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. In the passage, the author develops the argument by ['speculating about how structures observed in present-day creatures might have developed from similar structures in creatures now extinct', 'using an analogy with a known phenomenon to draw a conclusion about an unknown phenomenon', 'reinterpreting evidence that had been used to support an earlier theory', 'forming the hypothesis that best explains several apparently conflicting pieces of evidence']