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Re: STEGOSAUR CHEEKS..



In a message dated 95-10-11 18:25:41 EDT, Stang1996@aol.com writes:

>Didn't Greg Paul in a paper a few years ago argue that all 
>Ornithischians (including Lesthosaurs and such) had cheeks that 
>they inhereted from a common phytodinosaurian ancestor with 
>segnosaurs?

Right. Rudimentary cheeks along the rear of the mouth are a possible
synapomorphy uniting advanced prosauropods, segnosaurs, and ornithischians in
a monophyletic group within Phytodinosauria. The cheeks developed fully in
the so-called "genasaurian" ornithischians. There is no evidence of extensive
cheeks in segnosaurs, but there is at least room for the same kinds of cheeks
that may have existed in prosauropods. Needless to say, there is no evidence
for cheeks of any kind within Aves (including Theropoda).

There is similar evidence for a gular pouch in phytodinosaurs generally,
supported by the downwardly declined dentary and prominent chin seen in
sauropods, prosauropods, segnosaurs, and stegosaurs. Only a trace of the
declined dentary remains in other ornithischians. Among theropods,
ornithomimids show a downwardly declined dentary, and in the newly discovered
_Pelecanimimus_ are preserved traces of the pouch itself. Troodontids do not
show this kind of dentary--they have typical theropod dentaries, straight or
upwardly inclined--so I presume it arose apomorphically in ornithomimids and
is not a synapomorphy uniting, say, ornithomimids and segnosaurs.

G.O.