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hollow bones (revisited)

jdharris@teal.csn.net (Jerry D. Harris) wrote: 

>        There is a _big_ difference between a hollow bone and a pneumatized
>bone.  Virtually all long bones are hollow; that's just where the marrow
>is.  The point of contention with many dinosaurs is whether or not the
>hollow areas in some bones were pneumatic cavities -- that is, places where
>"air sacs" existed, as in birds.  However, in discussions of theropods, it
>often becomes easier to just say "hollow bones" when "pneumatized bones" is

  Is there any theoretical way to distinguish between the two types of
medulary cavity?  In birds, for instance, which have medulary cavities that
are partially occupied by pneumatized air sacks, the *walls* of the medulary
cavities in the long bones are distinct boundaries, and are smooth.
In many dinosaurs, the medulary cavities have walls that are *transitional*
into a spongy-type of bone, which is presumed to house the marrow.  Some
dinosaurs have no major voids in long-bones at all (neither "marrow voids" nor
"possible pneumatized air sack voids").  This is even more prevalent in some
non-dinosaurian reptiles.  Could "spongy medulary cavities" in some
dinosaurs (particularly the non-theropods) indicate that the bones
are *non*-pneumatized, whereas *distinct* wall boundaries could be suggestive
of air-sack voids? 

  One last point:  field-ID of theropod "scraps" has been based historically
on whether the medularity cavity of the bone is "open" (or in Horner's book,
_The Complete T. rex_, he says that "only theropods have hollow bones <of
the dinosaurs>").  Should this criterium be thrown out?