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How did pterosaurs fly using an elongated fourth finger?

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A painted turtle (Chrysemys picta) with a dark shell and red and yellow markings rests on a rock near water, surrounded by green aquatic plants. — reptile Painted turtle (*Chrysemys picta*).

Pterosaurs were the first vertebrates capable of flight, achieving this through a specialized wing structure. Unlike birds, which rely on feathers, pterosaurs developed a wing surface formed by a membrane of skin.1

This membrane was supported solely by an exceptionally elongated fourth finger. The first three fingers remained as slender, clawed structures used for clutching, while the fifth finger was absent. To provide strength and maneuverability, the wing membrane contained a system of fine, parallel keratinous fibers, similar to the shafts of bird feathers. Additionally, an accessory membrane stretching between the shoulder and wrist helped reduce turbulence during flight. When not in use, this wing apparatus could be folded rearward along the animal's flanks.1