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Re: Scientific American feathers paper
Alan Brush <email@example.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.
Prum, R.O. (1999), Development and evolutionary origin of feathers. J.
Exp. Zool. 285: 291-306
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