Tim,
The aerodynamic function was undoubtedly the last major exaptation.
Before that was insulation. But the tendency of feathers to grow in tracts
would indicate some function even before the insulation phase. Predator
evasion (or display) function first, then insulation (incl. brooding), and
finally aerodynamics (gliding and flight).
Birds almost certainly evolved from primitive "theropods" of which we
have no fossil record (yet), but if some version of BCF is correct, then
these early forms I am talking about were neither one nor the other. That
is why the term "dino-bird" is often used.
As for Microraptor or any other "feathered" theropod, there is no way
to know what the feather distribution was on their Triassic ancestors. We
just don't know, and that is why I don't want to get too attached to an
insulation-first hypothesis. I think some other function preceded
insulation, and evolution (especially exaptations) will make discovering
such a function very very difficult from post-Triassic fossils.
I'm certainly not going to say Longisquama is one of these, because I
think such featherlike structures could have evolved more than once.
However, the placement of the structures found on Longisquama could be
telling us what early dino-birds were like in a vague sort of way (i.e. with
parallel feather tracts along the back).
-------Ken
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From: "Tim Williams" <twilliams_alpha@hotmail.com> To: kinman@hotmail.com, dinosaur@usc.edu Subject: Re: feather tracts Date: Wed, 11 Jul 2001 16:18:51 -0500
Ken Kinman wrote:
But those are mostly Cretaceous (i.e., very derived Johnny-come-latelies), including secondarily flightless forms.
I wouldn't regard _Microraptor_ has "very derived" - it looks a lot like (but not exactly like) what I would expect of a pre-late Jurassic maniraptoran on the line to _Archaeopteryx_ and modern birds.
And which forms are secondarily flightless? _Caudipteryx_, just because it has remiges formed into a wing?
I'm talking about Triassic and Early Jurassic, very primitive forms.
Forms of theropods or forms of birds? If the latter, I wouldn't hold your breath waiting for Triassic birds to turn up. :-)
Once feathers were exapted for insulation, there would then have been a tendency to develop a more scattered pattern on the body, especially as they spread ventrally.
I think you might be putting the cart before the horse. Why couldn't feathers have evolved for insulation and _then_ exapted for aerodynamicity?
Tim
------------------------------------------------------------
Timothy J. Williams
USDA/ARS Researcher Agronomy Hall Iowa State University Ames IA 50014
Phone: 515 294 9233 Fax: 515 294 3163
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