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<<All that having been said, I have read the above quoted paragraph
several times, and I still cannot follow your logic. According to your
example, it seems that you are trying to demonstrate that pneumaticity 
evolved FOR weight reduction, since you apparently don't need 
pneumaticity to have air sacks (accoring to your example, which I just 
tried to weaken).>>

My point was that air sacs (that is of the pulmonary variety) seem to 
have evolved for a more efficient respiratory system.  Yes, pulmonary 
air sacs do pneumatize bone, this is something that all air sacs 
accomplish, but you cannot say that this ability of pulmonary air sacs 
to pneumatize evolved JUST for lightening weight.  There are far easier 
ways to lighten weight without evolving such a system, such as evolving 
a smaller size, or hollow bones (which need not necessarily have gotten 
that way through excavation by air sacs; some hollow bird bones are 
filled with marrow (Buhler 1992)).

But you are right, we cannot prove that pulmonary air sacs first evolved 
for a better respiratory system.  The only function that air sacs 
(including craniofacial sinuses) may have FIRST evolved for was just to 
pneumatize and remodel bone (read arguments and further expansions in 
Witmer 1997). 

But my original point, which I still stand by, was that you cannot 
ascribe increased pneumatcity in theropodian dinosaurs as a climbing 
adaptation that lightens weight because there are so many functions that 
pneumaticity can achieve.  Pneumaticity may not be the easiest way to 
lighten weight; bone fusion, bone loss, and miniturization are 
(theoretically) easier ways to lighten weight.  

I hope that I have made myself clear... (If I haven't I can restate).

Matt Troutman

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