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George Olshevsky wrote:
<< Has nobody ever wondered why stegosaurs and ankylosaurs look so different,
 they're supposed to be related to each other? I have, and I'm pretty sure
 stegosaurs are more closely related to the clade formed by marginocephalians
 and ornithopods than they are to ankylosaurs.>>

I have been curious as to why you think that stegosaurs are closer to cerapods
than to ankylosaurs.  The only support I have ever seen for this is Maryanska
and Osmolska (or M & O?) 1984.  They state that because Ankylosaurs have an
imperforate acetabulum, and all other ornithischians have a perforate
acetabulum, then ankylosaurs are the most plesiomorphic ornithischians.

This is flawed in many ways, most noticibly the fact that the common ancestor
of Dinosauria as a whole probably had a perforate acetabulum (didn't I just
write this last night?).  Additionally, work on basal ornithischians has shown
that Pisanosaurus is actually the most plesiomorphic ornithischian, rather
than some sort of Heterodontosaur because of its primitive ankle structure, as
well as its PROpubic pelvis.

Pisanosaurus has a perforate acetabulum which shows that the imperforate
acetabulum of ankylosaurs is a reversal (de ja vu?), thus effectively
dismantelling Maryanska and Osmolska's argument.

 <<That is, the "clade" Thyreophora
 is an illusion cast by the presence of dermal armor, together with a few
 minor characters,>>

Dermal armour is not the ONLY feature uniting Thyreophyra as a clade.  An
additional feature for instance, is the loss of the obturator process on the
ischium (also seen in marginos and heteros) which is pretty abnormal.

 <<in both of its supposed subclades, and dermal armor occurs
 in many other members of Ornithischia and Sauropoda.>>

I would like to know what other ornithischians are known to have dermal
scutes.  Some workers have been claimed that Hypsilophodon and Thescelosaurus
had scutes, but these have been shown to be artifacts or from crocodiles.

Additionally claiming that ossified dermal armour is primitive for sauropoda
because it is found in* three or four* titanosaurs is HARDLY convincing.  One
would assert that the presence of dermal armour in those few genera would
support a close relationship between them.

Realize that archosaurs do primitively have a single row of dermal
armour/scutes along the back, but only in a few titanosaurs, crocodiles, and
thyreophyran ornithischians does the armour appear elsewhere.

<<_Scutellosaurus_ and
 _Scelidosaurus_ are basal ankylosaurians, not basal thyreophorans; they share
no stegosaur apomorphies that I can think of (can anyone?).>>

A second and third superorbital element.  The palpabral ancestoral to all
ornithischians is the anteriormost element, a second element (convergently
seen in Iguanodon bernessartensis and Maiasaura peeblesorum) posterior to that
one, as well as one wedged between those two and the frontals is seen in
Stegosaurs and ankylosaurs.

Additionally Gargoyleosaurus (Ankylosauridae) and Huayangosaurus (Stegosauria)
have seven premaxillary teeth, which is abnormal for the Ornithischia.
Primitively, Ornithischians have six teeth, and cerapods reduced that to five.
Pawpawsaurus (Nodosauridae) has at least five teeth but part of the
premaxillae are broken, so it can't be known how many teeth it really had.

Peter Buchholz