Based on the shape of the wings of the basal chiropteran _Onychonycteris_, Simmons et al. (2008) suggest that it had an undulating flight style in which fluttering alternated with gliding, and, therefore that _Onychonycteris_ represents a transitional stage between gliding and continuous flapping. So on this basis, bats (or rather their ancestors) passed through a gliding stage.
Undulating flight means alternating between fluttering and a purely ballistic phase with completely folded wings. Lots of small birds fly that way. No gliding in there.
> If bats did go through a gliding phase, then that could be > rephrased as: 'the mammals that never made it past gliding are > exclusively herbivores'. :D That's an interesting point.
Too bad that *Volaticotherium* destroys it.