Re: true semilunates & carpometacarpi (was Caenagnathiformes)

["Mickey_Mortimer11" <Mickey_Mortimer11@email.msn.com>]

Ken Kinman wrote-

>      Now, referring to Mickey's statement below, I am glad to see no
mention
> that the "true semilunate" may have arisen twice (although I am aware of
> that possibility, however remote I think it may be).  Unfused carpals in
> Segnosauriformes don't worry me too much, since I don't use the words
fused
> or unfused in my definition (I did that on purpose since fused carpal
> elements occur in some primitive forms that lack true semilunates).  But
it
> occurs to me that perhaps unfused "true semilunates" is another reason to
> keep Segnosauriformes in a clade separate from Caenagnathiformes (in other
> words "enigmosauria" may be paraphyletic, or at least it's holophyly still
> remains unconvincing to me).

Unfused semilunates won't help enigmosaurs be paraphyletic because more
basal forms like Acrocanthosaurus, Sinosauropteryx, Scipionyx, some
Allosaurus and some Syntarsus have them fused.  Of course, Nqwebasaurus
doesn't, but it's a subadult.

>      I'm not sure exactly what Mickey means by non-semilunate form in
> Caudipteryx and Protarchaeopteryx, but assume it is the shape.  So to
> account for that, perhaps the  definition should say: "usually" has a
> distinctive semilunate shape.  I listed the size criterion first, since
the
> relatively large size seems to be a little more diagnostic than shape
> anyway.

Based on Padian et al. (2001), who report the distal carpal 1+2 of
Protarchaeopteryx is lozenge-shaped and that of Caudupteryx is "not as
distinctly semilunate as Archaeopteryx and Deinonychus".

>      What *does* concern me is the possibility that the fused carpals
found
> in some non-maniraptors could be mistaken for one of these fused "true"
> semilunates (i.e. carpometacarpi).  Is this a potential problem (I am not
> aware of any known forms that would give us such problems at the present
> time)?

The carpals don't extend to cover almost all of metacarpal II in the
non-maniraptorans where they are fused (eg. Acrocanthosaurus,
Sinosauropteryx, Scipionyx, Gallimimus).  So confusion at this point seems
unlikely, but once we find some basal maniraptoran with a semilunate that
extends _almost_ across metacarpal II, there will be trouble.
Also, just because distal carpals I and II are fused to form a "true"
semilunate does NOT mean a carpometacarpus is present.  The latter is the
term for metacarpals being fused together and to the semilunate.

> distal carpal structure which (1) is enlarged so as to fully cap
> metacarpals I and II; (2) usually has a distinctive semilunate shape; and
> (3) has a pronounced trochlear groove on the proximal side.

I'd be weary of saying "fully caps metacarpal I", as this is not the case in
Deinonychus or Alxasaurus for example, where the proximal end of that
metacarpal actually helps form the trochlear surface.

Mickey Mortimer