Re: Therizinosaurs

["Timothy Williams" <twilliams_alpha@hotmail.com>]

Jack Conrad wrote:

> Anyone have any good (read: demonstrable) apomorphies for
> Therizinosauroidea?  The abbreviated metatarsals might be one, but I can't
> accept the downturned dentary.

You're wise not to accept that one.  An anteriorly downturned dentary is 
also seen in _Ornitholestes_, ornithomimosaurs and prosauropods.  As for the 
"abbreviated metatarsals", I wouldn't be too sure of this one.  Sure, 
therizinosaurs have shorter metatarsals than other coelurosaurs, but this is 
probably a reversal - like the tetradactyl pes, not seen in _Beipiaosaurus_ 
(basal therizinosaur) which has a "normal" tridactyl pes.

> Most other characters seem homoplasious at
> best or characteristic of larger clades.

True, some therizinosaur characters are characteristic of sauropodomorphs 
(e.g. foot, pelvis - especially the ilium).  Since therizinosaurs are NOT 
sauropodomorphs but theropods, that makes them very distinctive characters 
for therizinosaurs.
True, some other characters are characteristic of ornithischians (e.g.
mandible, with the lateral "shelf" and inset tooth row - suggestive of
"cheeks"; opisthopubic pelvis, also seen in eumaniraptorans).  Again, this 
makes them very distinctive characters for therizinosaurs among theropods.

There are a whole lot of good apomorphic therizinosaur characters seen in 
the skull and postcranium of these beasties, including morphology of the 
humerus and (as George mentioned) the distinctive, laterally-compressed, 
scythe-like hand-claws.  The relevant chapters in the _The Complete 
Dinosaur_, _Encyclopedia of Dinosaurs_, _The Dinosauria_ (although this is a 
little out-of-date), plus the fairly recent paper in _Nature_ on 
_Beipiaosaurus_, all cover what makes this group so unique.

> It seems that the best character
> to support therizinosauroid monophyly is "fragmentary skeleton."

The _Alxasaurus_ material is good (although it's missing a skull), and a LOT 
of information can be drawn from the known skeletons of _Segnosaurus_ and 
_Erlikosaurus_.  The rest (_Enigmosaurus_, _Nanshiungosaurus_, 
_Beipiaosaurus_, _Therizinosaurus_) are "fragmentary", although 
_Beipiaosaurus_ has provided some excellent insights into therizinosaur 
evolution.  But let's not throw the baby out with the bathwater.  There is 
some pretty decent therizinosaur material floating about.


Tim

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