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Steve Brusatte wrote-

> >Otherwise, though continuously suggested as stem-marginocephalians,...
> I know this idea has been thrown around a lot on the list, and is favored
in _Dinosaurs of the Isle of Wight_, but has there been one solid cladistic
analysis that has shown it?

The question is "has there been a solid cladistic analysis that has shown
heterodontosaurids are ornithopods?".  I think the only ornithischian
phylogenetic analyses with this conclusion have been Sereno's, which did not
test a marginocephalian position for heterodontosaurids, as they left out
potential synapomorphies.

Jaime Headden wrote-

> >...heterodontosaurs have also been allied to ornithopods upon nearly one
> >feature, the contact between premaxilla and lachrymal. This has been
> >Sereno's contribution to the group.

Sereno (1986) gave four synapomorphies linking heterodontosaurids with
1. premaxillary tooth row below maxillary tooth row.
This is absent in Bugenasaura, Jeholosaurus and Zephyrosaurus, but is
present in pachycephalosaurs and Liaoceratops.
2. paroccipital processes strongly pendent.
3. ventral tip of quadrate extends below level of maxilla.
Absent in "Agilisaurus" multidens, but present in Scelidosaurus,
Huayangosaurus, pachycephalosaurs, Psittacosaurus and Liaoceratops.
4. premaxilla contacts lacrimal.
Absent in Agilisaurus, "Agilisaurus" multidens, Jeholosaurus Hypsilophodon,
Tenontosaurus and Parksosaurus, but present in Psittacosaurus.
I included 1, 2 and 3 in my ornithischian phylogenetic analysis (
http://www.cmnh.org/dinoarch/2001Aug/msg00405.html ) , but placing
heterodontosaurids in the Ornithopoda takes 4 more steps than placing them
as stem marginocephalians.  In most of those constrained trees,
heterodontosaurids actually come out as iguanodonts!

Mickey Mortimer