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RE: An Odd Question on Tyrannosaurs.....



> From: MKIRKALDY@aol.com [mailto:MKIRKALDY@aol.com]
>
> Posted for Chris Brochu:
> _________
>
> Nick Longrich said:
>
> >Brochu showed at the mexico SVP meeting that Lambe's Gorgosaurus
> >"sternum" is actually two fused pairs of gastralia,
>
> A note of caution - I showed that the Albertosaurus "sternum" *might*
> be the anteriormost fused gastralia.  Since I didn't see the
> "sternum" during my last visit to Ottawa, this needs to be
> cross-checked.  My conclusion was based on the fact that a big
> triangular mass of bone is present between the anteriormost gastralia
> in Tyrannosaurus (at least 2 specimens) and Tarbosaurus (at least one
> figured by Maleev), and the bone figured by Lambe *looks* like it
> could be this.  Moreover, other very complete tyrannosaurid skeletons
> lack an unambiguous bony sternum.  But, no firm conclusions have been
> drawn yet.

Since the cat is, as they say, out of the bag, I can add to what I said
before.  The Gorgosaurus "sternum" DOES have the same bone texture
appearance and structure as the middle portion of the anterior gastral
"plate" of Tyrannosaurus, and I think it is indeed the same element.
(Incidentally, the element is folded over in the Gorgosaurus libratus type:
the most commonly seen illustrations of it are restored).

>From this, and from the lack of definite ossified sterna in otherwise well
articulated tyrannosaurids, ornithomimosaurs, allosaurids, ceratosaurs,
etc., I think that the evidence is fairly good at present that none of these
basal forms had ossified their sterna.  (The mass in Pelecanimimus *might*
be the only one of the lot, but even that is a bit questionable now).

So what about the elements in Baryonyx, Sinraptor, Xuanhanosaurus, and
Scipionyx?  More study is needed, especially of the nature of the bone
tissue itself, but my initial hypothesis is that the first two are fused
gastral masses (although there are alternative hypotheses waiting in the
wings).  Rauhut and Dal Sasso & Signore, respectively, have identified the
"sterna" of the other two as midentified other elements: Rauhut's ID comes
out in his forthcoming megagraph on theropod phylogeny, so wait for the gory
details!

                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): 301-405-0796