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Re: bauplan convergence



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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