[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: four winged Archaeopteryx

On Tue, Sep 26, 2006 at 01:51:29AM -0700, Jaime A. Headden scripsit:
> Graydon (oak@uniserve.com) wrote:
> >What keeps Archie from folding the tail feathers against the central
> >shaft?
> I should have been more cautious in what I wrote, so to clarify, let
> me rephrase:
> "The tail in *Archaeopteryx*, unlike those of modern birds, have
> their raches based on bones, rather than embedded in flesh and
> muscles, much of which are analogous to the caudofemoral
> musculature. 

Feathers are integumentary tissue; if (as in the primary feathers of
some modern birds) the feather anchors to bone, that doesn't obviously
imply that it's a rigid anchor, and indeed given that the anchor would
have had to evolve as the aerodynamic function of the feathers
developed, it is very hard to see how it could be a rigid anchor -- the
first proto-bird whose tail fan hung them up on the underbrush when
fleeing a predator would tend to be deselected compared to its
compatriots whose tails kept folding..

> *Archaeopteryx* would retain these muscles only at the base of the
> tail, which would be very useless in altering the ENTIRE tail form.

Not if the bases of the feathers were connected into ligaments, which is
not observed but would not necessarily be, even in cases of
fossilization which preserve feathers.

Nor is it likely that original contour feathers from which the
aerodynamic feathers evolved lacked all connection to some mechanism of
mobility; even an emu can fluff its feathers.

> and unstraightening of the arm (or flexion of the elbow joint), and
> the musclar action required to fan the tail is absent in the wing. A
> similar condition may exist in *Archaeopteryx*, but the anatomy would
> be novel and there is no evidence to suggest it exists. Thus it would
> seem that fanning of the tail feathers would most likely have occured
> AFTER the shortening of the tail in pygostylians."

Individual wing feathers can be moved by extant birds.  I can't tell
you what the mechanism is, but watching a large soaring bird fly makes
it quite obvious that individual feather position adjustment is taking

It's -- to my mind -- rather astonishing to suggest that a bird would
evolve *any* aerodynamic structure and have no ability to control it.

Less refined ability than extant birds, certainly; maybe fundamentally
different than the way extant birds do the same thing, too.  But *no*
control mechanism, in the face of substantial selection pressure for
flight control, seems extremely far-fetched as an hypothesis.