From: jjh00@outs.ccc.amdahl.com (Joel J. Hanes)
Newsgroups: talk.origins
Subject: Gastralia (WAS Re: Keeping time twice a day
Summary: Some large dinosaurs had abdominal stiffeners, maybe
Date: 3 Sep 92 21:50:38 GMT
Reply-To: jjh00@OUTS.ccc.amdahl.com (Joel J. Hanes)
Organization: Amdahl Corporation, Sunnyvale CA

>>  rh@ishmael.UUCP (Richard Harter) wrote (anent Ted, I think):
>>>
>>> But the whole sauropod proposition is an interesting one.  I suspect
>>> old bronty didn't lie down unless she had to -- the legs could support
>>> her weight but that big belly couldn't.


>  I responded:
>
>> Are you aware that many dinosaur species had a whole set of
>> bones, "belly ribs", running down the abdomen between the
>> sternum and the pubis?


  Richard replies:
>
>If I'm not mistaken, the belly rib critters were nodosaurs and ankylsaurs
>and the like -- low slung, broad bodied, and armored.  I don't think that
>sauropods or theropods did.


Sorry it took me so long to reply -- I had to go dig the relevant
book out of the boxes ... I'm referring to _Dinosaurs_ by David
Norman, Crescent Books/Crown Publishers 1987, ISBN 0-517-65263-3
a large-format book, with a double-page spread for most major
families of dinosaurs; plausible in-life reconstructions on the
upper page, with line drawings of fossil skeletons and fairly
detailed explanatory text on the lower facing page.


Anyway, Richard is right about the diplodocids -- neither
Apatosaurus ("Brontosaurus") nor Diplodocus has the
"belly ribs", which are more properly called gastralia.
On the other hand, both skeletons are remarkable for the
degree to which the barrel of the rib cage encloses the
abdomen, and for the strong, downward-projecting pubis.
Perhaps the shoulder girdle, the normal ribs, and the pubis
together bore the weight of the recumbent animal -- perhaps not.
Of the other sauropods, neither camarasaurids
nor brachiosaurids have gastralia.

However, gastralia seem to be common among the saurischian
family, to which the sauropods belong.  The 10-meter prosauropod
Plateosaurus from the early Jurassic has gastralia, (and also has
much more delicate proper ribs than Apatosaurus).
Among the large bipedal saurischian meat-eaters, Tyrannosaurus and
Albertosaurus have many short gastralia in a double row that runs
all the way from sternum to the pubis, but the theropod carnosaur
Allosaurus does not have them, nor does the smaller (3-4 meter)
dromaeosaur, Deinonychus.

The gracile small (and toothless) biped coelurosaur Struthiomimus
has long gastralia, apparently bunched up near the middle of
the abdomen -- over the "solar plexus".  Next to an enlarged
drawing of these, Norman says:

  "Belly ribs, or gastralia, are found in various reptile
   groups, although they are absent from mammals.  They are
   not formed in the same way as vertebral ribs, and should
   not strictly be called by the same name.  No one really
   knows what their function is, but there have been several
   suggestions.  They may help to support the abdomen,
   especially important in an herbivore with a large gut."

Why he chooses to place this paragraph on a page of small,
non-herbivorous animals I don't know -- perhaps the gastralia
are particularly well-preserved in the type specimen of
Struthiomimus -- in the associated drawing, they are
disordered as if drawn from the specimen _in_situ_.


Among the ornithischia, the pachycephalosaur Stegoceras
shows well-developed gastralia in graduated sizes, the
smallest up by the shoulder girdle, and the largest
deep in the belly.


None of the low-slung dinosaurs illustrated (ceratopsians
like Triceratopsis; Stegosaurus, nodosaurs, Ankylosaurus)
are shown with gastralia.


Didactically

---
Joel Hanes