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> I have long maintained that small ornithischians are probably divisible into 
> small unspecialized ornithopods, small unspecialized marginocephalians, and 
> so forth, but poor quality of remains makes them difficult to separate. 

As noted by Drs Maryanska and Osmolska, pre-Cretaceous 
marginocephalians may be so "unspecialized" as to be unrecognisable as 
basal members of the Marginocephalia rather than as "fabrosaurs" or 
"hypsilophodonts" or whatever.

Take _Psittacosaurus_ - its skull may be highly evolved, but 
its postcranium is quite unspecialised, especially for an 
ornithischian of the Cretaceous.  It wasn't even very cursorial.  
There's a few derived features in the hips, and some psittacosaur 
specimens show ossified tendons; but overall, the postcranial 
elements of _Psittacosaurus_ are more typical of a lower Jurassic