[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
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.
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com