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air sacs



>As long as we are on the subject, what are the latest theories about
how/why air sacks evolved?

I don't know about the latest theories, but if dinosaurs indeed had them
(do they have the foramina on the long bones you see in birds, indicating
where the air sac penetrates the bone)?  I would suspect that there were
two forces involved: lightness and, more importantly, respiratory
efficiency.  In birds the air sacs are part of a breathing system that
allows unidirectional flow of air through the lungs, a much more efficient
system than our own giving far higher rates of oxygen extraction.  This
might have been as useful to a Tyrannosaurus on a tear as to a bird on the
wing.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net