Re: Tar and feathers

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

Mike Taylor <mike@indexdata.com> wrote:

> Hi Tim.  I wouldn't be _confident_ about this at all: I can't think of
> a single living analogue, that is, an extant animal those young has
> integument that is discarded in adults.

Apples and oranges, friend Mike.  These are dinosaurs we're talking about, 
not mammmals.  I baulk at using modern mammals as analogs for what might 
have gone on on in the "early days" of feather evolution.

Here's some good ol' just-so-ing to emphasise my point...

In birds, we know that the natal down of nestlings is very soon replaced by 
other feather types: contour feathers, adult down, semiplumes.  The latter 
are both aerodynamic and thermoregulatory in function, although these are 
not their only functions: e.g. contour feathers are often highly modified 
for display.  These feather types (and the filoplumes, bristles, powder 
downs, etc) vary enormously in morphology.  However, they are all appear to 
be derived from closed pennaceous feathers.  Thus, they roll off quite late 
in the developmental trajectory, irrespective of how simple or complex they 
are in structure.

Combining the developmental evidence with the fossil evidence, the following 
evolutionary scenario emerges:

(1) Non-avian theropods: Feathers evolved for insulation in nestling 
theropods, and was abandoned in ontogeny beyond this stage of life.

(2) Maniraptoriforms (or earlier?): Feathers persist into adulthood for 
insulation, and differentiate (e.g., to tufts and "sprays"; Stage II/III of 
Prum), perhaps to improve insulation.

(3) Maniraptorans: Feathers differentiate in adults for aerial locomotion - 
parachuting, gliding, then sustained flight.  The adult integument becomes 
dedicated to aerial flight.

(4) Birds (post-_Archaeopteryx_): Feathers differentiate even further in 
adults, producing the diversity feather types we see today.  However, the 
primordial function of feathers (insulation) is "reborn" by modifying 
(secondarily simplifying) the pennaceous feathers (Stage IV), not arresting 
feather development at Stage II/III.  This last avenue, although 
developmentally more parsimonious, was closed off in early birds, 
necessitating a more circuitous develomental route to generate adult down.

The above is an hypothesis; I'm not claiming this is what *did* happen.  It 
is congruent with the evidence, but other scenarios may explain the evidence 
more parsimoniously.  As I said, it was a burst of just-so-ing.  I'm just 
putting it out there.

Cheers

Tim



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