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David Peters brings up a few point about the mechanics and evolution of
> Drag forces are a problem evidently. (see Brown, Baumel and Klemm, 1995).
I have an abstract by Brown and Klemm 1989 (it would appear to be the
preliminary presentation for the citation given above) where they state that a
high level of drag would be encountered if a propatagium were absent, and that
propatagia counter this:
'The elongation of primary and secondary flight feathers increased the surface
area but not the shape and configuration necessary to produce significant lift.
Further, without additional modifications, the wing bones would have presented
a blunt, irregular surface to the air flow resulting in a high level of drag.'
Do they later suggest/prove that the propatagium is actually *responsible* for
drag? Surely the two points are in contradiction.
David's speculation that a propatagium may have been useful in minimising elbow
overextension in predators and climbers is indeed interesting. Alternatively,
the structure may have evolved purely to provide lift in volant forms - and in
this case, if you have a propatagium you are either volant or descended from
volant ancestors. Some authors (including Brown and Klemm as quoted above) have
stated that the enlargement of wing area by feathers or (non-propatagial)
patagia may have increased surface area beneficial to flight, but does not
provide a configuration necessary to provide significant lift. So a propatagium
is probably essential for fully volant tetrapods. Of course, the question is the
same as the one for feathers: did the structure evolve for flight, or was it
exapted from an existing structure (that may have had a previous use)?
The importance of the propatagium in generating lift would suggest that
_Archaeopteryx_ had one too (assuming that it was capable of volancy, and I
believe it was).
Further worthy of note is that avian propatagia incorporate a unique system of
collagenous and elastic tissue ligaments. These structures make the propatagia
an efficient lift-producing device, but, again, whether such structures would
be beneficial in a propatagium employed for activities not associated with
volancy is an untestable (?) option. It is interesting that, amongst mammals,
both flying squirrels and bats - both certainly relatively unrelated taxa - have
exceedingly similar propatagia and perhaps even homologous muscles spanning the
leading edge (the platysma II and occipito-pollicalis) (Gray and Sokoloff 1992).
They do not help to answer any questions, as (1) the earliest, fully volant bats
have a well developed propatagium, and (2) ancestors of volant mammals may also
have been hanging from tree trunks before flight evolved.
It would presently appear most parsimonious to accept that propatagia evolved
convergently amongst volant taxa. Propatagia may have been retained by non-
volant descendants of volant forms, so unfortunately the discovery of a non-
volant form with a propatagium would not prove much. You need to borrow the time
machine that the U.S. government is presently hiding in Arkansas, and go back
and watch tetrapods evolve volancy. Do they have a propatagium *before* they
start flying, or does it come along after they begin to get airborne? I dunno.
I'd like to hear George's thoughts on this, as if non-volant theropods descended
from gliding/flying forms, and we would expect the volant, arboreal ancestors
(dino-birds) to have propatagia (all extant taxa that glide with their forelimbs
do), the terrestrial descendants would initially retain the feature. Contrary,
sadly, to David's magnificent dromaeosaur restorations, I think we can say that
dromaeosaurs did *not* have propatagia. I'm not relying on the above logic here,
but a fossil: the Benevento juvenile dromaeosaur I have previously mentioned on
this list, while preserving a complete and extensive body impression, clearly
has no patagia of any kind. Its forelimbs are well preserved and, even in the
atrocious photocopies I have, easily discernible (Leonardi and Teruzzi 1993,
also article in _Oggi_). Of course, I don't see why a non-volant descendant of
propatagia-furnished ancestors couldn't *lose* any patagia. Such a reversal is
surely well within the limits of possibility.
I'd better sign off before I start thinking about wing morphology in flightless
"He's my brother"