Psuedo-Arctometatarsaly [the Thai "Ornithomimosaur"]

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

Tim Williams wrote:

<<On a completely separate topic, a while back
somebody mentioned that the new ornithomimosaur from
Thailand does NOT have an arctometatarsal pes. It
does. Buffetaut and Suteethorn's preliminary
descriptions and figures spell that out.>>

and Mickey Mortimer wrote:
 
<This is only because they use the term
"arctometatarsal" incorrectly. Note that their
description reads "proximally, however, the third
metatarsal of the Thai form is still visible in
anterior view, as a thin sliver of bone between the
proximal ends of metatarsals II and IV". And indeed,
the figure illustrates this well- you can still see
metatarsal III just fine between II and IV in both
proximal and anterior views. Correct me if I'm wrong,
but isn't the arctometatarsalian condition that in
which metatarsals II and IV contact each other
proximally?>

  Among other things, yes, including the sigmoid
curvature of the distal half of mtIII in both tyrants
and mimics, triangular cross-section of mtIII along
most of its length, and reduction of posterior aspect
of mtIII to the distal 1/5 or so. (pretty much from
Holtz, 1994 and 1995)

<The Thai form is pretty close to arctometatarsaly (as
would be expected in an ancestor of truly
arctometatarsalian forms), but isn't quite there yet.>

  A comment on the condition of compression mtIII in
coelurosaurs: aspect of mtIII on the extensor face of
the metatarsus is plesiomorphic for all of
coelurosauria, with dromaeosaurs, "coelurids" (as
defined elsewhere), compsognaths, segnosaurs, and some
ovis not exhibiting the reduction of mtIII on the
flexor face, which is exhibited in tyrants, mimics,
and some ovis. I argue (and elsewhere, check the
archives) that arctometatarsaly should be restricted
to that condition as exhibited by mimics and tyrants;
ovis, dromaeosaurs, and "coelurids" exhibit a similar
condition wherein mtIII is apparent flexorly and
extensorly, with troodonts having a derived condition
that is unique, whether "bullatosaurian" or
"deinonychosaurian." *Harpymimus* exhibits a condition
more similar to oviraptorosaurs et al. than to
arctomet taxa (Barsbold, 1988; Barsbold and Osmolksa,
1990). The condition seen in alvarezsaurs is different
again in that mtIII is contained withing mtII and mtIV
proximally, and is not apparent on the proximal face
(Clark et al., 1994; Novas, 1994a,b); double-checking
shows that the "avimimid" tarsometatarsus is more
similar to oviraptorosaurs (elmisaurs) than to
alvarezsaurs, including the form and arrangement of
the proximal metatarsals, and that mtIII is nearly
completely exposed extensorly and flexorly, as in
*Chirostenotes* and *Elmisaurus* (Kurzanov, 1981;
Currie, 1990; Holtz, 1994); it is very slender and
narrow at midshaft and proximally, which owes the
alvarezsaur/avimimid similarities honor, at least in
this respect.

  Not having seen the paper or the mts themselves of
the Thai form, I have not real take on it, but it
sounds pretty plesiomorphic, as a maniraptoriform.
Phalangeal or other prostcranial evidence would be of
considerable need here.

<Add this to Harpymimus and you've got fairly good
evidence of the ornithomimids developing
arctometatarsaly independently of other coelurosaurs.>

  I, among others, am under the impression that Harpy
is not an ornithomimosaur, for simple reason that the
pes, manus, arm, etc. do not exhibit any apparent
ornithomimosaur features. Barsbold (1988, pers. comm.
1999) disagrees.

  There is very little reason to doubt
arctometatarsaly evolved once, as the Arctometarsalia,
with troodonts closer to dromaeosaurs than to
ornithomimosaurs (arguments for the placement of
Troodontidae et al. notwithstanding [Norell et al., in
press; Barsbold, Maryanska, and Osmolska, 1990; Dong
and Russell, 1993]). That there is reason to doubt it
means only that there will be more publications
clarifying the evolution of the condition and the
phylogeny therewith. Heads up, Tom!


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

"Come the path that leads us to our fortune."

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