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Re: Tar and feathers

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.



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