Re: Scientific American feathers paper

["Tim Williams" <twilliams_alpha@hotmail.com>]

Alan Brush <brushes2@juno.com> wrote:

(And thanks for your on-the-mark answers.)

> > In other words, modern down feathers are 'trimmed down' pennaceous > 
> feathers, not a more primitive intermediate stage.
>
> Not necessary. Looking at the fossil record implies that the stages
> appeared in roughly the same order in phylogeny as they do in ontogeny.

Ah yes, I have no doubt the plumule-like 'tufts' came before pennaceous 
feathers in feather phylogeny.  However, Prum (1999) is very careful to 
distinguish the hypothetical down-like Stage 3B from the down of modern 
birds.  The latter appears to be derived from the Stage 4 feather morphology 
(closed, pennaceous).  Thus, the down feather of modern birds has a similar 
developmental trajectory to modern filoplumes (post-Stage 4), and not a 
hypothetical tuft stage (Stage 2 or 3B).

The reason I find this interesting, is that birds (neornithines, at least) 
may have re-invented the wheel when it came to the evolution of their down 
feathers.  Instead of holding the feather at Stage 2 (tufts) or Stage 3B 
(tufts + barbules), development continued on and passed through Stage 4 
(open pennaceous + barbules) before arriving in the morphology of a modern 
down feather.  Thus, modern plumules converge on a Stage 2 overall 
morphology (tufts, not vanes) that existed in non-avian theropods, but 
differ in having barbules.

The adaptive advantage of this scenario is perhaps that the plumule barbules 
keep individual barbs from twisting on themselves (as you suggest) or the 
extra branches provide superior insulation (I recall Ostrom mentioning this 
in a paper).  Or may all be a consequence of developmental constraints: the 
first birds (or first neornithines) lost the genetic ability (plasticity?) 
to generate down-like feathers, and so modern down came about by simplifying 
a subset of pre-existing pennaceous feathers?

Finally, you could argue that there was no Stage 3B in feather evolution, 
and the feather went from Stage 2 --> Stage 3B --> to Stage 4 to produce a 
closed pennaceous feather.

Phew, I'm exhausted.  This feather-evolution stuff is fascinating though.


Reference:

Prum, R.O. (1999), Development and evolutionary origin of feathers. J.
Exp. Zool. 285: 291-306


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