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-----Original Message-----
From: GSP1954@aol.com <GSP1954@aol.com>
To: dinosaur@usc.edu <dinosaur@usc.edu>
Date: Friday, October 17, 1997 3:38 AM

><large snip> Among crocodilians a it became highly
>developed to the current condition as a lumbar region and mobile pubis
>allowed extreme abdominal shifting. Perry has suggested that some form of
>abdominal ventilation was present among ornithischians. I noted that in
>ornithopods the presence of a very well developed lumbar region as well as
>absence of gastralia and a procumbent pubis suggest that a mammal-like
>diaphragm arrangement had developed. Tridatcyl theropods and sauropods
>largely abandoned the old system in favor of air-sacs as they lengthened
>increased the mobility of the posterior ribs (opposite the development of a
>lumbar region in crocs) in order to ventilate abdominal air-sacs. The
>intermediate stage may have been present in intermediate theropods such as
>Sinosauropteryx. Birds "remember" the old arrangement via their

Modern birds ventilate without much change in either the lung or the
thoracic cage although the ribs move forward slightly during breathing.  The
horizontal septum closes the caudal end of the thoracic area.

Three questions:
     1.  When did gastralia begin to develop in the tridatcyl theropods?
      2.  Did they develop in conjunction with the expansion of the rib cage
or did one follow the other?
      3.  Modern reptiles show a pressure gradient between the thoracic and
abdominal cavities even tho they
           do not have a diaphragm.  Did a septum have to develop in the
intermediate forms before air sacs
           could be ventilated or did the abdominal air sacs develop first.
Or from the septum itself?  In other
           words, how did this system start to evolve in your opinion?