February 8, 2011

Researchers Study How Snakes Became Legless

A new form of X-ray imaging technology and a nearly 100 million year old fossil have helped researchers gain a better understanding of how snakes lost their legs through evolution, according to a study published Tuesday in the Journal of Vertebrate Paleontology.

In the study, Alexandra Houssaye from the Museum National d'Histoire Naturelle (MNHN) in Paris and colleagues used synchrotron X-ray machines at the European Synchrotron Radiation Facility (ESRF) to create 3D images of a 95 million year old Eupodophis descouensi fossil.

The fossil, discovered ten years ago in Lebanon, was approximately 50cm long and "exhibits a small leg, about 2 cm long, attached to the animal's pelvis," according to a February 7 statement released by officials at the ESRF.

"This fossil is key to understanding the evolution of snakes, as it represents an intermediate evolutionary stage when ancient snakes had not yet completely lost the legs they inherited from earlier lizards," they said.  "Although the fossil exhibits just one leg on its surface, a second leg was thought to be concealed in the stone, and indeed this leg was revealed in full detail thanks to synchrotron X-rays."

"The researchers hope the new data will help resolve a heated debate about the origin of snakes: whether they evolved from a terrestrial lizard or from one that lived in the oceans," the press release added.

Using synchrotron laminography, a special imaging technique similar to computer tomography (CT) scans that was developed recently and is used to study large, flat samples, Houssaye and her associates created a high-resolution 3D image that shows "the fine detail of the buried small leg" discovered in the fossil.

Analysis of the picture suggests "that this species lost its legs because they grew more slowly, or for a shorter period of time," the ESRF officials said. "The data also reveal that the hidden leg is bent at the knee and has four ankle bones but no foot or toe bones."

In an interview with BBC News Science Correspondent Jonathan Amos on Tuesday, Houssaye said that the study revealed "the degree of regression of the legs."

"This could not be clearly determined based only on the visible leg, as some tiny bones such as ankle and foot bones might have been broken or removed from the surface," she added. "However, as all the second leg was preserved in the rock, we can now be almost completely sure of the degree of resorption of the leg."


Image 1: This is a photograph of Eupodophis descouensi, a fossil snake from the Cretaceous Period (95 million years ago) of Lebanon. The black scale bar at the bottom right equals 1 cm. Credit: A. Houssaye

Image 2: This is a 3-D reconstruction from synchrotron X-ray images of the previously hidden second leg of Eupodophis. The bones are artificially colored to highlight the internal structure of the bone and show how the snake's leg grew. Credit: A. Houssaye


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