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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 
Lesothosaurus,
 [snip]
 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.>>

It is.

<< And also the palpebral bone is much more similar to that of Dryosaurus or
 no? >>

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.

Peter Buchholz
Tetanurae@aol.com