Images Courtesy of Alexandra Houssaye

ScienceShot: Snake Legs—Going, Going, Gone

Snakes evolved from four-limbed ancestors, and scientists have long wondered how they lost their legs. A new type of CT scan of a 95-million-year-old fossil suggests how genetics stunted the limbs’ growth. Using a newly developed x-ray technique called synchrotron-radiation computed laminography, researchers probed details of the vestigial hind limbs of the ancient marine snake Eupodophis descouensi. Each pixel in images of the rock-cloaked bones was barely more than 30 micrometers across. By virtually reconstructing an 0.8-centimeter-long limb, layer by thin layer, the researchers found that—unlike the snake’s vertebrae and most of its other bones—the leg bones didn’t have thick-walled shafts. That’s a clue, the researchers say in a paper scheduled to be published online today by the Journal of Vertebrate Paleontology, that the limbs were small because they grew either much more slowly or for a much shorter time than did the other bones.

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