This site presents the idea that birds developed from pterosaurs.
This is a credible alternative to the current, mainstream idea that birds developed from land-based dinosaurs.
How could a non-pterosaur descendant develop a propatagium?
Not sure I know what you mean.
What I mean is other creatures have propatagiums who aren't descendants of pterosaurs. How could they get them if they didn't descend from pterosaurs?
Any dinosaurs have a propatagium?
No but why would that matter? Are dinosaurs purported ancestors of pterosaurs?
I will stop wasting time on you.