Re: THYREOPHYRA IS REAL

["Allan Edels" <edels@email.msn.com>]

All:

    My understanding was that marginocephalians were the only dinos to have
the marginal shelf overhang at the back of the skull (larger in some,
obviously); and that ceratopians (pro- and neo-) also had a rostral bone on
the front of the skull (where the beak would be attached).  Unless stegosaur
skulls possess either one or both of these characters, how can you align
them so closely with the marginocephalia??

    Allan Edels

-----Original Message-----
From: Tetanurae@aol.com <Tetanurae@aol.com>
To: Dinogeorge@aol.com <Dinogeorge@aol.com>; 102354.2222@compuserve.com
<102354.2222@compuserve.com>
Cc: dinosaur@usc.edu <dinosaur@usc.edu>
Date: Tuesday, July 28, 1998 3:02 AM
Subject: THYREOPHYRA IS REAL


>George Olshevsky wrote:
><< Has nobody ever wondered why stegosaurs and ankylosaurs look so
different,
>if
> they're supposed to be related to each other? I have, and I'm pretty sure
>that
> 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
>other
> 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
>Tetanurae@aol.com
>