I was originally just going to link to a Colugo, which I think are still pretty cool, and probably good evidence of what early stages of vertebrate flight looked like. Functionally they're not doing full flight like a bat, but their flight is still quite a bit better than a flying squirrel, a sugar glider, or one of the assorted other gliding mammals can do. You can see them flapping.
Also tiny cameras have come a long way since that video was made.