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Re: Obturator processes
In a message dated 11/30/99 10:20:46 PM Pacific Standard Time,
firstname.lastname@example.org writes (off-list, but I hope it's OK if I send
it to everyone):
> This is one possibility, but Yangchuanosaurus also has an ischial foramen
> and is an allosauroid. The presence of an obturator process in Allosaurus
> and Sinraptor would suggest the ischial foramen can evolve from an
Well..._Yangchuanosaurus_ has been *restored* with an ischial foramen, but
the restored pelvic area looks quite different from photos I've seen of the
type of _Y. shangyouensis_. And if you compare the structures at the base of
the ischium in _Yangchuanosaurus_ and _Sinraptor_, they don't match up.
Where _S. dongi_ shows a beautiful obturator flange, the restored _Y.
shangyouensis_ shows only a smooth, curving ventral margin on the ischium.
Also, the restoration of the _Yangchuanosaurus_ ischium indicates that the
bone bridge across the ischiadic "foramen" in _Y._ is incomplete--thus,
particularly after comparison with the very closely related _Sinraptor_, it
may be that the bridge was not there at all. This may also apply to the bone
bridge shown as closing off the pubic foramen. _Sinraptor_ shows an
incomplete bridge, and the bridge looks incomplete in photos of _Y._ as well.
When _Yangchuanosaurus_ was described, the beautifully preserved type
material of _Sinraptor_ still had years to go before it was even discovered.
My guess is that the ischium of _Yangchuanosaurus_ was very much like that of
_Sinraptor_ and that the obturator flange was either lost entirely in this
species, at least partially broken away, or misinterpreted by the illustrator
(Dong?). I prefer to use _Sinraptor_ when examining allosauroid pelvic