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Re: Therizinosaurs



Many thanks to Tom Holtz (or Tim Hiltz, or...) for a lively reply to my post
on segnosaurs (or therizinosauroids).

In a message dated 97-12-04 12:56:39 EST, th81@umail.umd.edu writes (quoting
me, quoting him):

<<
 ><< 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.>>

Except that when segnosaur resemble prosauropods, that's of no consequence!

The point I was making is that in the abbreviated palates of oviraptorosaurs,
everything is close to everything else, everything overlaps with everything
else. So when you find something overlapping something else in segnosaur
palates, it doesn't mean you can use the same feature in oviraptorosaur
palates to support a conclusion of relatedness.

If oviraptorosaurs are related to caenagnathids, which apparently have
(primitively?) long snouts, let's look at caenagnathid palates to see whether
they resemble segnosaur palates. Unfortunately, these don't seem to be
available...
 
<< >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.>>

I've looked at the character matrix in Russell & Dong's paper. What I see are
various patterns of 1's among the 0's, distinctive for each different
theropod group, and a distinctly different pattern of 1's among the 0's for
segnosaurs. Given only 1's and 0's, there is bound to be >some< theropod
group among those listed that has the largest number of 1's matching those of
the segnosaurs. I don't think this is a sufficient condition to demonstrate
phyletic relationship between that theropod group and segnosaurs.
 
<< ><< 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).>>

To me, this suggests the coronoid in oviraptorosaurs is a neomorph.

Unfortunately, I don't have my copy of Clark et al with me, so I can't check
the photos for this feature. I have been going by the illustrations in
_Dinosauria_, which may be out of date, though I think still serviceable. But
check also the photos on p. 235 of _The Complete Dinosaur_ (there's more than
one way to skin a cat here); _Segnosaurus_ mandible shows a clear coronoid
process, but _Erlikosaurus_ doesn't. Unless you consider that little ridge
along the top of the surangular significant.
 
But why am I talking like this? It's better for my case if segnosaurs have
>no< coronoid while oviraptorosaurs do! The point here, I think, is that the
distribution of the coronoid is variable in both groups, and is therefore of
dubious significance to the analysis.

<< ><< 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").>>

Actually, "it is unclear in any illustrations available to me" hits pretty
close to the mark.
 
<< ><< 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.>>

As I've said before, I have yet to see any characters or character suites
that definitely place segnosaurs within Maniraptoriformes, Coelurosauria,
Avetheropoda, Tetanurae, and Theropoda. The "best" we can do are features of
the forelimb, things like big sharp claws, tridactyl manus, semilunate
carpal, straplike scapula, which could be really powerful evidence, except
that the >rest< of the skeleton is quite non-theropod, or is at best only
vaguely theropod--like any other dinosaur skeleton might be.
 
<< >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. >>

Well, I can't argue with this, except to say that so far I've found the new
analyses vague and unconvincing, and that I'm not the only one who feels this
way.