Ancestors of snakes lost their limbs to help them wriggle through burrows, research suggests.
A new study of a 90 million-year-old fossil skull from Argentina may have solved the mystery of what happened to snake legs, scientists believe.
It challenges another theory that snakes originally became limbless in order to live in the sea.
Scientists used Computed Tomography (CT) scans to examine the inner ear of Dinilysia patagonica, a two metre (6.5ft) long relative of modern snakes.
They found a distinctive structure in its bony canals and cavities that was also present in modern burrowing snakes and lizards.
But the structure, which may assist the detection of prey and predators, was missing from snakes that live in water or above ground.
Lead scientist Dr Hongyu Yi, from the University of Edinburgh's School of GeoSciences, 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."
The findings, published in the journal Science Advances, confirm Dinilysia patagonica as the largest burrowing snake ever known.
Co-author Dr Mark Norell, from the American Museum of Natural History, said: "This discovery would not have been possible a decade ago - CT scanning has revolutionised how we can study ancient animals.
"We hope similar studies can shed light on the evolution of more species, including lizards, crocodiles and turtles."
A CT scan is a form of X-ray that generates detailed 3D images of internal organs and skeletal structures.