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RE: Ornithischian opisthopuby (was RE: Giant birds)



> From: Martin Barnett [mailto:martin.barnett3@virgin.net]
>
> Thomas Holtz writes:
> > In fact, the opisthopuby of ornithischians is commonly supposed to have
> made
> > additional gut space for digesting plants (basic aspect of physiology is
> > that plants take longer to digest than meat).  The new design allows an
> > increase of gut volume without making the critter extra-wide.
> Are there any advantages to the Saurischian hip design?  In the
> early stages
> of hip development are there any fundamental morphologic reasons why an
> ornithischian hip could not be built straight off?

Sort of an odd question when you think about it: are there any advantages
for the primitive condition?

Something we need to distinguish between (as I tried to mention in that
earlier posting): you have to distinguish between "propuby" (the primitive
condition, where the pubis is subvertical or anteriorly oriented) on the one
hand... er, hip, and particular specializations WITHIN saurischians on the
other.  There are all sorts of derived functional features in particular
saurischian pelves (the dolichiliac condition, the deep brevis fossa, the
preacetabular fossa, the particulars of some macronarian pelves mentioned
earlier, etc.).

However, for whatever reason the ancestral condition for tetrapods is
propuby.

> If the answers to both
> these questions are "no", then why (apart from random semi-functioning
> usefulness) aren't we seeing ornithischian hips from day one?

The easy answer (although not as satisfying to rampaging selectionists) is:
because they hadn't evolved it yet.  Regardless of how useful an advantage a
particular adaptation is, only creatures lucky enough to be descended from a
particular variant with that structure will have that structure.  Monstrous
brains deforming the skull shape (as in humans) are a wonderful adaptation,
and would be useful for all sorts of animals, but unless your ancestor (or
you) had developed that trait, you aren't going to have it.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843