Re: large fossil birds

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Monday, November 28, 2005 8:36 PM
Subject: Re: large fossil birds


> The only catch is that azhdarchids didn't have particularly long wings 
> relative to their size.  Thus, the issue does not seem to be one of 
> getting the wings large enough for the given weight (ie. maintaining a low 
> wing loading);

There is always an advantage in having the highest wingloading commensurate 
with the ability to launch, land, and extract energy from the atmosphere (if 
a soarer).

> at least not by increasing span alone.  Part of the issue might be a 
> relatively longer chord for giant pterosaurs (than in large birds),

Chord appears to have decreased relative to span in the larger azhdarchids. 
Since the largest birds seem to have been rather low aspect ratio (like 
vultures), I'd say that giant pterosaurs appear to have had a relatively 
shorter chord than large birds.

> resulting in a more favorable wing loading:mass scaling relationship.

That particular trend seems to me to have been in the other direction.  But 
I don't see that as necessarily unfavorable.

>  Again, however, I find this to be a dubious hypothesis because 
> azhdarchids didn't minimize wing loading and in fact have several 
> adaptations associated with increased use of powered flight (rather than 
> soaring).

Azdarchids seem to me to have had adaptations associated with increased 
power for burst flapping, not powered flight per se.  They had quite high 
aspect ratios (about 16.5 for Qsp and maybe roughly about 17.1 for Qn) and 
appear to have been optimised for dynamic soaring rather than for convective 
soaring.  I doubt that they flapped for more than 15  to 30 beats at a time, 
and would expect fewer beats most of the time.

> Still, the ideas on wing weight constraints are quite intriguing.  I'd 
> like to pose the following question, however (for the sake of discussion 
> if nothing else): Would we actually expect birds to reach the same size as 
> the largest pterosaurs?

If by size you mean span, I wouldn't.  Pterosaurs used a flight style that 
doesn't require endothermy (though they do seem to me to be endothermic). 
So they don't have to carry as much fuel load for a given span.  They also 
seem to have shifted more of the flight muscles from the torso out into the 
inner half of the wings.   Shifting weight into the wings is called 
spanloading, and it greatly reduces the constraints on maximum wingspan. 
Perhaps it was associated with the increased volume of the neck/head in some 
of the azhdarchidae, as a compensating mechanism.  I also suspect that birds 
may have had structural and weight issues with feathers as they increased 
the length and chord of the feathers and the loading upon them.

>  Pseudodontorns and teratorns reached spans and masses similar to very 
> large pterodontids.

Not quite  so large as sternbergi.

>  Perhaps azhdarchids were simply doing something unique that birds haven't 
> done.

I agree totally with that.

>  After all, birds include a wide variety of morphotypes/ecotypes that were 
> never seen in pterosaurs.

Which is, I think, why birds are still with us and pterosaurs aren't.  If it 
hadn't been for that big rock whacking us, I think pterosaurs would still be 
with us, and birds might not have morphed to incorporate high aspect ratio 
soaring..

JimC