RE: Psuedo-Arctometatarsaly [the Thai "Ornithomimosaur"] & announcements

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

Having finally dug myself out from under 15" (that's 45 cm) of snow, it's
back to business...

With regards to the Thai ornithomimosaur:

> <<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 warning about interpreting "arctometatarsality": line drawings (and for
that matter, photographs) can be misleading sometimes.  Given that the
metatarsi are not always perfectly flat planes on back (no kidding), and
given that drawings and or photographs of metatarsi tend to be done by lying
specimens down on a bed of sand or other substance, in which they can spread
slightly...  Well, this leads to misinterpretations of the metatarsi of taxa
such as _Garudimimus_ (in which the topmost parts of mtII and mtIV DO
contact on the anterior surface, figures to the contrary notwithstanding)
and the Thai ornithomimosaur (at least for the photos I've seen of it, again
line drawings of the specimen notwithstanding).

I predict, however, that the common ancestor of tyrants and ostrich dinos
had only this slight contact at the top, and the more extensive contact on
the anterior surface did evolve independently in both lines.

Note that, contra statements to the contrary earlier this month (to which I
did not directly reply, having to wade through a few hundred emails that
accumulated when I got back from a couple of trips...), Arctometatarsalia is
not solely supported on metatarsal characters.  Leaving aside the issue that
as a phylogenetically-defined taxa this clade is recognized independantly of
its diagnosis, tyrant dinosaurs and ostrich dinosaurs share numerous
skeletal features not otherwise found in other coelurosaurs (save for some
also found in the ever-problematic troodontids).  For more information on
that, see the forthcoming paper by me in the Theropod Paleobiology issue of
Gaia, which...

[Announcement I] WILL BE AVAILABLE THIS SPRING!!!!  More specific details
soon.

As for Announcement II: The U.S. release of (a modified version of) the
documentary version of Walking with Dinosaurs will be April 16 from 7-10 pm
on The Discovery Channel.  The whole shebang will be shown in a single
sitting.  Unlike the release elsewhere, various talking heads will be
intercalated coming into and out of commerical breaks.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843