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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.