[no subject]

[Nick Longrich <longrich@phoenix.Princeton.EDU>]

Erect limbs:
        Chameleons have them (but not the four-chambered hearts), bt
probably aren't a fair comparison since they probably evolved them for
very different reasons. 

 Differences in patagia:
        Differences in patagia are seen in bats living in the same
habitat, e.g. tropical rainforest, but vary widely based on behavior.
Among the bats, the insectivores have very large uropatagia. Fruit-eaters
do not, and the attachment of brachiopatagium is reduced and the
uropatagaium very reduced in the vampires, which are the most terrestrial
of all bats except perhaps New Zealand short-tails; vampires also have
really big feet and thumbs. The putative presence of a uropatagium in
Sordes, therefore, says little if anything about its presence in other
pterosaurs (ditto with the wing attachment) but it does imply that Sordes
was pretty maneuverable like insectivores, it isn't inconcievable (but
also not demonstrable) that Sordes could have used it to help capture prey
mid-flight like insectivorous bats- then again, bats legs articulate very
differently so it might not have had this ability. Although this raises a
good point, if we look at modern seabirds their tails are very small
compared to their wings (e.g. the albatross) compared to your
stereotypical landbird, so Sordes vs. Rhamphorhynchus probably does
reflect their different habitats. 

Gliders:
        The avian wing, or a bat wing, or a pterosaur wing, can be divided
into two main functions: lift and thrust. The downstroke's main purpose is
not the generation of lift. Flying squirrels, colugos, sugar gliders and
whatnot can all generate lift just fine. Their problem is what the
downstroke solves: providing additional thrust. the only way they can gain
forward motion is by sacrificing height: gliding. 
        In modern birds, bats, and
presumably in pterosaurs, the proximal part of the wing really doesn't
have much to do with providing thrust- it's still doing what the patagia
of flying squirrels and colugos do, providing lift. It's the distal part
of the wing that is different, however- the downstroke allows the already
present system of providing lift to continue to provide lift longer by
generating more forward thrust, so the animal no longer has to sacrifice
height. 
        Granted there are no animals making the transition today, but they
have made it partway by developing the ability to generate lift, which
means that we can view some of the transitional stages postulated for a
gliding origin in today's world, vs. none in a cursorial or flapping-first
scenario. What would flapping accomplish in an animal without wings,
even so? What would be the benefit of producing forward thrust? Perhaps
you could run faster in theory, but how would this promote leaving the
ground? Leaping farther by leaping faster is a possibility, but it would
be energetically very expensive and glide planes are an existing solution
to leaping farther. With enough thrust you could just become airborne,
soemthing like how a helicopter is just a big vertical propeller, but the
energy required for this would be phenomenal.