A recently posted bat take-off video documents how a tiny bat (not a vampire) does a magnificent pushup, reaches the acme of its trajectory then produces its first flap. Bat take-off has been used as an analogy for pterosaur take-off. But this bat comes with a difference.
This video comes with a difference.
This bat goes belly down to ‘cock’ the launch and it is also using its finger/wing tip to assist the launch by providing “second stage” thrust. The wing tip is the last part of the bat to leave the ground.
This differs from hypothetical pterosaur launches
in that most pterosaurs would be unable to ‘belly down’ due to limb proportions. And current forelimb launch hypotheses do not involve the extension of the wing finger to providing additional thrust for the initial leap.
In bats extending that wing finger during the initial phases of that leap also prepare the wing for that initial down flap. In pterosaurs the complete extension of the wing during that initial leap not happening fast enough is one of the main problems with the forelimb launch hypothesis. The other problem is most pterosaurs were overall larger-to-incredibly-larger than the bat in the above video. Doubling the height cubes the weight. That’s why giraffes don’t leap like gazelles despite their similar proportions.
We still have no launch tracks, fore or hind
So the fore limb launch for pterosaurs remains hypothetical. For that matter, so does the hind limb launch hypothesis. But if pterosaurs did launch with their fore limbs, then they might as well act like bats and use those big wing fingers to not only provide additional “second-stage” thrust, but to deploy those big wings prior to the acme of the launch trajectory. Now, whether or not pterosaur wings could do this is yet to be resolved.
And maybe only those pterosaurs the size of the bat above were capable of such launches. Mass matters.