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Re: Vertebrae of Early Sauropods



> Date: Wed, 24 Mar 2004 17:16:34 -0800
> From: "PATRICK JOHNSON" <mrpatjohnson@msn.com>
> 
> From my meager sources, I've read that the vertebrae of cetiosaurus
> is relatively solid in comparison with "more advanced", larger
> sauropods.  This leads me to suspect that the same can be said for
> other early sauropods, particularly vulcanodon and barapasaurus. And
> then, by backwards extension, it seems to me that the same should
> apply to the prosauropods.
> 
> I'd appreciate it if anyone would confirm or correct me regarding
> this.

Yes, this is pretty much true.  The trend throughout sauropod
evolution is towards progressive lightening of the vertebrae,
especially in the neck.

Intermediate sauropods such as _Camarasaurus_ achieve this by having
"chambered" vertebra, consisting of several large, hollow spaces in a
branching pattern, each of them roughly round, separated by thick
sheets of bone.  This condition is called camerate.

In Brachiosaurs and Titanosaurs, this morphology has changed: instead
of a few large spaces, there are many small ones, not in any
particular pattern, separated by much thinner bone layers and
irregular in shape.  This condition is called camellate.
_Sauroposeidon_ has cervical vertebrae that are a meter and a quarter
in length, but are made of bone which is nowhere thicker than 2mm, and
mostly the thickness of an eggshell.

Diplodocids tend to have intermediate camerate/camellate morphology.
There are two terms for rather subtly differentiated version of this
morphology: polycamerate and procamellate.  One could argue that this
is at least one too many.

The camellate condition is taken to the extreme in the vertebrae of
derived Titanosaurs such as _Saltasaurus_, in which the bone has
essentially the texture of a sponge, permeated by very many tiny
bubbles.  This is called the somphospondylous condition.  The same
thing appears to have evolved independently in _Mamenchisaurus_, so it
may be a general long-neck adaptation, and so less phylogenetically
useful that it would first appear.

The key paper is probably:

        Wedel, Matthew J.  2003.  The Evolution of Vertebral
        Pneumaticity in Sauropod Dinosaurs.  Journal of Vertebrate
        Paleontology 23(2):344-357, June 2003

but you may also want to read

        Wedel, Mathew J.  2003.  Vertebral pneumaticity, air sacs, and
        the physiology of sauropod dinosaurs.  Paleobiology, 29(2),
        2003, pp. 243-255

And especially

        Wedel M.J., Cifelli, R.L. and Sanders, R.K. (2000) Osteology,
        paleobiology, and relationships of the sauropod dinosaur
        Sauroposeidon. Acta Palaeontologica Polinica 45 (4)343-388

which is freely available for download at
        http://www.snomnh.ou.edu/pdf/2000/00-28.pdf

Finally, an observation.  I don't think anyone's stated this formally
yet, but it appears that throughout the camerate -> polycamerate ->
camellate -> somphospondylous progression, the weight-bearing strength
of vertebrae relative to the amount of bone in them does not actually
change very much.  That suggests that the evolutionary advantage of
the more derived morphology may lie elsewhere than in simple
strengthening and lightening.  I don't know where that would be, but
one can imagine that perhaps somphospondylous vertebrae would be less
prone to catastrophic failure when damaged than camerate vertebrae of
similar size and mass.

Hope this helps.

 _/|_    _______________________________________________________________
/o ) \/  Mike Taylor  <mike@indexdata.com>  http://www.miketaylor.org.uk
)_v__/\  "The only way to learn a new programming language is by
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