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The term "dinosaur" is restricted to just those reptiles descended from the last common ancestor of the groups Saurischia and Ornithischia (clade Dinosauria, which includes birds), and current scientific consensus is that this group excludes the pterosaurs, as well as the various groups of extinct marine reptiles, such as ichthyosaurs, plesiosaurs, and mosasaurs.

Like the dinosaurs, and unlike these other reptiles, pterosaurs are more closely related to birds than to crocodiles or any other living reptile.

This membrane may have incorporated the first three fingers of the hand, as evidenced in some specimens.

The brachiopatagium ("arm membrane") was the primary component of the wing, stretching from the highly elongated fourth finger of the hand to the hind limbs (though where exactly on the hind limbs it anchored is controversial and may have varied between species, see below).

In some later pterosaurs, the backbone over the shoulders fused into a structure known as a notarium, which served to stiffen the torso during flight, and provide a stable support for the scapula (shoulder blade).

Reconstructed wing planform of Quetzalcoatlus northropi (A) compared to the wandering albatross Diomedea exulans (B) and the Andean condor Vultur gryphus (C). northropi, the largest known flying animal, was more than three times as long as that of the wandering albatross.

Pterosaurs are the earliest vertebrates known to have evolved powered flight.

Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a dramatically lengthened fourth finger.

There has been considerable argument among paleontologists about whether the main wing membranes (brachiopatagia) attached to the hind limbs, and if so, where.Early species had long, fully toothed jaws and long tails, while later forms had a highly reduced tail, and some lacked teeth.Many sported furry coats made up of hair-like filaments known as pycnofibers, which covered their bodies and parts of their wings.Depending on their exact composition (keratin, muscle, elastic structures, etc.), they may have been stiffening or strengthening agents in the outer part of the wing.As shown by cavities in the wing bones of larger species and soft tissue preserved in at least one specimen, some pterosaurs extended their system of respiratory air sacs (see Paleobiology section below) into the wing membrane.

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