Re: Obturator processes

[NJPharris@aol.com]

In a message dated 11/30/99 10:20:46 PM Pacific Standard Time, 
mickey_mortimer@email.msn.com 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 
obturator
>  process.

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 
characters.

--Nick P.