Obturator processes

[NJPharris@aol.com]

In a message dated 11/29/99 2:09:58 PM Pacific Standard Time, 
nrlongri@midway.uchicago.edu writes:

> 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,

Nick P.