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AGILISAURUS AND OTHER TINY ORNITHISCHIANS THAT GIVE US HEADACHES
Alessandro Marisa wrote:
<< However, I'm a little skeptical with the obturator process of
for certain that the process was as Thulborn figured or Agilisaurus like.
Can you tell me what is the story?>>
I believe this was in some arguments between Dr Thulborn and myself about a
year ago. Lesothosaurus does indeed have an obturator process, but its size
and shape are not entirely known. Sereno and Galton have illustrated it as
being a small, triangular rise and not much like the large flanges seen in
most basal ornithopods. Thulborn has suggested that the distal edge of the
process is broken bone and may have, in fact been a bigger flange.
<<Please tell me if I'm right, but for me the coronoid process of Agilisaurus
is much high than that of in Lesothosaurus.>>
<< And also the palpebral bone is much more similar to that of Dryosaurus or
No it isn't. The palpabral of Agilisaurus is very strange and unlike those
seen in any other ornithischians period. It is long and robust and makes a
fused contact with the postorbital at the rear of the orbit. The palpabral
is a weird weird weird bone. I am not sure what it was used for and no one
else does either...
Peter Galton has made the best attempt at explanation that I can get my hands
on (and believe me I've looked). To paraphrase, the palpabral is in fact in
two parts, one an ossified part and the second a ligament that goes from the
point of the palpabral to the postorbital. This whole palpabral setup seems
to have been there to keep the head wide enough so that the eyes a) could be
big and fit into the head and b) so the eyes wouldn't fall out. This
happened in ornithischians because they frontals, which traditionally do both
of those jobs, have become extremely long and thin in basal forms. WHY this
happened is beyond me.. and in fact seems to have lost "favor" because every
derived ornithischian group lost its palpabrals and widened its frontals
(ceratopians, hadrosaurs, ankylosaurs etc etc etc).
In fact, this whole animal is weird. The cheek teeth are tall and closely
packed and very reminiscent of heterodontosaurids', but the premaxillary
teeth show a very primitive state and the face lacks an upper jaw diastema
which is present in marginos and ornithopods.
George Olshevsjy wrote:
George fails to mention however that this is only his opinion and has yet to
be demonstrated by any study or analysis, and his own arguments are weak and
misrepresent opposing viewpoints.
Thyreophyra IS a monophyletic taxon because of, among other things, an
additional premaxillary tooth (for a total of seven), the addition of two new
superorbital elements (for a total of three), and dermal armour.
George will go on and on about how this last character is bad because it is
basal for the Ornithischia but fails to show where else dermal armour is
found in the Ornithischia, in Sauropoda besides about 3 very derived very
Late Cretaceous titanosaurs and Theropoda besides Ceratosaurus. Until it's
shown that ANY other dinosaurs, especially ornithischians that demonstratibly
belong to any non-thyreophyran group, had dermal armour, the most
parsimonious solution is to have dermal armour a thyreophyran synapomorphy
and a reversal to the basal archosaur condition.