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Nancy Huang

Oct 8th 2021

A Pterosaur That Flew in the Age of Dinosaurs Had Opposed Thumbs

Scientists may have just discovered the earliest animal with opposed thumbs in a seemingly unlikely candidate: the pterosaur.

Pterosaurs were flying reptiles that lived in the age of dinosaurs. Now, a Current Biology study reports that a newly discovered species of this animal appears to have had opposed thumbs — an adaptation that might have allowed this winged wonder to grasp tree branches like a monkey.

The species has been nicknamed “Monkeydactyl,” but its scientific name is Kunpengopterus antipollicatus, which means “opposite thumb” in ancient Greek. As the study reports, this newly discovered species was found in China in a fossil bed rich with biodiversity from a subtropical Jurassic forest from 160 million years ago.

Masters of the Air

Pterosaurs and dinosaurs lived during the Mesozoic era (251 million to 66 million years ago), which includes the Triassic, Jurassic and Cretaceous time periods. Pterosaurs were the first known vertebrates to be capable of powered flight, which they used to dominate the air and hunt for prey in spectacular fashion. As Live Science explains, about 130 pterosaur species have been identified from around the world, even though their hollow bones didn’t fossilize well. Among the largest pterosaurs was Quetzalcoatlus, which stood as tall as a giraffe, had a wingspan of 35 feet and weighed 500 pounds. The smallest pterosaurs were the size of a sparrow.

Thumbs for Grasping

“Monkeydactyl” had a wingspan of approximately 85 centimeters, according to the Current Biology report. The evidence for its opposed thumbs comes from a single fossil encased in rock, which was subjected to microcomputed tomography (microCT) to generate an accurate 3D image of the entire fossil. Both thumbs are facing the other fingers, similar to the position of a human hand when grasping a tree branch.

Humans have “opposable” thumbs that are also capable of facing the same direction as the other fingers, such as when a hand is flat on the ground. In contrast, the Current Biology authors believe that Monkeydactyl’s thumbs were “permanently opposed in a natural position.” This is similar to the permanently opposed big toe that bald eagles use to catch prey and perch on branches.

Digits and Flight

Pterosaurs, dinosaurs, birds and humans all belong to an ancient class of animals known as tetrapods, which started with four limbs and five digits on each limb. As a paper published in Biological Reviews describes, some tetrapods increased their dexterity and ability to navigate in trees by developing opposable thumbs (e.g. primates, koalas, tree frogs, chameleons) and sometimes opposable toes (e.g. birds, chimpanzees, koalas, tree frogs).

As an article in the Journal of Evolutionary Biology explains, pterosaurs, birds and bats are the only tetrapods known to have evolved flapping flight. Two of these groups have usable thumbs. In birds, the finger bones have become so highly modified to support the wings that they are almost unrecognizable as fingers. (Think of trying to find the finger bones in a cooked chicken wing.) Grasping ability in birds has been relegated to the feet and beak.

In bats, the bones in their second, third, fourth and fifth fingers have become extremely elongated to support the membranous wing. However, the thumb remains free and functions as a grasping claw. In pterosaurs, the fifth finger has been lost, while the fourth finger massively expanded to support the membranous wing. The first three fingers formed a grasping claw. In Monkeydactyl, the thumb appears to have become opposed, which is often associated with living in trees.

Life in Trees

The ability to grasp narrow branches can help animals navigate complex habitats and reach fruits, insects and other food sources. The tooth structure of Monkeydactyl suggests that it may have eaten insects, as the Current Biology authors note.

The researchers who discovered Monkeydactyl compared its features with those of 25 other pterosaurs and 156 other species, including many that live primarily in trees. They found that Monkeydactyl was well suited to life in the trees — more so than two closely related pterosaur species that lived in the same environment around the same time. This suggests that these closely related pterosaurs may have engaged in niche partitioning, meaning that they ate different foods and used different resources to minimize competition.

Pterosaurs died out 66 million years ago. Nonetheless, it is fascinating to learn about their adaptations and strategies for success, since variations of their opposed digits are being used by animals today.

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