Re: Placement of Segnosauria

["Jaime A. Headden" <qilongia@yahoo.com>]

David Marjanovic wrote:

<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.>

  My point was that the carpal block has some work to do, sorting out
scenarios and homology. As you note, de Klerk et al. state that Kirky
has exhibited some displacement in the carpus and manus. The close
appression of one element the the distal end of the radius makes it fit
the radiale interpretation for carpals, and thus a proximal carpal. The
second is looser and could thus be both either proximal or distal.
Theropods typically have five carpals, with three proximal and two
distal, with ornithomimosaurs shifting one distally (my conclusion,
synthesized from recent interpretive works on carpals including Chure's
presentation at last SVP, and so I forget if this was mentioned by
someone else -- Holtz may have brought this up, so I will hunt for
those posts myself, but I have some of the papers).

<...as I just wrote, ornithomimosaur carpals are more or less reduced
beyond recognition.>

  Not neccessarily, just very derived from the primitive and norm.
Problem is, no ornithomimosaur exhibits a different morphology except
*Harpymimus,* so an evolutionary sequence is not attainable.
Compsognathids and tyrannosauroids may be consistent, but their carpus
is the coelurosaur norm, from which eumaniraptorans, oviraptorosaurs,
therizinosauroids, and troodontids are derived (and note that compared
to tyrannosauroids and compsognathids, segnosaurs have derived carpal
blocks, with a strongly-developed hinge [tall condyli]; there may be
others, but this depends on the evolutionary placement of
therizinosaurs)

<...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.>

  How? What is your support for a hypothesis that states that known
segnosaur material is non-theropodan?

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

  About what? The ascending process is tall with a laterally directed
flange at least proximally (closer to the hinge of the ankle in this
case) flush with the lateral margin of the tibia, astragalus comprises
well over a fifth the tibial length (nearly 25% -- general
off-the-top-of-my-head estimate, my actual figures are buried
somewhere), distal end of fibula reduced in diameter distal to the
anterior fibular flange and is subsequently much less than 1/4 the
width of the tibia, and greatly reduced in relative diameter, anterior
iliac blade longer than posterior iliac blade, ventral margin of
posterior blade curves ventrally in lateral/medial views, lesser
[=anterior] trochanter flush at the apex with the height of the greater
trochanter and its form is wing-like, tibia with fibular crest, pubic
apron relatively shallow [pubis is incomplete distally, but this
feature is in marked contrast to prosauropods who have very deep aprons
and shallow pelvic canals.], metatarsal I does not contact distal
tarsals, medially flattened proximal ends of metatarsals III-IV, manal
claws very mediolaterally compressed, hinged distal carpal block
shallowly semi-lunate in extensor/flexor aspects, ventral margin of
anterior blade of ilium curves ventrally and the blade itself is deeper
than the posterior blade, even though it is incomplete dorsoventrally
in both ilia, dorsals and cervicals bear lateral depressions,
zygapophyses splayed diagonally relative to the horizontal plane of
each bone, the postzygapophyses expanded distally ... and that's just
the main block, assuming the posited hypothesis of non-homologous
blocks. The diagnosis for *Beipiaosaurus* is simple, and the only
reasonable autapomorphies given (I can find more in text and in
personal observation) are 1) posterior carinae of dentary teeth
unserrate with 2) posteriorly convex margins and bulbous bases, 3)
tooth crowns nearly mesiodistally as long as the crown is high above
the basal constriction [rephrased from Xu et al., 1999: p. 351
{fig.2a,b}; _Nature_ 399: 350-354]. So far, these are limited to the
problematic dnetary, but I doubt prosauropods progressed into the
Cretaceous either, and therizinosauroid affinities are again stronger
if you interpret this form as a theropod; prosauropod, and you would
have extensive postcranial convergences related to carnivory and
increased cursoriality. I will discuss this form more completely in the
near future, including my own observations of autapomorpies. Including
the forlimb material indicates a raptorial form with exceptionally long
forelimbs, long and slender antebrachium and robuts manus nonetheless
longer than the antebrachium. I mean, I could go on.... *Beipiaosaurus*
is a theropod, and a member of the gropup that includes
therizinosauroids [not a therizinosauroid == {*Alxasaurus* + other
forms commonly known as Therizinosauridae sensu Russell and Dong,
1994}, and therefore the term Segnosauria can be resurrected and
defined to include *Bepiaosaurus* as the most inclusive taxon, but
probably only after a more concrete phylogenetic hypothesis is
established; in refering to B, I will use the term segnosaur, in
refering to non-B segnos, therizinosauroid is therefore appropriate]. 

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

  Xu et al., 1999, address the jaw as perhaps too incomplete for
analysis, but it is not likely to be significant as no clear
therizinosaur autapomorphies can be distinguished from it, and the
teeth themselves are more similar to plateosaur teeth anyway and thus
not useful in consideration of phylogeny.

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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