Science reporter, BBC News
What was lost tens of millions of years ago is now found.
A fossil animal locked in Lebanese limestone has been shown to be an extremely precious discovery - a snake with two legs.
Scientists have only a handful of specimens that illustrate the evolutionary narrative that goes from ancient lizard to limbless modern serpent.
Researchers at the European Light Source (ESRF) in Grenoble, France, used intense X-rays to confirm that a creature imprinted on a rock, and with one visible leg, had another appendage buried just under the surface of the slab.
"We were sure he had two legs but it was great to see it, and we hope to find other characteristics that we couldn't see on the other limb," said Alexandra Houssaye from the National Museum of Natural History, Paris.
The 85cm-long (33in) creature, known as Eupodophis descouensi, comes from the Late Cretaceous, about 92 million years ago.
Unearthed near the village of al-Nammoura, it was originally described in 2000.
Its remains are divided across the two interior faces of a thin limestone block that has been broken apart.
Synchrotron-radiation computed laminography produces fine detail
A portion of the vertebral column is missing; and in the process of preservation, the "tail" has become detached and positioned near the head.
But it is the unmistakable leg bones - fibula, tibia and femur - that catch the eye. The stumpy hind-limb is only 2cm (0.8in) long, and was presumably utterly useless to the animal in life.
Current evidence suggests that snakes started to emerge less than 150 million years ago.
Two theories compete. One points to a land origin in which lizards started to burrow, and as they adapted to their subterranean existence, their legs were reduced and lost - first the forelimbs and then the hind-limbs.
The second theory considers the origin to be in water, from marine reptiles.
This makes the few known bipedal snakes in the fossil record hugely significant, because they could hold the clues that settle this particular debate.
The top picture is a synchrotron view of the visible snake leg
Synchrotron light in the bottom view illuminates the hidden limb
"Every detail can be very important in establishing the great relationships and that's why we must know them very well," explained Ms Houssaye.
"I wanted to study the inner structure of different bones and so for that you would usually use destructive methods; but given that this is the only specimen [of E. descouensi], it is totally impossible to do that.
"3D reconstruction techniques were the only solution. We needed a good resolution and only this machine can do that," she told BBC News.
EUROPEAN LIGHT SOURCE
Electrons are fired into a linac, or straight accelerator. They're boosted in a small ring before entering the storage ring. The superfast particles are corralled by a train of magnets. Energy lost by turning electrons emerges as intense light (X-rays).
The 850m-circumference ring has 32 magnet clusters, or cells. Electrons turned by plain magnets produce 'standard' X-rays. Particles 'wiggled' at undulator magnets emit stronger X-rays. X-rays can't turn with electrons and head straight down beamlines.
Experiment 'hutches' receive the most intense X-rays in Europe. The light probes materials on the atomic and molecular scale. Robots can place many samples in the beam for rapid science. ESRF data leads to new materials, drugs, electronics, etc.
That machine is the European Synchrotron Radiation Facility. This giant complex on the edge of the Alps produces an intense, high-energy light that can pierce just about any material, revealing its inner structure.
For this study, the fossil snake was clamped to an inclined table and rotated in front of the facility's brilliant X-ray beam.
In a process known as computed laminography, many hundreds of 2D images are produced which can be woven, with the aid of a smart algorithm, into a detailed 3D picture.
The finished product, which can be spun around on a computer screen, reveals details that will be measured in just millionths of a metre.
The E. descouensi investigation shows the second leg hidden inside the limestone is bent at the knee.
"We can even see ankle bones," ESRF's resident palaeontologist Paul Tafforeau said.
"In most cases, we can't find digits; but that may be because they are not preserved or because, as this is a vestigial leg, they were never present."
To modern eyes, it may seem strange to think of a snake with legs.
But look at some of the more primitive modern snakes, such as boas and pythons, and you'll see evidence of their legged ancestry - tiny "spurs" sited near their ends, which today are used as grippers during sex.