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A new hypothesis for the evolution of the pterosaur wing is presented. It is based on the observation that many aspects of pterosaur non-wing
anatomy were present in their non-volant, prolacertiform sister taxa. In pterosaurs alone, metacarpal IV was twisted 90° medially, so the wing digit flexed in the plane of the metacarpus, rather than towards the palm as in other tetrapods. While grappling a tree using manual digits I-III, a pre-pterosaur with this character would have been free to flex and extend digit IV in the plane tangential to the circumference of the tree. The addition of a small dermal extension that opened like a Japanese fan would essentially have completed the development of the proto-wing. Its subsequent increase in size was brought on by selective competition. Successful powered flight would have been possible only after a critical wing size had been achieved.

Peters, D. (2001). A New Model for the Evolution of the Pterosaur Wing--with a twist. Historical Biology: A Journal of Paleobiology 15 (4): 277 - 301

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