Perhaps flying fish became the first vertebrates to colonize the air, to avoid predators? Scales to feathers? Are there any fossils? I know this sounds fishy but I'm serious. The body plan of a fish is already suited for flying. And no predators to worry about. Except maybe insects. Or perhaps the ancient flying fishes ate the flying bugs?
Think about it.
New Paradigm - Fish to Bird?
Fish to Dinosaur to Aquatic Dinosaur to Bird?
Old Paradigm - Fish to Lizard to Dinosaur in Tree to Bird?
Note the flying fish in this video only hovers above the water. Modern flying fish don't go any higher because a bird would certainly eat them. But if there were no birds in the air yet any brave fishes that jump in the air wouldn't worry, and fly up and higher.
This would explain why flying fish don't fly any higher - because bird'zl eat'm up.