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SOME ORNITHISCHIAN MUSINGS
Jaime had an hypothesis about ornithischian palpabrals coming in four
distinct styles, which I don't think is really the case. If you look at a
basal ornithischian skull in dorsal view, you see that the frontals are very
narrow and that the palpabrals mirror the outline (kind of) of the J/QJ bar.
This condition also is seen in basal ceratopians (although many people think
that Protoceratops etc didn't have PAPs), though the point of PRF/PAP contact
has migrated dorsally in the orbit.
The eyes in basal ornithischians were probably very large as demonstrated by
the gigantic scleritic rings found in Hypsilophodon and others. They may or
may not have been spherical, but if they weren't and the eyelids attached to
the frontals, they would have been shaped like sanddollars based on the width
of the head and the scleritic diameter.
As for stegosaurs..... I have a feeling that the super-orbital roofing is
just a PAP arch being completely ossfied. SO1 would be the palpabral and SO2
and SO3 would be different ligaments, perhaps being homologous to two
seperate ocular ligaments in basal taxa.
These same three bones are also found in basal ankylosaurs and, along with 7
PMX teeth, is a good synapomorphy linking the groups in a monophyletic
This is markedly different from the condition in Agilisaurus louderbacki,
where there is a single PAP element that just goes all the way back to
contact the PO. Dryosaurus altus *may* also have this condition according to
some of Galton's drawings, but I am not sure. Some specimens of Iguanodon
bernessartensis also seem to have a second PAP element floating in space
This animal is based on some ornithischian leg bones from, I think, the Hell
Creek FM. They may or may not belong to Bugenasaura or Stygomoloch, which
are both sympatric leg-less ornithischians of the right size to own the legs.