Re: Waimanu & avian evolution (comments)

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Friday, May 19, 2006 12:40 AM
Subject: Re: Waimanu & avian evolution (comments)


> A long decline is important, but given the diversity of late Cretaceous 
> large-bodied pterosaurs (a diversity that was much higher than previously 
> thought, it now seems), I think the bolide would have to remain pretty 
> relevant.

I tend to think it likely that if the bolide hadn't come along,  the big 
pterosaurs would have remained with us till man came along to do the 
bolide's work.

> Okay, I'll buy that; though for the sake of debate I'd like to toss out 
> the idea that loss of small-bodied forms and gain of large-bodied forms 
> isn't really an overall decline in diversity.

I find it interesting that smaller pterosaurs declined as birds diversified, 
but that soaring birds don't seem to have developed till after the 
pterosaurs were out of the way.  At the end, they each seem to have been 
successfully filling different niches.

> Also, the importance of tail fans for agility and maneuverability is 
> complicated; some of the most agile birds have almost no tail fan at all. 
> Raptors do use their tails extensively, however (though use varies quite a 
> bit between taxa).

The birds are short-coupled, so the tail fan doesn't add much tail volume 
and therefore not much tail force.  Note that in this sense, the word 
'volume' doesn't have its usual meaning -- it refers to the moment that can 
be developed, not the size.  A small tail area on a long moment arm has a 
similar tail volume to a large tail area on a short moment arm.  However, 
when agility is important, every little bit helps.