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Caution- "down-like" feathers
Before we go too far discussing "down," it might be wise for all those
interested in the subject of feather evolution to read Prum's theoretical
paper (Prum, RO, 1999, Development and evolutionary origin of feathers. J.
Exp. Zool. 285:291-306) if you haven't already.
When Ji, Norell and others mention "down-like" feathers, they definitely
are not talking about modern natal down unless they say so. Prum describes
the likely phases of feather evolution, including an early, down-like phase,
which he takes pains to distinguish from down. That is, the feathers have
some basic elements in common with natal down, such as lack of barbules, but
this by no means says they were down.
My read of Prum suggests he leaves open the possibility of long threads as
seen in dromaeosaurs and troodons (by not excluding them, at least). Thus,
something built on a SIMILAR plan to modern natal down might have entirely
different surface properties (stiff, bristly, matted, furry, oily,
water-shedding, air-pocketing etc. etc.) compared to the fuzzy wuzzy stuff we
are familiar with.
The point Ji and others seem to agree with is that the observed, basally
branched structures in the fossils fit well with Prum's evolutionary model of
a down-like branching pattern evolving BEFORE the pinnate branching pattern.
Fuzzy natal down is just a limited sub-class of down-like feathers. Those on
the new dromaeosaurs and troodon are different, and seem very un-down-like to
our (my) modern, natal-down-conditioned sensibilites. But they are still
called "down-like" for the reasons I just mentioned.
Did the foregoing help, or just confuse matters more?
Anyway, read Prum's paper. You'll be glad you did.
Thomas P. Hopp
Author of DINOSAUR WARS, a science fiction novel published by iUniverse
Now Humans are the Endangered Species! http://members.aol.com/dinosaurwars