I think there's more to the difference between pterosaurs and dinosaurs than how they stand. I believe the number of holes in their skull is another key difference, and I think its one of the things that's used to separate things like dimetrodon and mammal-like reptiles, as well as sea goers like mosasaurs, plesiosaurs and ichthyosaurs from true dinosaurs as well.

Probably something similar seperating True Dragons from creatures like dragon-turtles, wyverns and so on.