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Darren Naish wrote:

> They (crocs) do have air-filled sinuses in skull bones and vertebrae, but
> these are closed (unlike pleurocoels which are linked to the lungs and air
> sacs).

Closed air-filled cavities in animal tissues will not persist but will
eventually collapse or become filled with fluid.  This results from the
uneven exchange of CO2 and O2 between the tissues surrounding the
cavity and the cavity lumen.  Respiratory exchange ratios are rarely
exactly 1, and the cavity will experience a net loss of volume because
the CO2 produced will not match the oxygen removed.  A major
exception is the fish swim bladder, but in physoclist fish with bladders
that do not communicate with the trachea there is an elaborate
countercurrent exchange system present in the 'rete mirabile' which
elaborates gas through a combination of lactic acid secretion and the
Root effect present in fish hemoglobin.  I don't know of any evidence for
such a structure in the vicinity of dinosaur pleurocoels or the putative
pleurocoels in crocodilian skulls.  If anyone has evidence that the croc
pleurocoels are permanently  air-filled, please let me know of it.

This argument has relevance for dinosaurs because of the lung
architecture that must be associated with an air-sac system.  If some
dinosaurs had pleurocoels, they must have had an air-sac system,
which in turn requires an avian-style parabronchial lung architecture. 
Their parabronchial lungs may not have been as sophisticated as those
of modern birds, but the realities of ventilation and gas exchange seem to
require it.

Feedback greatly appreciated.

James M. Norton, Ph.D.
University of New England
11 Hill's Beach Road
Biddeford, ME  04005
phone: [207]283-0171 x2270
fax: [207]283-3249
email: jnorton@mailbox.une.edu