It may not sound like the obvious place to look when trying to determine how snakes lost their limbs, but the inner ear of a fossilised skull could finally solve the mystery.

CT scans of a 90-million-year-old Dinilysia patagonica fossil show the ancestor of modern-day serpents had a large, spherical canal in its ear - a feature typically found in burrowing creatures.

This suggests snakes lost their legs when their ancestors evolved to live and hunt in burrows and not to help them live in the sea, as previously thought.

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CT scans of a Dinilysia patagonica fossil (left) show the ancestor of modern-day serpents had a large, spherical canal in its ear (illustrated in purple right) - a feature typically found in burrowing creatures. This suggests snakes lost their legs to live and hunt in burrows and not to live in the sea, as previously thought

The Cretaceous period fossil from Argentina is from a creature called Dinilysia patagonica.

Dinilysia patagonica was 6.5 feet-long (two metres) and is thought to have been the largest burrowing snake the world has ever known.

The extinct species is closely linked to modern snakes and bony canals and cavities, like those in the ears of modern Typhlops brongersmianus - a modern burrowing snake found in South America - controlled its hearing and balance.

SNAKES ON A PLANE CT scans (also called computerised axial tomography or CAT scans) work by taking a series of virtual X-ray slices through an object. Each plane or section can be microscopically thin, and these are then assembled into 3D computer images to reveal detail hidden to the eye or a conventional microscope. In this case, the scan involved taking more than 2,000 slices through the skull, each just 26 micrometres (0.026mm) thick - about a third of the thickness of a human hair. Mark Norell, of the American Museum of Natural History, who took part in the study, 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.' Advertisement

Dr Hongyu Yi from the University of Edinburgh, and Mark Norell from the American Museum of Natural History, built computer models to reveal the tiny hidden structures inside the extinct animal’s skull.

They then compared these scans to those taken of modern reptiles.

The CT scans showed that the inner ear, or vestibule, of the ancient serpent, is distinct to ancient and modern burrowing reptiles and quite unlike those of snakes that live in the ocean or above ground.

The vestibule contains the sensory organs that control balance and hearing within a basket-like structure of three narrow semi-circular canals.

The scientists discovered this area was large and spherical in Dinilysia patagonica and said this is a 'morphological signature' for burrowing.

By comparison, aquatic reptiles have smaller and less defined vestibules, according to the study published in the journal Science Advances.

The findings will help scientists fill in gaps in the story of snake evolution and offer clues about a hypothetical ancestral species from which all modern snakes descended, which was likely a burrower.

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 CT scans showed that the inner ear, or vestibule, of the ancient serpent contains the sensory organs that control balance and hearing within a basket-like structure (shown in pink) of three narrow semi-circular canals. The scientists said this area was large and spherical and that this is a 'morphological signature' for burrowing

By comparison, aquatic reptiles have smaller and less defined vestibules (shown in orange in the lower image). A model of a snake skull is also shown

‘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.’

He told MailOnline: 'We know Dinilysia is limbless from the rest of its skeleton, but this doesn't tell us its behaviour.

'Its skull shape is not specialised as a burrower, but the surprise here is that the inner ear is very specialised and shows clear resemblance to modern burrowers.'

'This means we can now say that snakes lost their legs when their ancestors became adept at burrowing, rather than swimming.'