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Re: AGILISAURUS AND OTHER TINY ORNITHISCHIANS THAT GIVE US HEADACHES
In a message dated 5/5/99 10:09:35 PM EST, Tetanurae writes:
<< 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. >>
These are extraordinarily weak characters and I seriously doubt whether
they're synapomorphies for anything. For example, the seventh premax tooth is
seen only in Huayangosaurus among stegosaurians, and the earlier and more
primitive huayangosaurid stegosaurian Emausaurus has fewer than seven, so
seven is derived for Huayangosaurus. Don't recall >any< ankylosaurians with
seven premaxillary teeth (but spinosaurian theropods have seven; does this
mean Huayangosaurus is a spinosaurian?); I'd certainly like to know which
ones these might be, if any. Another thyreophoran synapomorphy is supposedly
a transversely broadened postorbital process of the jugal, but I don't see
this feature in any stegosaurian skull (and good luck trying to find it in
any advanced nodosaurid or ankylosaurid skull!); it seems to be there in
Scelidosaurus, but since I consider Scelidosaurus an akylosaurian but not a
stegosaurian, this has little impact on stegosaurian relationships.
Stegosaurians and ankylosaurians are profoundly different ornithischians,
easily distinguishable even among their most basal forms; besides which
stegosaurians share quite a number of postcranial characters with the
marginocephalian and ornithopod clades to the exclusion of Ankylosauria.
Presence of osteoderms has misled many people, including Peter, into
postulating a closer relationship between stegosaurians and ankylosaurians
than the remainder of their skeletal anatomies warrants. If you get rid of
the dermal armor--which could easily have developed independently in both
groups, if it's not a plesiomorphy--stegosaurians and ankylosaurians aren't
even in the same ballpark.