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Ornithischian opisthopuby (was RE: Giant birds)

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Rob Meyerson
> I have thought that the radical difference in hip designs implied
> *two separate* instances of bipedal evolution (I envision several
> protodinosaur groups existing, where only the ornithischians and
> saurischians survived).  Personally, I can't see a saurischian
> evolving an ornithischian hip, even with a truckload of
> generations.

However, if one looks at the (admittedly fragmentary) pelvis of
_Pisanosaurus_, from the Late Triassic Ischigualasto Fm. of Argentina, there
is NO evidence that this ornithischian had a backwards pointing pubis!  So,
not all ornithischians are known to have opisthopuby.

Additionally, the "saurischian" pelvis (in the sense of a propubic pelvis)
IS the primitive condition: everything from _Lagosuchus_ to pseudosuchians
to lepidosaurs to anapsids to synapsids to amphibians have pubes which
either point forward or directly downward (or barely point).  So unless you
envision Ornithischia as a clade of non-Terran aliens (or at least an
independant radiation from fish...), they had to develop from a propubic

Note that basalmost saurischians do NOT have the standard model "saurischian
pelvis" (a _Ceratosaurus_ to which an _Allosaurus_ pubic boot was added)
found in most textbooks.

> There doesn't seem to be any advantage to the "new"
> hip design.

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.

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