What Is a Pterosaur? The First Vertebrates That Really Flew

Three vertebrate groups have evolved powered flight and each solved the wing differently. A bat stretches membrane across four elongated fingers. A bird's wing is supported by the arm bones with feathers providing the surface. A pterosaur elongated a single finger, the fourth, into an enormous strut running along the leading edge of a membrane that attached to the body and to the hind limb. That design leaves the other three fingers free and clawed, used for climbing and walking. The membrane was not a simple sheet but a structured surface containing stiffening fibres, blood vessels and muscle, visible in exceptionally preserved specimens, which allowed it to be tensioned and shaped rather than flapping loosely. A unique bone at the wrist, the pteroid, supported a forward membrane in front of the arm that acted as a control surface. The skeleton was extensively pneumatic, with air sacs invading the bones, which is a feature shared with birds and dinosaurs and inherited from a common ancestor rather than evolved for flight.

The group ran for over one hundred and fifty million years and varied enormously:

• •Early forms from the late Triassic, small with long tails, and already fully capable of flight when they first appear, which leaves the origin of flight in the group essentially undocumented
• •Short-tailed pterodactyloids from the late Jurassic onward, which include most of the familiar forms and most of the large ones
• •Filter feeders with hundreds of fine bristle-like teeth, which strained small organisms from water in the manner of a flamingo
• •Forms with enormous head crests, in some cases larger than the rest of the skull, used for display and possibly for steering
• •Azhdarchids, the giants of the late Cretaceous, with wingspans estimated at ten to eleven metres and a standing height comparable to a giraffe
• •Evidence of fur-like body covering called pycnofibres, now argued by some researchers to be related to feathers, which would push the origin of feather-like structures back considerably and remains contested

The largest pterosaurs exceed what many people assume is possible, and explaining their flight has required rethinking assumptions. Their skeletons were extraordinarily light, with bone walls sometimes under a millimetre thick supported internally by struts, so a ten-metre animal may have weighed only a couple of hundred kilograms. Launch was the hard problem, since a bird that size would be unable to take off using its legs alone, and the favoured explanation is a quadrupedal launch in which the animal vaults off its powerful forelimbs, using the muscles that also power the downstroke, which produces far more thrust than legs could and is supported by the proportions of the limbs. Once airborne, soaring rather than flapping would have dominated, and modelling suggests they could cross oceans. Whether the largest could flap sustainably at all remains debated, and estimates of their mass and flight performance have varied by a wide margin between studies, which reflects how much depends on assumptions that cannot be measured directly.

Trackways settled a long argument about posture. Fossil footprints attributed to pterosaurs show a quadrupedal gait with the hands placed outside the feet, walking on the folded wing fingers and the three free digits, which contradicted older reconstructions showing them upright or awkwardly sprawling and established that they were competent on land. That has implications for their ecology, and the large azhdarchids in particular are now generally reconstructed as terrestrial stalkers, walking across landscapes picking up small animals in the manner of a storm stork rather than skimming the sea as earlier images showed, which is supported by their neck and skull proportions and by the environments their fossils come from. Their extinction at the end of the Cretaceous was complete, with no survivors, and the reasons are debated, with competition from birds proposed and generally regarded as insufficient, since the two groups had coexisted for tens of millions of years and the large pterosaurs occupied roles no bird then filled.

The wing hangs from a single enormously elongated fourth finger, which is neither the bat nor the bird solution, and the membrane contained stiffening fibres and muscle rather than being a loose sheet. They flew fifty million years before birds and produced animals with eleven-metre wingspans that probably launched by vaulting off their forelimbs. Trackways show them walking on all fours with the hands outside the feet.