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definition of dinosaur

I believe I read somewhere (I think it was here) that we should define 
dinosaurs as all the descendants of the most recent common ancestor of 
_Megalosaurus_ and _Iguanodon_.

I can provide that definition to my students (but I will have to follow 
up with an explanation as to how they can recognize a dinosaur when they 
see one).  I predict that these questions will be asked about the 
cladistic definition:

Why are those two genera singled out?  Isn't it true that after all these 
years we still do not have even one complete composite skeleton of 
_Megalosaurus_?  Why not choose _Allosaurus_, instead?

Do we really know when ornithischians branched off (separated?, 
whatever--what would the correct cladistic term be?) from saurischians to 
know that we're not leaving behind something like _Eoraptor_ or a 
herrerasaurian?  Let me rephrase that so no one here will jump on me for 
misunderstanding _Eoraptor_ and herrerasaurs, which has nothing to do 
with the point.  Might we be leaving something behind that many people 
would label as a dinosaur based upon its over-all anatomy (hips, sacrum, 
limbs, feet, shoulder, neck, etc.)?

What is the closest thing we have discovered to that most recent common 
ancestor of _Megalosaurus_ and _Iguanodon_?

As always, I (we) will appreciate any pointers.

Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu