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Re: Therizinosaurs





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chapters in the _The Complete Dinosaur_, _Encyclopedia of Dinosaurs_, _The Dinosauria_ (although this is a little out-of-date), plus the fairly recent paper in _Nature_ on _Beipiaosaurus_, all cover what makes this group so unique.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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