One of evolution's biggest mysteries is why the modern snake has no legs. There are two general hypotheses that have been proposed: one claims snakes descended from terrestrial burrowers, and the other pegs modern snakes as descendants of marine swimmers. In either case, it was clear that snakes lost their through habitat specialization, but which habitat they were adapting to was a mystery.
Recently, two researchers from the University of Edinburgh and the American Museum of Natural History turned to a 90-million year old reptile skull to look for answers to this longstanding conundrum and may very well have found them. In a study published Friday in Science Advances, co-authors Hongyu Yi and Mark Norell claim that modern snakes likely evolved from landlocked, burrowing ancestors.
"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 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."
To arrive at this answer, Yi and Norell used CT scans to examine the bony inner ear of Dinilysia patagonica, a reptile that lived during the Late Cretaceous period in South America, grew up to 2 meters in length, and is closely linked with modern snakes. Just like modern snakes, D. patagonica used its bony ear canals for controlling balance and detecting vibrations in the ground, making it a good candidate for comparison.
Using their CT scans of the fossilized skull of D. patagonica, Yi and Norell built virtual 3D models to compare with the ear canals of modern snakes. What they found was a distinct structure in the inner ear of species that burrow, a trait which might help them detect prey and predators. When they sought this structure in modern snakes that lived above ground or operated primarily in marine environments, it was notably absent.
In addition to confirming D. patagonica as the largest burrowing snake that ever existed, Yi and Norell's results also help fill in the picture of a hypothetical species of snake from which all modern snakes descended.
"This discovery would not have been possible a decade ago—CT scanning has revolutionized how we can study ancient animals," said Norell in a statement. "We hope similar studies can shed light on the evolution of more species, including lizards, crocodiles and turtles."