X-rays Reveal Ancient Snake's Hidden Leg

Eupodophis descouensi
The Eupodophis descouensi specimen found in Lebanon. The snake's inch-long legs are in the tail section to the left of the snake's head. (Image credit: A. Houssaye)

A new look at a 95-million-year-old fossilized snake reveals two tiny leg bones attached to the slithery creature's pelvis. A three-dimensional reconstruction of the bones could help researchers understand how snakes evolved to lose their legs.

The fossil, found in Lebanon, is from an era when snakes had not yet completely lost the hind limbs left by their lizard ancestors. A much-debated question among paleontologists is whether these leggy ancestors were ocean-living swimmers or land-dwelling burrowing lizards.

The new finding takes a stab at answering that question.

One-inch-long fossilized leg bone is visible on the surface of the fossilized Lebanese snake, but half the pelvis (where another leg would be expected) is buried in rock. The 19-inch-long (50 centimeter) snake (called Eupodophis descouensi) is one of only three snake fossils with its hind limbs preserved, so breaking it open to look for the other leg was out of the question, said study researcher Alexandra Houssaye of the Museum National d'Histoire Naturelle in Paris.

Instead, the researchers used a technique called synchrotron-radiation computed laminography (SRCL). Like a medical computed tomography (CT) scan, SRCL uses X-rays to image the internal structure of an object, but at 1,000 times higher resolution.

"Only 3-D [scanning] could reveal the inner structure of the bones without damaging them, and the same is true to observe the complete second leg," Houssaye told LiveScience.

The scanning revealed a hidden leg, bent at the knee but lacking foot and toe bones. The setup of the bones is similar to that of terrestrial lizards, Houssaye said, adding that one study couldn't settle the "land ancestor versus water ancestor" debate. However, she said, the anatomy of the bones suggests that evolution took snakes' legs not by altering the way they grew. Instead, Houssaye said, it looks as though the limbs grew either slower or for a shorter period of time.

This experiment was the first to use the SRCL technique in paleontology, Houssaye said, and there's much more to analyze. The next steps, she said, include analyzing other hind-limb snake fossils, studying the limbs of living snakes and lizards and analyzing the fossils of the oldest snakes known.

The researchers report their results in the Feb. 8 issue of the Journal of Vertebrate Paleontology.

You can follow LiveScience Senior Writer Stephanie Pappas on Twitter @sipappas.

Stephanie Pappas
Live Science Contributor

Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.