Re: Placement of Segnosauria

["David Marjanovic" <david.marjanovic@gmx.at>]

> <But in these, the distal carpals 1 + 2 are never
> fused, whereas they are in ALL (neo?)theropods...>
>
>   My two cents though this was in reply to Pete
> Buchholz (sorry, dude :)):

It was. I didn't want to write two mails for one discussion (sometimes
people tell me I'm lazy).

>   Huh? Ornithomimosaurs and tyrannosauroids do seem to
> indicate separate distal carpals 1 + 2, as does Kirky,
> that funky little *Nqwebosaurus.*

Hm. I don't know much about ornithomimosaur hands, except that their carpals
are tremendously reduced. AFAIK, tyrannosauroids have _one_ carpal, which is
a fusion of the semilunate + radiale + ulnare. The original paper of Kirky
says:

"The carpus and manus are well preserved on both sides, exposed in dorsal
view on the right and palmar view on the left (Fig. 4C, D). The carpus
consists of two [!] adjacent [TINY] carpals, both preserved somewhat out of
articulation on the left and right sides. The medial (radial side) carpal is
slightly larger than the lateral (ulnar side) carpal, and is proximodistally
compressed. It has a concave [!] proximal end that presumably capped the
distal radius. The distal surface is convex, presumably fitting into the
transversely concave surface on the proximal end of metacarpal (MC) I. The
lateral carpal is nearly cuboidal and, on the right manus, appears to
contact the lateral side of the distal radius and the medial half of MC II.
    The two carpals are not fused to each other and, as preserved, are not
closely appressed to either the metacarpals or antebrachium [more shortly:
"We don't know whether they are proximal or distal"]. Although the convex
distal surface of the medial carpal may be interpreted as an incipient
trochlea, there appears to be no strong development of either a semilunate
carpal or a fused semilunate carpal block with a well developed distal
trochlea (sensu Gauthier, 1986), as reported in many other coelurosaurians.
However, the two carpals of *Nqwebasaurus* appear [well, look at the figures
if you have them; I could scan them if someone wants them] to be situated in
positions homologous to those forming the semilunate carpal block in other
tetanurans. We therefore tentatively identify tham as distal carpals 1 and 2
(Chure, 1999). Due to its subadult status, it is not known how carpal
morphology, ossification, and fusion would have progressed ontogenetically."

To me, the carpus of *Nqwebasaurus* looks extremely reduced. This may be a
reason to include it in Compsognathidae, but not one to infer much on
homology, as reduction of bones often brings about either fusion (leg of
*Basilosaurus*) or secondary separation.

Chure, Holtz, and
> others have been reevaluating the form of the carpals
> in theropods and such, but lack of fusion is hardly a
> decisive feature anymore. At least as far as the
> presented evidence indicates. Avian-style carpal block
> appears in several forms, with the pulley-like
> proximal hinge and semi-lunate profile, palmar [volar]
> condyle shorter and lower than the extensor [outer]
> condyle, so on, is a feature that also persists
> throughout Theropoda, Sereno and Novas (1995) and
> Sereno (1999) described it for *Herrerasaurus;* Chure
> (1999) has recently described it for *Allosaurus*, the
> condition in ornithomimosaurs, including
> *Deinocheirus* and *Pelecanimimus* is unique as they
> are all flattened discoid "thingies," and the
> condition of fused, giant block covering the proximal
> surfaces of the first and second metacarpals occurs in
> only troodontids, dromaeosaurids sensu stricto [_not_
> including those newbie and wierdie things like
> *Achillobator*, though this region is not preserved],
> oviraptorosaurs, birds, etc. Holtz recently ran a list
> of changes and some papers to follow up here on the
> list earlier this year or last last, sometime around
> the 59th SVP [Denver].

I'll try to get these papers; as I just wrote, ornithomimosaur carpals are
more or less reduced beyond recognition.

> The condition of the manus in
> *Therizinosaurus,* *Alxasaurus,* and *Beipiaosaurus*
> are all very homologous, with short third metacarpals,
> really long fingers, giant claws, no twist-thumb
> (Bakker and Galton, 1974?), dorsoproximal flexor
> "lips" on the claws, identical semi-lunate but unfused
> carpal blocks, first metacarpal appressed to second,
> and so on, that argue for sympatry at least.

Oh, I never doubted that *Therizinosaurus*, *Alxasaurus* and *Beipiaosaurus*
are segnosaurs (sensu Mortimer: Details on Nanshiungosaurus bohlini). What
the description of *Alxasaurus* did in the first place, AFAIK, was that
*Therizinosaurus* is a segnosaur, not that either is a theropod, and this is
what I doubt.

> There's more, and the biggest case for
> *Beipiaosaurus* being a theropod[...] is in the
> foot,

The illustration in the original paper doesn't convince me. I think it's too
ambiguous.

> previously mentioned on this list, thus in the
> archives, and the links Pete Buchholz provided will
> get you to the other threads, I'm sure.

I have looked there, I'm still not convinced.

:-)