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Re: CNN on Lungs and Feathers



At 01:35 PM 12/31/97 -0800, Augustus T. White wrote:
>
>Can we go from the observation of bone cavities directly to the 
>conclusion that a cervical air sac existed? 

I believe so.  See below.

> Even in modern birds I 
>understand that it is not clear how the cervical air sac functions, 
>i.e. how it is "powered," if at all, or whether it simply responds 
>passively to the pumping of the caudal air sac(s).

This is not actually an issue in regard to its *existence*.  The reason
that pneumatic vertebrae demonstrate the existence of an adjacent air-sac
is that such pneumatic cavities are only known to form by expansion of
diverticulae from such an air-sac.

Thus, no implication concerning the *function* of said air-sac need be
construed.  It need not even have participated in breathing.  It might have
served some totally unrelated function originally (though I rather doubt
that).

Given that the ventral thoracic air-sacs are not adjacent to many bones,
they may not show up in fossils.  My guess is that the non-abdominal
air-sacs in general precede the abdominal ones.  The thoracic air-sacs
could easily be powered by movement of chest ribs.

>  Given that modern birds power 
>their respiration primarilly from the caudal sac, the finding of a 
>potential source for a cervical airspace doesn't necessarily get us 
>anywhere.

This cannot be the ancestral condition.  It is too specialized, and too
derived.  The system has to have evolved through some other intermediate
state first.  The sequence in which pneumaticity appears in the bones of
theropods gives us a clue as to the sequence involved.
>
>In addition there are other potential functions for anterior air 
>spaces that don't involve a conflict with Ruben's model.  To name a 
>few: sound production, weight reduction, air filtration, 
>thermoregulation, and controlling loss of water.

Still, the mere existence of air-sacs at all means that his assumption that
the abdominal air-sacs are primary cannot be held certain.
>
>The potential for caudal air sacs seems a lot more important to the 
>bird issue, since they have fewer non-respiratory uses and are the 
>primary engine of respiration in modern birds. ...

However, the existence of posterior pneumaticity in some non-avian
coelurosaurs indicates that the evolution of these caudal air-sacs
*precedes* the transition to the avian condition (unless George Olshevsky
is right about BCF).
>
>Finally, it isn't enough to have air sacs.  Thre has to be a way to 
>power them, and its hard to apply mechanical stress to an air sac 
>that's encased in bone.

The pneumatic diverticulae that invade the bone are probably not involved
in breathing, even in living birds.  The main air-sacs from which the
diverticulae diverge are the key structure there.

Looking at the picture of the air-sac system of modern birds, I see five
pairs of air-sacs.  The abdominal (caudal) one and the cervical one are
rather different in placement and relationships than the others.

>  Ruben also points out (I lack the knowledge to 
>criticize this) that dinos do not have the rib modifications 
>necessary to power the caudal air sac in the same fashion as birds, 

Such a specialized mechanism cannot evolve all at once.  Some prior, less
derived, variant had to precede it.

>and that the dino 
>sternum seems inadequate to the task.  (The usual response to this 
>involves ratite respiration.  How *do* ratites respire if not by 
>use of the sternum?)

Oh, they have a sternum, just not a "powerful" one such as Ruben et alii
claim is necessary to drive the system.

I think the point is that the "inadequacy" of the dinosaur sternum rests in
assuming a need for an avian level of respiration.  Since the extremely
high level of respiration in birds seems to be associated with flight, it
is not at all clear that dinosaurs would need such a system, any more than
ratites do.

--------------
May the peace of God be with you.         sarima@ix.netcom.com
                                          sfriesen@netlock.com