, including lizards, crocodiles and turtles.”