Re: bauplan convergence

["Scott Hartman" <scott_hartman@hotmail.com>]

> > Gravity can get ANYBODY to the ground that leaps out of a tree.

> > Gravity, air, and a little extra skin on the arms gets you >>parachuting.

> > But parachuting doesn't require that you flap.

    Actually, while the term "parachuting" is bandied about by many when 
discussing incipient levels of gliding, this usage is innappropriate.  Balda 
et al. (1984 Archaeopteryx conference volume)provide a more aerodynamically 
correct definition of parachuting as a method to slow descent using only 
drag (or at least where lift is negligable).  Animals that can glide at a 
measly 45 degree angle from the horizontal are still creating lift at an 
impressive 70% of body mass.  Few, if any animals qualify as parachuters 
(although perhaps flying frogs are an exception?)  That's why flying 
squirrells and snakes have flat bottoms but round(er) tops.
    Competent parachuters generally favor a relatively slow descent, while 
competent gliders usually glide at quite high speeds, and often (but less 
stringently) in more horizontal directions (e.g. from branch to branch).  So 
there is a large functional and behavioral gap between gliders and 
parachuters, which I doubt is crossed often.  I suspect that gliding 
requires a scansorial habbitat, where slight flattening of the ventral 
surface, or slight enlargement of patagia will have an imediate impact on 
locomotion safety, and soon have a large impact on scansorial locomotive 
speed and efficiency.
    This is important, because theropods and Archaeopteryx have deep narrow 
bodies, and long limb elements that keep would keep their center of gravity 
high off their footing, which doesn't help when recovering from a boffed 
jump to the next branch.  I have no problem envisioning theropods climbing 
trees from a functional standpoint (grasping hands, lots of claws and what 
not), but there are simply no theropods (or Archaeopteryx) that show 
scansorial adaptations.  So I doubt scansorial gliding was important in the 
evolution of avian flight.

Scott
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