Re: Pro(to)avis

[jrc <jrccea@bellsouth.net>]

Thanks, Tim.  I had seen it.  I've added a couple of comments below, in the 
pertinent sections.

----- Original Message ----- 
From: "Tim Williams" <twilliams_alpha@hotmail.com>
To: <dinosaur@usc.edu>
Sent: Friday, September 16, 2005 12:25 PM
Subject: Re: Pro(to)avis


> The tail of _Archaeopteryx_, however, differs from this arrangement in 
> several important respects. _Archaeopteryx_ probably had a limited range 
> of lift forces and tight coupling between vertebral and rectricial 
> movement.

If uncascaded, Archie's tail would have functioned as a low-aspect ratio 
lifting device.  Consequently, it would have been capable of a large range 
of lifing forces.  I'm not sure about Archie's retricial movement, but if 
the tail was capable of cascading, it would have greatly impacted the 
lift/drag ratio, lowering maximum lift while greatly lowering maximum 
drag -- but that isn't pertinent to our present discussion.

> This would have made the tail of this primitive flier better suited to 
> stabilization than maneuverability.

This is correct, but not quite for the reasons that they say, as they said 
them above.  However, one of the primary functions of the long, fully 
feathered tail appears to me to have been its capability of supporting the 
weight of the long tail plus a substantial portion of the weight of the 
hindlimbs, thereby compensating for what appears to have been an otherwise 
rather noticible aft cg problem.  In 4-winged birds, the tip fan is capable 
of supporting the weight of the tail, but at first blush doesn't appear 
suited for also supporting the weight of the legs, so without the hindlimb 
feathers those birds would also have had a a bit of a cg problem.  Note that 
none of this applies to modern birds with brains evolved for more unstable 
flight and without the weight of a long, unfeathered tail.

> The capacity to significantly alter lift and manipulate and flight surface 
> without distortion may have been two factors favoring tail shortening and 
> pygostyle development during avian evolution."

I think they were as well, but again -- this isn't what I was talking about.

All the best,
JimC
P.S.  Gatsey & Dial do good work.