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pneumocoels in fossilized bones

A question for the group, with apologies to those who may receive
multiple postings:  

What are the criteria used to determine whether a fossil bone that
appears to be hollow was in life filled with air?  

A reduction in cortical thickness to produce a tube-like structure could
have been used as a strengthening and/or weight reduction strategy, but
the cavities of  such bones could still have been filled with lipid and not
air.  Clues such as perforations of the bone cortex would seem to be
ambiguous, since these could allow for the passage of blood vessels as
well as small branches of an air sac system.  Comparisons with extant
creatures with hollow bones (birds) is a powerful tool, but not foolproof. 
As a mammalian physiologist interested in the structure of the dinosaur
respiratory system, I would like to know the basis for statements
regarding the presence of pneumocoels in dinosaur bones, because it
makes a big difference in determining what type of ventilatory and gas
exchange structures dinosaurs possessed.  The cavities of air-filled
bones must have connections to the ambient air via an air-sac-like
respiratory system, since air pockets in tissues always collapse or
become fluid-filled without an active process to sustain them.

Any help would be 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