Networks of blood vessels in bats' wings serve only to disperse heat generated in flight  This heat is generated only because bats flap their wings
Thus paleontologists' recent discovery that the winged dinosaur Sandactylus had similar networks of blood vessels in the skin of its wings provides evidence for the hypothesis that Sandactylus flew by flapping its wings, not just by gliding
speculating about how structures observed in present-day creatures might have developed from similar structures in creatures now extinct