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Re: Obturator processes



In a message dated 12/1/1999 6:46:13 AM Pacific Standard Time, 
LOKICORP@compuserve.com writes:

>         I think this is simply because the triangular obturator process has
>  migrated towards the proximal end -- 

This is true as well.  But look at the ischia of at least some tyrannosaur 
specimens, and you will see a broad proximal portion with a prominent ob. 
proc., like a troodont or oviraptorosaur ischium, plus a long, distal rod.

>  it's the triangular shape and proximal
>  position of the process that are derived with respect to the generally
>  trapezoidal-shape and more distal position in more primitive theropods.

Not sure which "more primitive theropods" you mean here.  Carnosaurs (with 
the *possible* exception of _Yangchuanosaurus_) and _Afrovenator_ (?basal 
carnosaur?) have well-defined, proximally-positioned, quadrangular ob. 
flanges, attached to the ischium along one edge only.

_Compsognathus_ has a distally-positioned, broad-based ob. proc., the ventral 
margin of which is confluent with the ventral margin of the ischium.

_Ornitholestes_ has a proximally-positioned, broad-based ob. proc.

Basal tetanurans have no ob. procs. at all--at most, an ischiadic plate 
pierced by a foramen (e.g. _Monolophosaurus_).

--Nick P.

P.S.  I'll try my best to illustrate (if it looks funky, scroll down to the 
monospaced version):

__
__)    |
|__   <-Carnosaur ob. flange (e.g. _Sinraptor_)
    |   |                  /\
    |   |  <-ischium (| proximal, | distal)
    |__|                                 \/

__
    \
__  \_______   
   )_________|  <-ischium (<-proximal, ->distal)

    /\
     |  Coelurosaur ob. proc. (e.g. _Compsognathus_)


 |        
 \  o      <-Basal tetanuran ischiadic plate with foramen (e.g. 
_Monolophosaurus_)
   \     |
    |    |                   /\
    |    |   <-ischium (| proximal, | distal)
    |__|                                   \/


Monospaced version:

__
__)   |
|__   <-Carnosaur ob. flange (e.g. _Sinraptor_)
   |  |
   |  |             /\
   |__|  <-ischium (|proximal, |distal)
                               \/
___
   \
__  \______   
  )________|  <-ischium (<-proximal, ->distal)

    /\
     |  Coelurosaur ob. proc. (e.g. _Compsognathus_)


 |        
 \ o      <-Basal tetanuran ischiadic plate with foramen (e.g. 
_Monolophosaurus_)
  \   |
   |  |
   |  |             /\
   |__|  <-ischium (|proximal, |distal)
                               \/