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Re: Megaraptor Claw

Waylon Rowley wrote-

> I don't know the exact shape of the ulna, but if it is
> posteriorly bowed, that would be a strong indication
> of at least maniraptoran affinities. The manual
> phalanx is interpreted as I-1. If this is spinosaur
> material, then it should be a large easily diagnosed
> element -  phalanx length equal to 300% the depth at
> its proximal articulation. What would be helpful is if
> I knew the morphology of the distal metatarsal III
> (anyone care to volunteer some info?). I REALLY doubt
> someone would confuse a metacarpal for a metatarsal,
> but I suppose it's possible. One last comment on the
> claw. I've heard it sheath tracts/grooves  are
> asymmetric - just like some other dromaeosaurs. I
> don't know if any spinosaurs evolved this
> convergently, but i've never heard about it being
> present in spinosaur unguals. I think there is a good
> case for placing it with dromaeosaurs, but I would
> like to hear what HP Mickey Mortimer has to say about
> it since he has such a keen interest in these
> critters. I'm more of a carnosaur/basal tetanuran
> buff.....

Well, I suppose I'll elaborate then. :-) The ulna is not posteriorly bowed,
unlike most maniraptorans (alvarezsaurids and therizinosauroids being
exceptions).  The facet for the radius appears transversely expanded and
concave, which is a maniraptoran character.  The olecranon isn't nearly as
large as a spinosaurid, although it is much larger than in dromaeosaurids.
Also, Megaraptor has a triangular distal ulna, like dromaeosaurids, whereas
spinosaurids have transversely expanded distal ulnae.  Manual phalanx I-1 is
not known in spinosaurids, but that of the related Torvosaurus is extremely
stout, unlike the elongate phalanges of Megaraptor and dromaeosaurids.
Distal metatarsals of Baryonyx are known, but not illustrated or described
in sufficient detail.  Megaraptor resembles dromaeosaurids and differs from
non-coelurosaurian tetanurans in the ginglymoid distal articulation that
extends further proximally.  Also, the metatarsal narrows proximally as in
most coelurosaurs, but unlike torvosauroids and carnosaurs.  The ungual
resembles dromaeosaurid pedal unguals more than manual unguals.  This is
because it is tapered ventrally in cross section, has a small flexor
tubercle, and has a sheath groove higher on the medial side.  So nearly all
of the evidence suggests Megaraptor is closer to dromaeosaurids than
spinosaurids and that the ungual is from the pes.

> BTW, I got that spinosaur phalanx synapomorphy from
> Sereno, where he states "First manual digit length
> 300% the depth at proximal end." I'm not sure if he
> meant the entire digit or the 1st phalanx or
> ungual....anyone care to clear that up? thanks

It refers to the ungual- and is not a valid spinosaurid synapomorphy.  It's
actually supposed to be a torvosaurid + spinosaurid synapomorphy, but
doesn't show the correct distribution among taxa.  I wrote the following
back in September while discussing Chilantaisaurus-
The torvosaurid + spinosaurid subgroup is diagnosed by "manual ungual I
elongate (three times height of proximal articular end)", which is present
in C. tashuikouensis.  If Sereno's manual ungual length is along the curve
(as it must be for Baryonyx to exhibit the trait, so I assume it's implied),
Chilantaisaurus has a ratio of 3.17, while Baryonyx has a ratio of 3.2 and
Torvosaurus has a ratio of 3.4.  Neither Herrerasaurus, coelophysoids,
Afrovenator or Allosaurus show these proportions, although abelisaur and
carcharodontosaurid unguals are undescribed and I lack an illustration of
"Szechuanosaurus's" unguals.  However, several basal coelurosaurs have the
derived condition (Nqwebasaurus, Dryptosaurus, most ornithomimids), although
others don't (Compsognathus, Scipionyx, tyrannosaurids).  This shows the
character distribution to be more complex than previouly thought.

Mickey Mortimer