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Snake Limb Loss Linked to Ancient Ancestors Evolving to Live & Hunt in Burrows; Analysis of Inner Ear Structure in 90-Million-Year-Old Reptile Skull Provides New Evidence

Fresh analysis of a 90-million-year-old reptile fossil is helping scientists solve an evolutionary puzzle - how snakes lost their limbs. The skull is giving researchers vital clues about how snakes evolved. Comparisons between CT scans of the fossil and modern reptiles indicate that snakes lost their legs when their ancestors evolved to live and hunt in burrows, which many snakes still do today. The findings show snakes did not lose their limbs in order to live in the sea, as was previously suggested. Scientists used CT scans to examine the bony inner ear of Dinilysia patagonica, a 2-meter-long reptile closely linked to modern snakes. These bony canals and cavities, like those in the ears of modern burrowing snakes, controlled the ancient reptile’s hearing and balance. The scientists constructed 3D virtual models to compare the inner ears of the fossils with those of modern lizards and snakes. Researchers found a distinctive structure within the inner ear of animals that actively burrow, which may help them detect prey and predators. This shape is not present in modern snakes that live in water or above ground. The new findings help scientists fill gaps in the story of snake evolution, and confirm Dinilysia patagonica as the largest burrowing snake ever known. The findings also offer clues about a hypothetical ancestral species from which all modern snakes descended, which was likely a burrower. The study, published recently in Science Advances, was supported by the Royal Society. Dr. Hongyu Yi, of the University of Edinburgh's School of GeoSciences, who led the research, said: "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. 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." Mark Norell, Ph.D., of the American Museum of Natural History in New York City, who took part in the study, said: "This discovery would not have been possible a decade ago - 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." The image shows a modern snake skull, with inner ear shown in orange. (Credit: Dr. Hongyu Yi)

[Press release] [Science Advances]