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Re: Therizinosaurs



In a message dated 97-12-02 17:10:57 EST, th81@umail.umd.edu writes:

<< The characters in Dong & Russell which were unique to oviraptorosaurs and
 therizinosauroids among coelurosaurs:>>

Let me go through these one by one...
 
<< Basipterygoid processes abbreviated, fused to pterygoids;>>

>Everything< is abbreviated/fused in oviraptorosaur palates! Good grief!
Besides, the basipterygoid processes are short in prosauropods, so this could
well be a plesiomorphy for segnosaurs.

<< Vomers extend posteriorly to basicranium;>>

The amount of elongation of the vomers in segnosaurs is autapomorphic. In no
theropod (or other non-segnosaur dinosaur) does the vomer actually extend
posteriorly right to the basicranium; if anything, the basicranium might
extend anteriorly to meet the vomers! In segnosaurs, the vomers extend back
way beyond the palatines, which doesn't happen in >any< coelurosaur (or other
non-segnosaur dinosaur). The vomers of _Oviraptor_ are very short, to go with
the snout, and it is only the relative expansion of the basicranium that
brings the basicranium close to them.

<< Palatine fenestra closed;>>

The palatine fenestra is open in _Dromaeosaurus_, _Gallimimus_,
_Deinonychus_, and all other coelurosaurs I've looked at. Except _Oviraptor_,
whose relevant skull bones, as I've noted, are simply too
reduced/vestigialized to be of any use in these kinds of analyses.
_Oviraptor_ does, however, have "choanae" where the fenestrae ought to be.

<< Coronoid ossifications absent;>>

A decent-size coronoid is >present< in _Oviraptor_, and there is a remnant
low coronoid ridge in _Segnosaurus_ and _Erlikosaurus_. So this is not a
character of segnosaurs, but you might anyway want to argue that >presence<
of a low coronoid unites oviraptorosaurs with segnosaurs(!).

<< Ulnar facet of humerus expanded, merges with entepicondyle;>>

I think this character is too vaguely defined to be of any use.

<< Preacetabular ala of ilium greatly expanded vertically;>>

The amount of vertical expansion of the anterior ala of the ilium in
segnosaurs is enormous and is scarcely comparable to that of any theropod.
It's a segnosaur autapomorphy and has nothing to do with theropods.

<< Postacetabular ala of ilium acuminate. >>

The shape of the postacetabular ala of the ilium is also highly apomorphic in
segnosaurs. And anyway, it is acuminate in prosauropods and probably reptiles
in general.

Everyone should go back to reread Greg Paul's 1984 JVP paper on segnosaurs as
transitional forms between prosauropods and ornithischians. There is much
truth there, even if it's not expressed in today's beloved
cladistic/character-scoring terminology:

Paul, G. S., 1984. "The segnosaurian dinosaurs: Relics of the
prosauropod-ornithischian transition?" Journal of Vertebrate Paleontology
4(4): 507-515.