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Ancient Creature ‘Arm’ed In Unusual Fashion: Reptile Displayed Unique Forelimbs With Giant Claw

drepanosaurus_illustration.jpg
Victor Leshyk
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It resembles a chameleon, but it’s not. Nor is it a dinosaur or crocodile. In fact, it doesn’t have any living descendants or close relatives—it’s named Drepanosaurus and scientists find this prehistoric creature quite odd, but oh so fascinating.

Drepanosaurus isn’t exactly a household name,” said Randy Irmis, curator of paleontology at the Natural History Museum of Utah and associate professor of geology and geophysics at the University of Utah. “It’s a small reptile about the size of medium-sized lizard that lived around 212 million years ago. It’s got some pretty crazy forelimbs and that’s why we started studying it.”

Drepanosaurus had usually large forelimbs compared to a chameleon and donned a giant claw on its index finger. Irmis and colleagues uncovered that this creature possessed unique variations in the bones of its forelimb and recently published their findings in Current Biology.

Drepanosaurus fossils were first found in Italy decades ago, but the remains were badly crushed. Additional fossils found in New Mexico were three-dimensionally preserved, allowing scientists to examine the odd forelimb more thoroughly.

Irmis explained that nearly all four-footed animals—humans, lizards, even dinosaurs—have the same basic forelimb pattern. Attached to the shoulder is a long bone, the humerus. The humerus meets two elongated parallel bones termed the radius and ulna. Then you’ve got the wrist and hand bones.

Drepanosaurus doesn’t quite follow convention. While its limb is comprised of the same bones, the shape, proportion, and orientation are drastically different from the standard arrangement. For instance, the ulna and radius are not parallel and the wrist bones are enlarged and extended.

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Credit Adam Pritchard et al.
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The unique forelimb anatomy of Drepanosaurus compared to other reptiles.

The anatomical adaptations along with the massive claw likely served a specialized role.  

“We believe [Drepanosaurus] could dig into wood to get insects in the type of motion we call ‘hook-and-pull’ digging,” Irmis said. “This particular motion is something modern anteaters use to get at their food.”

Irmis described hook-and-pull motion similar to an old-fashioned can opener—hook the claw into wood or soil, pull on it, and open the nest full of insects. 

If Drepanosaurus isn’t already strange enough, it also had a claw on the end of its tail.

“You can imagine having a claw on your tail would certainly help with stability if you’re walking around in trees and on logs using your [front] claws to search for insects,” said Irmis. “It’s super cool, because it’s such a weird and wonderful creature.”

Beyond its unprecedented forelimb variation, Drepanosaurus provides the first evidence of a hook-and-pull digging organism evolving in the fossil record, demonstrating that survival strategies seen in modern species like anteaters go back at least 200 million years.

Additional information: This study was a collaboration between Yale University, Stony Brook University, University of Utah and the Natural History Museum of Utah, Virginia Polytechnic Institute and State University, and the Dinosaur Institute at the Natural History Museum of Los Angeles County.