What Were Pterosaur Wings Made Of? A Membrane With Fibres In It

A pterosaur wing was supported along its leading edge by the arm and by a single hugely elongated fourth finger, from which a membrane extended back to the body and, in most reconstructions, to the hind limb. A separate small membrane ran forward from the wrist, supported by a distinctive bone found in no other animal, and a further membrane stretched between the hind limbs in many species. That arrangement differs from both of the other vertebrate flight solutions, since bats spread a membrane across four elongated fingers and birds use feathers attached to a shortened arm, so the three groups solved the same problem three different ways. The single-finger support is the feature that most distinguishes the group and also its greatest structural vulnerability.

A handful of exceptionally preserved specimens reveal the internal structure:

• •Stiffening fibres running through the membrane, arranged roughly parallel and densely packed
• •Multiple layers, including muscle fibres and blood vessels within the membrane
• •A layer of skin over the structural layers
• •Evidence that the membrane could be tensioned and its shape adjusted actively
• •Attachment to the hind limb in several specimens, which constrains how the animal walked
• •Hair-like body covering separate from the wing, now widely interpreted as related to feathers

A plain sheet of skin makes a poor wing, because it flutters, stretches unevenly and cannot hold an aerofoil shape under load, and the preserved fibres solve exactly those problems. Running from front to back through the membrane, they resist stretching along that direction while allowing the wing to fold, which lets the animal maintain a stable curved profile in flight and keeps the trailing edge from flapping destructively. Combined with muscle within the membrane, they suggest active control of tension and camber during flight rather than a passive surface, which would give a degree of shape control that neither birds nor bats achieve in the same way. That capability is part of how animals reaching wingspans of ten metres or more could fly at all, since a passive membrane at that scale would be unmanageable.

Comparing the three groups that achieved powered flight clarifies what each arrangement costs. Birds carry feathers, which are individually replaceable, permit a wing that is slotted and reshaped continuously, and moult progressively so a damaged flight surface repairs itself over weeks. Bats stretch membrane across four fingers, which gives fine control of shape through many independent supports and produces a wing that is highly manoeuvrable and vulnerable to tearing. Pterosaurs used one finger and a fibre-reinforced membrane, which is structurally simpler, involves far less bone and appears to have scaled to sizes the other two never approached. Each solution came with different failure modes, and the fact that no group after the pterosaurs reached comparable size suggests something about their arrangement was particularly suited to it, though what exactly is not settled.

Several questions are genuinely unsettled. Whether the main membrane attached to the ankle, to the thigh or only to the body is disputed, and it matters enormously, since the attachment determines the wing's shape and area and therefore every calculation about flight performance, and different specimens have been read differently. Whether the animals launched from a standing start using all four limbs, which is now the leading proposal, or ran or dropped from height, bears on the same anatomy. How the largest species managed takeoff at their estimated masses is not fully resolved, and the mass estimates themselves vary by a factor of two between studies. And whether the membrane's repair capability was sufficient to survive the tears that must have occurred is a practical question with no direct evidence.

The wing was a membrane supported by one enormously elongated finger, which differs from both bats and birds and is the group's defining feature. Exceptional fossils preserve stiffening fibres, muscle and blood vessels inside it. The fibres resist stretching front to back, which holds an aerofoil shape under load and helps explain how ten-metre wingspans were possible.