Re: Pro(to)avis

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Tim Williams" <twilliams_alpha@hotmail.com>
To: <dinosaur@usc.edu>
Sent: Friday, September 16, 2005 1:34 PM
Subject: Re: Pro(to)avis


> It is my understanding of such things (which is not that much) is that a 
> posterior center of gravity (cg) is a problem for pasive (non-powered) 
> gliders as well as active (powered) fliers.

Correct.

> > 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.
>
> One curious aspect of avian evolution (at least, curious to me) is that 
> the shortening of the tail and development of a pygostyle occurred in 
> birds that had rather long legs (e.g.,_Sinornis_).  I think one of 
> Gatesy's papers discusses this too.  So early ornithothoracine birds 
> dispensed with the tip fan, but kept their "primitive" theropod legs.

I think that implies that the tip fan could have been used for supporting 
the weight of the tail, as well as providing directional stability.  As the 
brain develops the ability to control unstable flight, the tail shortens and 
the tip fan goes away.

> Also, wouldn't the long tail-frond and hindwings have interfered with a 
> running take-off?

Not necessarily.  Most bipedal dinos seem to have held the tail rather high. 
The hindlimb feathers would have been in trail.  I'm not much of a fan of a 
generic requirement of running takeoffs anyway.  Many birds leap well enough 
to to get most of the way to steady-state stall speed without having to run 
to do it.   Others run.  Several ways to skin that cat.

> > P.S.  Gatsey & Dial do good work.
>
> That they do.
>
> many traditional pre-flight models have been constrained by the perception 
> that the proavian either spent all of its time on the ground, or all (or 
> most) of its time in trees.  This did not help the formulation of 
> individual pre-flight models.

I don''t know who wrote this, but I agree with it.  As the ability to fly 
develops, the environmental envelope that accompanies its use enlarges.  I 
do think that the earliest precursors were probably cursorial and that 
arborality developed as preflight ability increased.
Jim