pterylae & tails

[brushes2@juno.com]

Dinolisters,

Some comments on a recent thread:

 PTERYLAE:    There are 7 major feather tracts (pterylae) in modern
birds. Their locations and boundaries are well established, and we even
know how their topology is determines at the molecular level in ontogeny.
The major contour feathers, including remiges and retrices (or flight
feathers) occur in the pterylae. However, feathers are not limited to
pterylae. Various other feathers, e.g. filoplumes, down, bristles etc
occur on other body surfaces. Feathers don't occur on scales surfaces.

There is no evidence that scales were transformed directly into feathers.
There is, however, considerable evidence  that feathers, while sharing an
epidermal origin with scales and other epidermal derivatives, are novel.
First, feathers are cylindrical, not plate-like in both structure and
development. Feathers contain a unique set of structural proteins and
genes. Their branching morphology derives from a series of fusions (often
referred to as branching) steps in production. Feather development is
based on the conical geometry of the follicle and is considerably
different from that of the archeosaurian scale (which actually also occur
in birds). The model of a dinosaur scale somehow elongating to form a
quill or becoming fragmented in situ is inaccurate and is actually
incapable of producing even the most simple feather structure. A
primitive feather, such as the filaments in Sinosauropteryx or the more
complex tufted structures in Dromeosaurs, is a consequence of the way the
follicle works, not the results of some selective force working on its
product. By the way, the  same follicle is completely capable of
producing feathers of different morphology--and function--in sequential
molts.


TAILS:   Steve Gatsey and coauthors have written extensively on therpod
locomotion. Among other topics they specifically address the changes in
structure of the tail (from Frond To Fan) and its evolution in
Archaeopteryx (Evolution  1996 50:2037-2048). His discussion of the
evolution of theropod flight (JVP 1997 17:308-329) is exemplary and
involves the concept of locomotor modules (Evolution 1996 50:331-340).
The whole idea of evolutionary modules is quite stimulating and can even
be successfully applied to the evolution of feathers.

Cheers,

Alan

Alan Brush
BRUSHES2@juno.com