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Re: FLIGHT VERSUS BROODING
The Nefarious GSP wrote:
>A better scenario has the strengthened shoulder girdle and enlarge muscles,
>pennaceous feathers and folding arm that so dramatically characterize birds
>and their immediate relatives first evolving for flight, not brooding.
It seems clear that the strengthened shoulder girdle developed in
concert with increased use in locomotion and/or feeding. Since arm-feeding
theropods such as _Allosaurus_(?) did not appear to have the same
overdevelopment of the shoulder girdle, I would suggest the former was the
case. This is NOT to say that the arms were not used in feeding, simply that
the features GSP describes may not be explainable only by increased
adaptation to arm-feeding.
(Shall we make up a new name for "arm-feeding" too? Maniraptoring,
perhaps? How about brachiomandibulosynergy? Or manulophagy? Jargon forever!)
What sort of locomotion are we talking about? Given the gracility of
the forelimbs, and what appear (emphasize "appear", I personally have no
data) to be wider angles of excursion of the forelimbs, and the
consolidation of the shoulder girdle into a single functional unit (q.v.
hadrosaurs, Lull and Wright 1941), I'd say that qaudrapedal locomotion is
less likely the purpose of these adaptations. This leaves flight and
arborality (or something else?).
Flight has a lot going for it, such as an "explanation" for the
folding arms and feathers. However, others (Chatterjee comes to mind) have
come up with scenarios showing how the arm/wing folding motion may be
exapted from a climbing motion. The origin of feathers is debateable, but I
have yet to hear a scenario which satisfactorally explains how they evolved
de novo for flight. Note that, like Dr. Orenstein, I do not buy the
neccessity of a gliding stage; indeed, I think the burden of proof is on
those who insist on a gliding intermediate stage.
Arborality may also explain the sickle claw of dromaeosaurs
(climbing peton), the reversed hallux (even though it may not have been
useful in perching per se), the highly mobile base of the tail (Dr. Holtz
has commented on the difficulty of climbing a telephone pole with a
broomstick up your butt) as well as the distal tail's rigidity (bipedal
animals balancing on branches might appreciate a dynamic stabilizer as much
or more than agile cursors). Indeed, Dr. James Norton and I have both
proposed (on this list) various scenarios involving the role of trees in the
dromaeosaur attack strategy (at least ancestrally) leading to fluttering flight.
Of course, this leads us around in a circle. The evidence is by no
means conclusive. Most of the arborial evidence may also be interpreted as
cursorial evidence of some sort. As of now, this is (in the words of the
great Alan Feduccia) an "intuitively pleasing" scenario. The truth of the
matter is that we have no non-equivocal evidence of arborality in non-avian
maniraptoriforms. I leave you with two questions: what should such evidence
look like and, if there is truly no possible form of "unequivocal" evidence,
isn't insisting that it be found before we accept arboreal maniraptorans
really another form of accepting negative evidence?
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"...To fight legends." - Kosh Naranek