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Origin of flapping (was: Re: Feduccia [was: polarity...])...)
Ronald Orenstein wrote (9/24/96; 5:45p):
>[snip]...we ar no further ahead at finding a gliding
>precursor for pterosaurs (or bats, for that matter) than for birds...
But there ARE gliding archosauromorphs.
>My alternate for birds was the idea of an arboreal animal (probably
>at least a functinal biped, but that is not an essential part of the
>idea) developing structures to assist, not in getting from tree to
>tree, but in fluttering or leaping upward within a tree (to reach,
>say, insects on leaves or branch tips). Otherwise, why develop a
WARNING: speculation ahead!
_Reaching_ may be the key here. It might exlain why the forelegs got so
long in both pterosaur and bird precusors. Foreleg flapping wouldn't do
much good in an animal having short forelegs. Most archosauromorphs have
longer hind legs, even the gliding Sharovipteryx. Why wouldn't flappers
have evolved to be like certain experimental airplanes that have larger
rear wings and canard wings up front for steering (a flapping
Sharovipteryx)? (Furthermore, the propeller in some of these planes is
in the rear.)
Animals that must reach for branches, perhaps because they are too large
to risk leaping, might be prime candidates to develop long, strong arms
(and small, light-weight bodies). Long arms frilled with anything--hair,
extended scales, a skin membrane--might be flapping wing precursors.
Anyway, it is possible that long, strong arms evolved first (maybe also
for catching prey, as Ron suggested).
Ron also said (sorry--I deleted the words) that we don't know how much
the hind legs of the first pterosaurs were involved in flight. I think
is is possible that the first pterosaurs had wing membranes that did not
reach the hind legs at all--that only later pterosaurs had wing membranes
stretching to the hind legs, as some fossil impressions show. Otherwise,
I wonder if the first flappers wouldn't have flapped both the front legs
and hind legs in unison, then with no clear aerodynamic reason (or is
there?) why the forelegs seem to have become so dominant over the
usually-dominant hind legs. I realize there may be anatomical
limitations in hind-limb wings, but are there any such
anatomical-limitation hypotheses that aren't after-the-fact ad hoc? Why
didn't the Sharovipteryx's of the world evolve into "metatarsopters"?
(Now, there's an alternative evolution for you!)
I like long, strong, reaching arms first, leading to wings. So I think
Ron is on the right track--for birds and maybe even pterosaurs and bats.
>As we know, non-flapping gliders can travel very long distances
>(over 100 feet for the colugo and some large Asian flying squirrels),
>and as gliding is so low-cost energetically there may be little
>pressure to add a flapping component (unless the forests thin out
>to a very great degree).
What if a glider is swooping down toward its prey but the prey hears the
rush of air at the last moment (or its companion prey see it and noisily
scatter) and tries to side-step or bolt ahead? Maybe a sudden, frantic
(hence, energetic) flapping might prolong the glide or turn it aside to
still catch dodging prey. If the forest thins out to a great degree,
trees will be farther apart, and gliders that can prolong their glides
using incipiently powered flight might have an advantage.
Norman R. King tel: (812) 464-1794
Department of Geosciences fax: (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712 e-mail: firstname.lastname@example.org