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Ken Kinman wrote:
<<The first five deal with the jaw (and plant chewing) so could have
arisen more than once.  I also wonder how many times the predentary bone may
have arisen, given its presence in at least one Cretaceous bird (EGAD!!!).>>

The ornithischian predentary is very unique among animals and in the
basal-most forms is almost identicle.  It's a base for a lower jaw beak, but
also seems to serve as an accessory joint for the two dentary rami to rotate
somewhat indepentantly of eachother (at least in the most basal forms).

We also can't forget however that all ornithischians are clearly dinosaurs.  
The all have an upright gait, perforate acetabula (lost in some later
ankylosaurs), and the very distinctibe dinosaurian hand where digits 4 and 5
have no claw.

It's very unlikely that stegosaurs, say were really theropods, and also
evolved a strongly opisthopubic pelvis, ornithischian teeth, "cheek"
depressions, and a perdentary.

<<Ornithischian phylogeny seems a whole look shakier than ever imagined,
and a paraphyletic (or even a polyphyletic) Thyreophora seems a real

A paraphyletic Thyreopora could be real, but since there is no evidence to
the contrary yet presented, we have to go with the conventional view that
Thyreophora is monophyletic and excludes all other known ornithischians.

Peter Buchholz