Re: Gasosaurus (was Re: MJ coelurosaurians)

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 01:03 PM 2/11/99 -0800, Nick Pharris wrote:

>_Monolophosaurus_ does not have an obturator flange of any kind, if the
>original description is accurate.  It retains a full ischiadic plate,
>pierced by a foramen.

Quick terminological comment: what _Monolophosaurus_ has is what Charig &
Milner (1997) coined as an "obturator flange" aka an ischiadic plate.  As
named and described (p. 51), the obturator flange is what ceratosaurs
possess, among others.  It is, indeed, pierced by a foramen, as in
_Yangchaunosaurus_ (but not species of _Sinraptor_).

>IMHO, that makes it a basal tetanuran or an
>avetheropod more basal than _Afrovenator_ or spinosaurs.

Don't see why that would put them basal to spinosaurs, which have a
obturator flange.

_Afrovenator_ does indeed have a good obturator process: _Afro._'s position
is somewhat ambiguous at present: it might be just outside of Avetheropoda,
it might be a basal carnosaur, etc.

>As discussed before, published illustrations of the pelvis of
>_Yangchuanosaurus_ are a mess.  I tend to rely more on the pelvis of
>_Sinraptor_ to illustrate sinraptorid characters.

Indeed.  However, from observation, _Yang._ and _Sin._ DO vary from each
other in some conditions, so don't ignore this variation.

>All right.  I'll give you that the quadrangular obturator flange seen in
>carnosaurs may be a basal avetheropod feature, further "whittled down" in
>coelurosaurs into a triangular obturator process.  But that means that
>spinosaurs are most likely basal avetheropods as well (or carnosaurs),
>rather than megalosaur relatives.

Again, I don't see why you say that, as spinosaurs lack an obturator process
(and instead have an obturator flange).  However, it is true that
spinosaurids could well be a basal carnosaur, since relationships among
basal tetanurines are highly unsettled.

>
>> although some (the obturator foramen) are lost in
>> allosaurid carnosaurs, 
>
>and carcharodontosaurids; sinraptorids show an intermediate stage, with
>the obturator foramen largely surrounded by a bony hook.

Under the taxonomy I use (see, for example, the Holtz & Brett-Surman
taxonomy & systematics chapter of _The Complete Dinosaur_), Allosauridae =
all taxa closer to _Allosaurus_ than to _Sinraptor_.  Under this definition,
and by the results of my studies, the "carcharodontosaurids" are a subgroup
of Allosauridae.

The "hook" condition is found only in _Sinraptor dongi_ among sinraptorids:
even _S. hepingensis_ has a closed obturator formamen, as do specimens
(species?) of _Yangchuanosaurus_.  It certainly does look as if _S. dongi_
documents how a true obturator notch forms: transition in action!  It is not
clear, however, that this is part of the same historical evolutionary event
that resulted in the condition in _Allosaurus_ and relatives: it may be
convergence to allosaurids (and coelurosaurs) within Sinraptoridae as well.

Ah, homoplasy...

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661