Gorgosaurus (was RE: Daspletosaurus temporal/stratigraphic range)

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

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> henry mendoza
>
> Hey Dr. Holtz,
>
> What is he current status of the genus Gorgosaurus.
> Carr continued to lump it in with Albertosaurus,
> apparently just accepting Russell's analysis, but not
> really elaborating on why. Yet, I have heard
> hints/comments throughout the last few years from
> Currie and you that it is distinct from Albertosaurus.

Okay, here's the skinny:
Nearly all the phylogenetic analysis of the tyrants I know of converge on A.
sarcophagus and G. libratus as sister taxa.  That is, they do share derived
features not found in other tyrannosaurids.

So of course we can now whip out our genericometers and...  Oh, yeah.
Right, they don't exist...  That being the case, it is as much an aesthetic
decision as anything else as to whether to include two sister taxa in the
same genus or to place them in separate genera.  Some very smart guys (Carr,
Williamson, Brochu, Russell, Paul) place sarcophagus and libratus in the
oldest valid available genus name, Albertosaurus; other very smart guys
(Currie, Hurum, Bakker, moi) feel more comfortable placing them in separate
genera.  And I mean that last part seriously: "*feel* more comfortable".
These decisions are not based on science; they are bookkeeping decisions.

If, on the other hand, (WARNING WARNING Entirely hypothetical situation not
to be taken seriously WARNING WARNING) someone demostrated that libratus was
closer to, say, Daspletosaurus than to Albertosaurus (End Warning), then
most would agree that, in order to keep genera monophyletic, "Gorgosaurus"
would have to be retained.

> While we are at it. Any comments on the
> Tarbosaurus/Tyrannosaurus generic problem too?

At a genericometer reading of XX.III, I find that... it currently pleases me
to include them in separate genera.  However, I have previously kept them in
the same genus.  They do seem to share a LOT of derived features, but as
Brochu has rightly cautioned, these may be size-related.

> My only concern morphologically would be the odd
> looking scapulocoracoid of one of the Tarbo skeletons.
> Namely the larger of the two skeletons displayed in
> the Great Russian Dinosaur Exhibit. The coracoid looks
> very different from any tyrannosaurid specimens I have
> seen. I figured it was just a restoration error. As it
> is obvious in the illustrations of the russian
> monograph that the coracoid was incomplete (but then
> why not restore it like all the others?). Any comments
> on that?

Without specimen numbers or photos I don't have much to go on there.  One
possibility: the coracoid was restored back in the old days and/or by people
unfamiliar with tyrant anatomy.  Furthermore, restorations can often be very
off (the foot of the AMNH T. rex, which is just a cast of the incorrectly
restored type specimen, now at Carnegie; the hand of the type of
Shantungosaurus; etc.) but because they were done on very large specimens
the museums don't feel that the effort to fix the restoration is worth the
expense (and possible damage to the specimen).

                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