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Snake fossil with four legs just wants a hug

The discovery of a fossil of a snake with four legs suggests that the modern-day reptiles evolved from digging, rather than swimming, ancestors.

Tetrapodophis amplectus, the snake with four legs for grasping. David Martill, University of Portsmouth

What's the difference between a snake and a lizard? If you answered "legs," you are now officially dead wrong. A snake fossil with four limbs each ending in five seemingly functional digits has been discovered, bringing with it a new theory as to how modern-day snakes evolved. The research was published today in the journal Science.

The snake, aptly named Tetrapodophis amplectus, or grasping four-legged snake,was discovered in the Museum Solnhofen in Germany by paleobiologist David Martill of the University of Portsmouth, the UK, while on a field trip with students.

"The fossil was part of a larger exhibition of fossils from the Cretaceous period," Martill said in a statement. "It was clear that no one had appreciated its importance, but when I saw it I knew it was an incredibly significant specimen."

The fossil was from Brazil and is around 110 million years old. And, although fossils of snakes have been found with legs before, the limbs in question were two useless hind legs. This is the first snake fossil discovered with four legs.

"It is generally accepted that snakes evolved from lizards at some point in the distant past," Martill said. "What scientists don't know yet is when they evolved, why they evolved, and what type of lizard they evolved from. This fossil answers some very important questions, for example it now seems clear to us that snakes evolved from burrowing lizards, not from marine lizards."

Tiny hands (left) and feet (right). David Martill, University of Portsmouth

The snake's legs were too small to have been used for crawling. The specimen measures some 20cm (7.9 inches), with a head the size of an adult fingernail. Its forelegs measure just 1cm (0.4 inches), with tiny hands half that length. Its back legs and feet are slightly longer. While it is possible that the snake may have grown larger, the proportions would render the legs useless in any event.

"It is a perfect little snake, except it has these little arms and legs, and they have these strange long fingers and toes," explained Nick Longrich, a palaeontologist and senior lecturer at the University of Bath, who worked with Martill on the research.

Artist's concept of the snake giving its prey a deadly hug. Julius T. Csotonyi

"So when snakes stopped walking and started slithering, the legs didn't just become useless little vestiges -- they started using them for something else. We're not entirely sure what that would be, but they may have been used for grasping prey, or perhaps mates."

There are several features that categorise the tiny animal as a snake, rather than a lizard, according to the team: It has a long body, rather than a long tail, containing 160 vertebrae, and another 112 vertebrae in the tail; the implantation, direction and pattern of the teeth and bones of the lower jaw are typical of a snake, rather than a lizard; and it has a single row of enlarged belly scales characteristic of snakes, as opposed to the multiple rows seen on the belly of a lizard.

Interestingly, some salamander bones remain in the animal's stomach. This could indicate that snakes were carnivorous earlier than had been supposed.

"The preservation of the little snake is absolutely exquisite," said German palaeontologist Helmut Tischlinger, who also worked on the research with Martill. "The skeleton is fully articulated. Details of the bones are clearly visible and impressions of soft tissues such as scales and the trachea are preserved."