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Dear All,
Tracy asked me to forward the post (down below) to the whole list.
I was a little surprised at first, but I guess it is a question that should be considered. I'm not sure why stegosaurs in particular might be classified outside of an ornithischian clade, but if they were, I think ankylosaurs would probably have to be classified separately as well.
I'm not sure, but are you suggesting that the predentary might have evolved independently in two or more lines (stemming from different groups of primitive dinosauromorphs? I've never really considered that possibility----I guess it is possible, but doesn't seem very parsimonious. But then again, before last weekend I knew very little about ornithischian phylogeny, so I am a relative "newbie" on this subject. Will have to roll the idea of a "polphyletic ornithischia" around in my head. But if it is polyphyletic, that is kind of scarey. If the interrelationships of just three major groups of dinosaurs is problematic, think how messy it would get if we had even more independent groups to consider.
-----Cheers, Ken
From: "Tracy Ford" <dino.hunter@home.com>
To: <kinman@hotmail.com>
Date: Wed, 13 Jun 2001 13:20:25 -0700

-----Original Message-----
From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of Ken
Sent: Wednesday, June 13, 2001 6:01 AM
To: dinosaur@usc.edu

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....

Why does Stegosaurus HAVE to be an ornithischian? Just look at the diversity
of life today. Stegosaurs, and other dinosaur groups for that matter, may
not be as closely related as is thought. We have hundreds of million's of
years of evolution and trying to classify them. And they all may not be as
closely related.
Tracy L. Ford
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