RE:Origin of feathers

["Norton, Patrick" <Patrick.Norton@state.me.us>]

>Ah, but they >did< keep from falling out of the trees, ingeniously   
enough.
Which is why we now have birds instead of a bunch of smashed archosaur
skeletons.<

Perhaps we should pause to remember that the debate over arboreal v   
cursorial origin of flight and feathers goes back more than a century to   
O.C. Marsh (1880) and S.W. Williston (1879.)  Marsh's arboreal argument   
was carried forward primarily by Heilmann (1926) who argued against   
cursorial origin of avian flight and concluded that feathers were   
elongated scales which evolved for aerodynamic purposes and were   
advantageous in parachuting and gliding. Although some still argue   
Heilmann's theory today, one of the principal contemporary advocates of   
the arboreal origin of flight, Walter Bock, abandoned in 1986 the   
"aerodynamic origins" theory of feather evolution in favor of P.J.   
Regal's idea that feathers evolved for insulation in protobirds that were   
facultative homoeotherms. Regal concluded that avian feathers and   
homoeothermy were probably fully evolved prior to even the most   
rudimentary flight.

In the absence of any undisputed fossil evidence of protofeathers, I   
prefer Regal's theory on the evolution of feathers over Heilmann's not   
only because I think it has more intuitive appeal, but partly because it   
is independent of the arboreal/cursorial problem concerning the origin of   
flight. Accepting Regal's theory means you can have homeothermic   
feathered creatures that, whether on the ground or in the trees, are   
preadapted to responding to selection for gliding and flight.

Williston's theory of cursorial origins of flight were carried forward by   
Baron Franz Nopsca (1907, 1923), John Ostrom (1970's-80's) and a   
fascinating (at least I think so) 1983 article by Caple, Balda and Willis   
[Am. Nat. 121:455-476.]  Despite the attractiveness of the trees down,   
parachuting - gliding - flying scenario, I lean towards Ostrom's and   
Caple's cursorial theory--particularly if combined with Regal's theory on   
the evolution of feathers.  The initial aerodynamics-related selective   
pressure on feathers on the trailing edges of the arms of a   
homoeothermic, feathered proto-bird would be towards reduced drag and   
increased lift and thrust. Although the "ground up" argument is not   
without problems, it is also appealing to me because it seems to   
eliminate at least two evolutionary steps required by the "trees down"   
theory--tree dwelling and parachuting.

Reasonable people have disagreed on these issues for a hundred years and   
will presumably continue to do so--at least until the evidence is in.