I speculate that the evolution of flightlessness is at least plausible for azhdarchid pterosaurs. If they did become flightless, what might they look like? This is the hypothetical flightless form Shemhazai ptychocheirus, invented for use on Tet Zoo.
As you are probably already aware, we explored this idea even further with the Speculative Dinosaur Project, eventually leading to cursorial megapredators. I'm pretty sure the idea originally came from you.
I feel so very stupid for taking half a day realize what the name came from. Leader of the fallen angels, very clever! I guess it's that I had never before seen Shemyazaz spelled that way.
They're vestiges, and perhaps redundant. But perhaps they could be used in sociosexual display - the animal is dull overall, but the relictual bit of membrane on the wing finger is brightly coloured and can be 'flashed' when required.