Pterosaur brachiopatagium to bird postpatagium

Lets look at the brachiopatagium of the pterosaur and see how it changed in the development to bird.

http://books.google.ca/books?id=nQ3vLQkEWQMC&pg=PA56&lpg=PA56&dq=postpatagium&source=bl&ots=F45R6-nU20&sig=wAVGEOUZ5wVoLpwKjqQyaQJ0c_g&hl=en&sa=X&ei=69HIUMCDFvDlyAGSy4H4BA&ved=0CDkQ6AEwAQ#v=onepage&q=postpatagium&f=false

"Several sheets of tough, tendinous tissue form significant parts of the wing structure: the patagium and the patagialis longus muscle and tendon actually form much of the leading edge of the wing"

Note page 128 of this reference:
http://books.google.ca/books?id=KG86AgWwFEUC&pg=PA128&lpg=PA128&dq=bird+postpatagium&source=bl&ots=RtK3TnXIZI&sig=1E_t2KtzJvsO5J1risrR6Ujv52I&hl=en&sa=X&ei=gGvcUsuOGqTu2AW-xYD4BQ&ved=0CFwQ6AEwBA#v=onepage&q=bird%20postpatagium&f=false

^{http://www.wordnik.com/words/postpatagium}

Postpatagium:
^{n. In ornithology, the triangular fold of skin, just back of the shoulder-joint, which runs from the side of the body to the upper posterior face of the upper arm.}

It may well be the case that the development from the pterosaur  brachiopatagium  to the primitive bird wing skin took place in one step through a mechanism such as "facilitated variation".

Also Scansoriopterygidae presents as an excellent transitional wing skin membrane, between pterosaur and later members of Paraves.

See the following references for more info on the fascinating topic of facilitated variation:
http://pterosaurnet.blogspot.com/2011/09/pterosaur-wing-to-bird-wing.html
http://pterosaurnet.blogspot.com/2011/09/more-on-facilitated-variation-and-wing.html
http://en.wikipedia.org/wiki/Facilitated_variation
http://www.pnas.org/content/104/suppl.1/8582.full