Bright Idea Sheds Light on Snake Legs

Monday, June 20, 2011

Novel three-dimensional (3D) imaging technology has provided an unparalleled view of the legs of an ancient snake. The study, published in the latest issue of the Journal of Vertebrate Paleontology, suggests that snakes lost their legs by growing them more slowly or for a shorter period of time. The researchers hope the new data will help resolve a heated debate about the snake origins: whether they evolved from a lizard that burrowed on land or swam in the oceans.

The new imaging technique used in this study, called synchrotron-radiation computed laminography (SRCL), uses an intense, high-energy beam of X rays to deeply penetrate dense materials. The fossil snake, named Eupodophis descouensi, was rotated in this high-energy beam, resulting in thousands of two-dimensional images. These images were compiled into a high-resolution, three-dimensional model of the snake's hips and small (0.8 inch or 2 cm) legs, which are otherwise inaccessible.

"Synchrotrons are enormous machines and allow us to see microscopic details in fossils invisible to any other techniques without damage to these invaluable specimens," says Paul Tafforeau from the European Synchrotron Radiation Facility and co-author of the study.

In the ten years since Eupodophis’ discovery in 95-million-year-old rocks of Lebanon, paleontologists have sought a way to study the fossil snake's growth. To do so, they collaborated with the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and physicists developing SRCL from the Ångströmquelle Karlsruhe (ANKA) in Germany. This imaging technique is similar to computed tomography known from medical x-ray imaging, but providing approximately 1,000 times higher spatial resolutions. Employing the much stronger x-ray source of a synchrotron accelerator, the technique is particularly adapted to 3D imaging of such flat plate-like specimens presented by fossils.

The new data reveal a Eupodophis leg that is bent at the knee and has four ankle bones but no foot or toe bones. More importantly, it reveals the internal architecture of the leg bones, which strongly resembles that of modern terrestrial lizard legs. It is this information that has been used to better understand growth in extinct snakes.

"The revelation of the inner structure of Eupodophis hind limbs enables us to investigate the process of limb regression in snake evolution." says Alexandra Houssaye of the Museum National d'Histoire Naturelle, lead author of the study.

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