How Hard-Headed Lizards Lost Their Legs

The original and only known specimen of the Messel lizard Cryptolacerta. The skeleton is only a few centimeters long and is almost complete, lacking only parts of the tail. Its hands and feet were very small, showing the first steps of limb reduction. CREDIT: Robert Reisz View full size image

Image of a currently-living amphisbaenian, which only has two legs.
CREDIT: Gary Nafis

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The mysterious "worm lizard" has finally found its evolutionary home; the legless animals are closely related to a group of lizards named the lacertids, a new fossil intermediate indicates.

The worm lizards, also called amphisbaenians, look so similar to primitive snakes that researchers weren't clear which group they belonged to, the snake or lizard family.

"They look like, basically, worms, or a group of snakes that we call blind snakes," said study researcher Robert Reisz, of the University of Toronto, Mississagua, in Canada. "They burrow for a living and eat grubs."

This new fossil, named Cryptolacerta hassiaca, has an amphisbaenian-like reinforced head but retains chunky, lacertid-like limbs. The fossil was discovered in the so-called Eocene Messel site near Frankfurt, Germany, and provides the first skeletal evidence that the two groups are most likely related.

A three-dimensional scan of the fossilized Messel lizard Cryptolacerta as revealed by X-ray computed microtomography. This approach allowed the researchers to study not only the originally exposed bone surface in great detail, but also the internal anatomical regions that were otherwise obscured.
CREDIT: Robert Reisz

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"These animals first evolved a shovel-like head, burrowed headfirst with a shovel-like mechanism, then after that lost its limbs," Reisz told LiveScience. "There are lots of things about the head that tell us it was well on its way to be a amphisbaenian."

The 47-million-year-old fossil is well preserved and represents the only known example of its species. At the time of its death it would have been considered a "living fossil," having survived unchanged from its ancestors for millions of years. There are other, older fossil examples of the amphisbaenians, but this is the first example of the intermediate stage between the lacertids and the amphisbaenians.

The fossil organism most likely lived in the leaf litter on the forest floor. It used its reinforced head to dig in the dirt and leaves, though it most likely didn't live completely underground.

A life reconstruction of the Messel lizard Cryptolacerta. Comparison of the animal's body proportions with those of modern lizards revealed that this little reptile most probably lived under leaf litter in the forest, and occasionally dug burrows to hide.
CREDIT: Robert Reisz

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Throughout evolution, animals have lost their limbs several times, so it's not unfathomable that these two species developed similar body plans and burrowing strategies separately, the researchers say.

"We have two groups of animals — the blind snakes and these limbless lizards — resembling each other to a great degree, but they developed this body design independent of each other," Reisz said. "There are lots of ways you can lose your limbs, and this one gives us an example of how it happens in these strange little subtropical lizards."

The study was published today (May 18) in the journal Nature.

You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.