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Re: Therizinosaurs



At 07:56 PM 12/2/97 -0500, Dinogeorge wrote:
><< The characters in Dong & Russell which were unique to oviraptorosaurs and
> therizinosauroids among coelurosaurs:>>
>
>Let me go through these one by one...

Well, I'm not going to defend all of Dong & Russell's characters (I may not
entirely agree with them).
 
><< Basipterygoid processes abbreviated, fused to pterygoids;>>
>
>>Everything< is abbreviated/fused in oviraptorosaur palates! Good grief!

But not so in the long-snouted therizinosauroids.  A resemblance is still a
resemblance.

>Besides, the basipterygoid processes are short in prosauropods, so this could
>well be a plesiomorphy for segnosaurs.

It could well be.  However, the distribution of ALL the characters suggest
otherwise, even in cladograms (such as Dong & Russell's, or my new one)
which include prosauropods to help sort things out.

><< Coronoid ossifications absent;>>
>
>A decent-size coronoid is >present< in _Oviraptor_, and there is a remnant
>low coronoid ridge in _Segnosaurus_ and _Erlikosaurus_.

Clark et al. cannot find any evidence of a coronoid in their examiniation of
_Erlicosaurus_, and the photographs do not reveal any.  Earlier line
drawings show something there, but the photos of the specimen do not.  I
have not personally seen the skull of _Segnosaurus_, and photographs are not
currently available.

As for oviraptorosaurs, the coronoid cannot be found in any caenagnathid jaw
(it may be fused with the other bones, however).  Although there is a great
coronoid process in the jaws of _Oviraptor_ and other oviraptorids, the
coronoid bone is difficult to resolve as a seperate ossification (even in
young individuals, like the "_Ingenia_" on tour with the Russian Dinos exhibit).

><< Ulnar facet of humerus expanded, merges with entepicondyle;>>
>
>I think this character is too vaguely defined to be of any use.

In other words, it is present and supports this hypothesis? :-)  (The
primitive condition in D&R's paper is "ulnar facet on humerus small or absent").

><< 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.

With the two above characters, oviraptorosaurs (including _Microvenator_)
are the only non-therizinosauroid theropods which approach the (admittedly)
more derived condition in therizinosauroids.  And, with the latter feature,
it indeed is probably a reversal of sorts (as is the lack of a
well-developed intramandibular joint).  However, the presence of these
particular conditions AS WELL AS the derived features which place both
oviraptorosaurs and therizinosauroids within Maniraptoriformes, and
Coelurosauria, and Avetheropoda, and Tetanurae, and Theropoda wind up
uniting these two groups.

>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.
>
Greg's article is not rejected because it used a some-what different
(although essentially cladistic) character scoring.  In fact, a few
cladistic analyses following that paper supported a sauropodomorph position:
Gauthier, 1986 and the unpublished paper presented by Sereno around 1989.
This is why Weishampel, Dodson, & Osmolska have "segnosaurs" outside of
Theropoda in _The Dinosauria_, and (following that) why most recent books
(Fastovsky & Weishampel, Farlow & Brett-Surman) give these beasties their
own chapter.

The reason that therizinosauroids are now considered theropods comes from
new specimens, new data sets, and new analyses.  It could well be that
additional data will place them back into Sauropodomorpha.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661