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Re: CNN on Lungs and Feathers



Stanley Friesen wrote:

> And Ruben is almost certainly wrong.  There is solid evidence for pneumatic
> cavities in cervical and anterior dorsal vertebrae in all known theropods
> (except perhaps the herrerasaurids and _Eoraptor_ - I do not remember for
> certain).  This means that the cervical air-sac already existed *long*
> before _Sinosauropteryx_ evolved.   And pneumatic spaces in the posterior
> dorsal, pelvis, and hind-limbs is known from some non-avian coelurosaurs.
> This means that the *abdominal* air-sac existed by then.

Can we go from the observation of bone cavities directly to the 
conclusion that a cervical air sac existed?  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).  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.

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.   
 
> Note, also, that the fact that the cervical air-sac seems to have preceded
> the abdominal one eliminates Ruben's objection to air-sacs co-existing with
> a diaphragm.  (Though, of course, the abdominal air-sac could have existed
> prior to invading the posterior bones).

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, their relative 
infrequency is certainly a sign that we might be erecting an evolutionary 
edifice on an uncertain foundation.  

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.  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, 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?)

I must admit that most of my knowledge in this area is of very recent and 
uncertain vintage.  I'd appreciate any corrections you may have.  
However, from what I've seen, neither side has made a very convincing 
case on this front of the BADD debate.

  --Toby White