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In a message dated 5/5/99 5:20:34 AM EST, darren.naish@port.ac.uk writes:

<< We should expect a major revision in our understanding of small 
 so-called ornithopods over the next decade. I am wondering when 
 someone will show that marginocephalians fall into a late Jurassic 
 clade of euornithopods:) Unless their precursors really are the 
 heterodontosaurs. >>

One small problem with heterodontosaurids being exact precursors of 
marginocephalians is that in Heterodontosaurus the posterior premaxillary 
process apparently extends all the way to the lacrimal, cutting off the nasal 
from the maxilla, as in most ceratopians and in most ornithopods (except, 
e.g., Hypsilophodon, where it >almost< reaches the lacrimal, and 
Tenontosaurus, where it's not even close), but not as in pachycephalosaurians 
or more primitive ornithischians (e.g., Lesothosaurus, stegosaurians, 
ankylosaurians). It would be nice to see more variability in ths character.

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. The 
story is far from simple, and may ultimately require discarding 
Marginocephalia as a useful group. (Thyreophora is synonymous with 
Ankylosauria and as far as I'm concerned can already be discarded.)