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I wouldn't go so far as to suggest that Thyreophora (sensu lato) is polyphyletic. I have a feeling that it is neither holophyletic nor polyphyletic, but simply paraphyletic (and therefore a natural group for traditional eclecticists). Right now I would say that armor in stegosaurs and ankylosaurs is probably something that is neither pure convergence nor pure parallelism, but something in between (since convergence and parallelism can grade into one another, and distingushing between the two may be next to impossible given an incomplete fossil record, and could be an arbitrary distinction in any case). However, I certainly agree with you that using armor as a synapomorphy is definitely a bad idea. However, if you could find synapomophies for a stegosaur plus ornithopod clade, I would certainly consider them, but I strongly doubt that such a clade exists.
I am presently inclined to classify thyreophorans (sensu lato) as paraphyletic (two separate, but adjacent clades) as follows:
1 Pisanosauridae
2 Lesothosauridae
3 Plesion _Scutellosaurus_
B Aykylosauridae (incl. Scelidosaurus)
4 Plesion Emausaurus
B Stegosauridae (incl. Huayangosaurus)
5 Thescelosauridae
6 Plesion Agilisaurus
7 Plesion Echinodon
8 Heterodontosauridae
...and so on for other ceropodans....
However, I will need a lot more convincing when it comes to transferring Pisanosauridae to the marginocephalian clade. Whether many ankylosaurids are primitively or secondarily in possession imperforate (or semi-perforate) acetabula is something that needs serious discussion. A perforate acetabulum is one of the weakest synapomorphies for a dinosaurian clade in my opinion (it probably happened gradually at least three separate times).
I have little problem accepting a pachycephalosaurian clade containing Heterodontosauridae, Stenopelix, and Pachycephalosauridae (as sister group to a Psittacosaurid-protoceratopsid-ceratopsid clade). However, inserting Pisanosauridae anywhere within this marginocephalian clade will require some very persuasive evidence. :-)
-----Way past my bedtime, Ken
From: Dinogeorge@aol.com
Reply-To: Dinogeorge@aol.com
To: dinosaur@usc.edu
Date: Wed, 13 Jun 2001 00:42:34 EDT

In a message dated 6/10/01 2:42:36 AM EST, Tetanurae@aol.com writes:

<< George Olshevsky insists that the only "major" synapomorphy linking
stegosaurs to ankylosaurs is the presense of armor, which he insists is
all over Orntihischia and Sauropoda. I'll ignore for the time being the
that no other ornithischians have ever been found that have any armor
preserved, as well as the fact that no sauropods, but a few very derived
titanosaurs had any sort of armor. With the sporadic disribution we see
here, it's somewhat similar to insisting that all sauropsids had beaks since
birds and turtles have beaks, and since we don'tm find them elsewhere,
they're just not preserved right, or cartilaginous. >>

I should clarify this position a bit. Right, we only find >armor< (that is,
bony dermal ossifications) sporadically in phytodinosaurs. The point I
haven't made very clearly yet is that phytodinosaurian skin (and indeed most
dinosaurian skin that I've seen and/or read about) has a beaded texture
something like that of Gila monster skin. The dinosaurian skin tubercles
could very easily transform into dermal ossicles and scutes quite
independently (and randomly, as needed) in different dinosaur groups. This
seems to have happened with ankylosaurians and stegosaurians, as well as with
certain titanosaurians. There are also dermal-armor-like structures, namely
the dorsal spines of Diplodocus, known in other sauropod groups. So presence
of dermal armor of itself does not impress me greatly as a character to
support a phyletic relationship. (I have a similar view of the other
characters supposedly supporting a monophyletic Thyreophora.)

>Shape< of the dermal ossifications might be a better character. E.g.,
presence of >keeled scutes< and >spines< in ankylosaurs and stegosaurs might
indeed support a phyletic relationship. But other than that, stego and ankylo
armor is rather different (can't really mistake one for the other, although
spines are similar), hence my conclusion that armor probably developed
independently in those groups, and keeled scutes & spines in both is a
coincidence (convergence). Generally, stegos and ankylos are so different
from each other that their common ancestor must have lived well before
Scutellosaurus (which is a good basal ankylosaurian and has no stego
features). All the dinos called basal thyreophorans these days are actually
basal ankylosaurians (except Emausaurus, which is clearly a huayangosaurid
stego). Right now I still cannot resolve the trichotomy
ankylos+stegos+cerapodans; stegos seem as different from ankylos as they are
from cerapodans. (Indeed, I have doubts about grouping cerapodans =
marginocephalians + ornithopods, so that the trichotomy should really be an
unresolvable tetrachotomy. Could build a decent case for grouping stegos and
ornithopods, for example.
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