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Tim Williams wrote-
> You're wise not to accept that one. An anteriorly downturned dentary is
> also seen in _Ornitholestes_, ornithomimosaurs and prosauropods. As for
> "abbreviated metatarsals", I wouldn't be too sure of this one. Sure,
> therizinosaurs have shorter metatarsals than other coelurosaurs, but this
> probably a reversal - like the tetradactyl pes, not seen in
> (basal therizinosaur) which has a "normal" tridactyl pes.
An apomorphy doesn't have to be unique to one taxon, just evolved separately
in that taxon. Besides, Proceratosaurus and Pelecanimimus lack this trait,
so it probably evolved separately and would count as a therizinosaur
apomorphy if not for it's presence in some oviraptorosaurs. Again, a
reversal can also be an apomorphy. Because of this, "shortened metatarsals"
may very well be a good therizinosaur apomorphy.
> 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.
I concur, Erlikosaurus's skull is very strange and
oviraptorosaur/troodontid/ornithomimosaur-like, but the skull is unknown in
other therizinosaurs, so it's features cannot be considered apomorphies for
the group. This is the problem with most "therizinosaur" apomorphies, they
are actually only apomorphies of less inclusive therizinosaur groups like
Alxasaurus+Therizinosauridae or Therizinosauridae. Until Beipiaosaurus is
better known or a more basal therizinosaur is described (which should be
soon), the standard pelvic and cranial features, etc. cannot be considered
synapomorphies of the entire clade.