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In a message dated 11/29/99 2:09:58 PM Pacific Standard Time,
> So what do you think makes the carnosaur
> obturator nonhomologous?
Well, if _Monolophosaurus_ is a basal carnosaur, rather than a basal
tetanuran (and I'd be pretty hard-pressed to tell you which is the case),
then the carnosaur and coelurosaur ob. procs. are convergent, since _Mono_
displays a full ischiadic plate, pierced by a foramen.
The two types of ob. procs. are very different. Carnosaur ischia are
straight and rodlike, though the end may hook slightly as in sinraptorids or
flare slightly as in _Allosaurus_, _Afrovenator_, and _Giganotosaurus_. The
obturator process (actually, for carnosaurs I think the term "obturator
flange" fits better) is a little tab of bone, rectangular in _Sinraptor_,
trapezoidal in _Allosaurus_ and _Afrovenator_, and described as "lobulate" in
_Giganotosaurus_. The proximal end of the flange is bounded by the embayment
left by the opening of the ischiadic foramen, and the distal end is separated
from the main body of the ischium by a small notch.
Coelurosaur ischia, on the other hand, are much broader and more platelike,
though the distal end is drawn out into a long rod in _Ornitholestes_,
tyrannosaurs, and ornithomimids. The tip of the ischium usually points
backwards, rather than down, though some tyrannosaur and ornithomimid
specimens show this better than others. The ob. proc. is a large hook, and
the ventral edge is smoothly confluent with the main body of the ischium.
This sounds very complicated, and it would be much easier to explain with
figures, but to make a long story short, a carnosaur ob. proc. looks like it
is *attached to* the side of the ischium, while a coelurosaur ob. proc. looks
more like it is *part of* the ischium.
This is one of the reasons why I favor interpreting _Siamotyrannus_ as a
carnosaur, rather than a coelurosaur. While the ob. proc. in that specimen
has been lost, it looks like it would have been a tab attached to the ventral
side of the ischium, rather than part of an expanded, platelike proximal
portion of the ischium.
Hoping that made any sense at all,