hollow bones (revisited)

[Phillip Bigelow <n8010095@henson.cc.wwu.edu>]

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
>meant.

  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?