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Re: Therizinosauroid apomorphies



Nick Longrich wrote-

> All of these are typical maniraptoran/eumaniraptoran features, absent from
> ornithomimids. There are some others that aren't published yet. To link
> therezinosaurs to ornithomimids, we would have to a) accept all of these
as
> convergent or else reversed in ornithomimids, and b) accept a similar
> amount of homoplasy for features linking oviraptorosaurs and
> therizinosaurs, particularly in the skull. This seems, to me, an
> insurmountable problem.

Yes, I agree.  In all of my phylogenies, therizinosaurs always come out as
maniraptorans.

> Sinosauropteryx shows a number of things that suggest it is more primitive
> than Tyrannosaurus. For one, it's got something like 65+ caudals, while
> tyrannosaurs and all known maniraptoriformes all have ~40 or fewer. For
> another, Sinosauropteryx lacks the anteroposteriorly elongate chevrons
seen
> in tyrannosaurs and maniraptoriformes. Tyrannosaurs also have a fairly
> proximal transition point- maybe 12 transverse processes in the
articulated
> tail I've seen; Sinosauropteryx has transverse processes out past 25 or
so.
> Furthermore, Sinosauropteryx appears to have short, robust hindlimbs and a
> very robust fibula (despite their small size), while tyrannosaurs are
> gracile in a typically coelurosaurian fashion. I'm not sure why there
seems
> to be this big consensus that  Sinosauropteryx is a coelurosaur at all,
> unless it's because it fits our prejudices by being small and fuzzy- there
> seems to be nothing to prevent Sinosauropteryx as now known from being
near
> allosaurs- or even lower down in the tree, along with the spinosaurs and
> torvosaurs? Why can't there be chicken-sized, furry allosaurs?

Ah, yes.  I remember this debate.  The large amount of caudals is probably
an apomorphy, as even Dilophosaurus and Ceratosaurus have less (~45 and ~55
respectively).  Other coelurosaurs like Alxasaurus also seem to lack
expanded chevrons.  Sinosauropteryx is described as having four transverse
processes, with bumps in more distal caudals that could be interpreted as
transverse processes.  Thus, unless you have unpublished information,
Sinosauropteryx has 4(+?) caudals with transverse processes and this feature
cannot be used to argue the phylogenetic position of Sinosauropteryx.  Any
robustness in the hindlimbs would have to be quantified to be useful.  I
believe I listed these features previously to show why Sinosauropteryx is
considered a coelurosaur:

- amphicoelous cervical vertebrae
- slender third metacarpal
- fourth metacarpal absent (convergent in Allosaurus, note Sinraptor has all
four)
- dorsal margin of ilium slopes caudoventrally
- obturator process placed more than 25% down the ischial shaft
- triangular obturator process
- ischium reduced in length compared to pubis

> RE: alvarezsaurs it seems unlikely that they are related to ornithomimids.
> Alvarezsaurs show enormous cervicodorsal hypapophyses, a downcurved
> posterior iliac blade, almost no brevis fossa, a well developed
> trochanter-antitrochanter articulation, phenomenally elongate hindlimbs,
an
> elongate fourth toe, a very proximal transition point, and humeral
condyles
> on the cranial aspect of the humerus. All of which are absent in
> ornithomimids but present in derived maniraptorans.

Yes, but they also have several ornithomimid-like characters (elongate
manual phalanx I-1, so many unserrated teeth, etc.).  They still group with
maniraptorans in my analyses though.

> As for why so damn many things keep turning up as ornithomimid
> relatives in the tree when they may actually not be (alvarezsaurs,
> therizinosaurs, troodontids) ...

For what it's worth, nothing ever shows up as the sister group to
ornithomimds (besides maniraptora, of course) in my cladograms.

> I was of this impression, but I think the double pleurocoels
> actually refer to double central pleurocoels, not to pleurocoels of the
> neural arch- as illustrated  the vertebrae attributed to Avimimus appear
to
> lack double central pleurocoels, although it may have been something that
> varied with ontogeny (if I recall, it can even vary from one side of a
> vertebra to another).

Really, it looks to me to be located on the centrum.  Oh well.  We'll get
the real answer once the redescription is published in the Mesozoic Birds
volume.

Mickey Mortimer