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Re: Therizinosauroid apomorphies



>Anyone have any good (read: demonstrable) apomorphies for
>Therizinosauroidea?  The abbreviated metatarsals might be one, but I can't
>accept the downturned dentary.  Most other characters seem homoplasious at
>best or characteristic of larger clades.  It seems that the best character
>to support therizinosauroid monophyly is "fragmentary skeleton."
>

Some possible synapomorphies:

        -obturator process contacting or fused to pubis (enigmosaurus,
segnosaurus, probably Nanshiungosaurus)
        -dorsoventrally expanded, laterally directed anterior iliac blade
(nanshiungosaurus, segnosaurus)
        -first metatarsal with a proximal concavity where it articulates
with the second metatarsal (Erlikosaurus, Segnosaurus)
        -spatulate teeth (Erlikosaurus, Segnosaurus, Alxasaurus)
        -pubic boot long anteriorly, short posteriorly (Enigmosaurus,
segnosaurus, also in oviraptoridae, caenagnathidae, avimimus)
        -pedal digit II most robust in pes (erlikosaurus, alxasaurus, also
dromaeosaurs, confuciusornis)
        -very strong lateral compression of pedal unguals (approached in
caenagnathidae)
        -manual ungual II longer than ph II-2 or MC II (Therizinosaurus,
Beipausaurus, ?Alxasaurus)
        -extreme expansion of distal humerus (Erlikosaurus, Therizinosaurus)
        -radial and ulnar condyles of humerus not offset from each other
(unlike in, e.g. birds or dromaeosaurs). (Therizinosaurus, Erlikosaurus).
        -astragalar condyles do not completely cover end of tibia
(Segnosaurus, Beipausaurus)
        -extremely tall anterior dorsals (height from top of centrum to
zygapophyses exceeds centrum height)(Alxasaurus, Nanshiungosaurus)
        -blocky pedal phalanges (Alxasaurus, Erlikosaurus)
        -short, robust manual phalanges and metacarpals (Alxasaurus,
Therizinosaurus)
        -edentulous dentary tip (Erlikosaurus, Segnosaurus)
        -prominent rugosity just dorsal to the posterior tip of the iliac
blade (Segnosaurus, Alxasaurus)


I seem to recall that there is a partial skull for Segnosaurus, showing the
same inflated basisphenoid of Erlikosaurus.


but the issue goes beyond simply trying to find apomorphies linking the
individuals to each other, it also centers around finding the phylogenetic
position of the individual taxa. For example, if we had a dozen features
linking them to each other, but the rest of the anatomy threw them up and
down Coelurosauria, this would complicate the argument for monophyly. If we
had just a half-dozen features linking them, but extremely strong support
for putting all the individual taxa in maniraptora and near
oviraptorosauria, this would improve the monophyly argument. In my opinion,
we have something like the latter. The skull of Erlikosaurus shows quite a
few oviraptorosaurian features (medially short nasals, wide, closely
appressed pterygoids, retracted nares, fused parietals, ectopterygoids
lateral to palatines), and you can find maniraptoran features in the
humerus (cranial distal humeral condyles, separated humeral head), and the
pes (elongate fourth digit).  Segnosaurus preserves  a distally placed
obturator arguing for maniraptoran status, along with the lack of a
squared-off posterior iliac blade. The pubic boot as noted above insinuates
oviraptorosaur status. Meanwhile, the neck vertebrae of therizinosaurs have
double pleurocoels like caenagnathids. The manual unguals of therizinosaurs
and alxasaurus are more like those of Maniraptora than anything else.
Russell has argued that therizinosaurs lack a supracetabular crest, which
would imply that they fit pretty high up the tree, with things like
oviraptors, dromaeosaurs, and birds, as does their trochanteric crest;
therizinosaurs also have a carpus which is derived from a maniraptoran
semilunate carpal although they may have lost fusion between the distal
carpals (which could constitute another therizinosaur synapomorphy).
        So even if the apomorphy list isn't overwhelming by itself due to
its spottiness, the rest of the anatomy is shoving them into Maniraptora
and in particular towards Oviraptorosauria- largely confined to the same
branch of the tree, they pretty much have nowhere to go except with each
other. At least that is how I interpret it.

        -N