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A while back there was the question of pleurocoels serving in part
to lighten bones, notably in sauropods and birds. The face came up that
birds have the same mass of bone to body weight as mammals. Feduccia
addresses this in his book: Yes, this is true. However, the birds do have
light skeletons, they just also have light soft anatomy as well.
If we wanted to judge "lightness" we might do well to look at
strength to weight ratio, in which case bird bones would certainly score
very well. I have a pelican humerus at home, and I can tell you it is one
of the most amazingly beautiful things to balance it on your finger and
feel how amazingly light it is, or look into a broken bird ulna and see
the little /\/\/\ zig zag struts which brace the inside of the bone. But
it has to bear half the weight of a pelican in flight, and they're damn
big birds (but to those of us in the know, smallish dinosaurs).
However, this is certainly not the only function of pleurocoels.
Compsognathus had hollow bones, but was not under pressures like those
faced by the pelican or by something like Camarasaurus.
And interestingly, the ornithischians seem to be without them- at
least, I've never heard of any having them. Since pleurocoels are also
found in pterosaurs and even in some crocs, if I recall, this would seem
to be a secondary loss (unless they are there. anyone know?).