RE: Big theropod olympics, part LXIV

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

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Larry Dunn
> Sent: Tuesday, August 31, 1999 10:42 PM
> To: dinosaur@usc.edu
> Subject: Big theropod olympics, part LXIV
>
>
> Various people wrote:
>
> > Rex is wider, bulkier, and proportionately
> > stronger in form and specifically in limbs and neck,
> > than Gigi.
>
> and
>
> > the T-rex is much bigger and really more powerfull
> > than the giganotosaurus,
>
> Where is this information coming from?  From what I
> have been able to gather, known specimens of
> _Giganotosaurus carolinii_ are not only longer but
> also *bulkier* than those of _Tyrannosaurus rex_.
>
> All I've been able to gather is that the skulls and
> teeth of tyrannosaurids were more robust than those of
> carcharodontosaurids.
>
> So what have I missed?  Is there a theropod specialist
> in the house?

Present.

To try and put this briefly:
The _Giganotosaurus_ limb material is bulkier than those of _Tyrannosaurus_,
and the femur of the former is longer than any present specimen of _T. rex_
by a few cms.  Femora seem to be the best predictor of (estimated) body mass
in nonavian dinosaurs, whereas distal limb elements can be more elongated or
more stout (presumably due to selective pressures for different locomotory
regimes).  Based on this estimator, the type specimen of _Giganotosaurus_
was more massive (and thus heavier) than any known _T. rex_ specimen.

As I wrote just yesterday, the total skull length of _Gig._ is considerably
longer than that of the biggest known _T. rex_, but this is due to the
backswept nature of the rear of the skull of _Gig._  Going along the
baseline (tip of premaxilla to end of occipital condyle) the skulls are very
close in size.

Tyrannosaurids in general have more heavily reinforced skulls than those of
carnosaurs such as _Gig._, and _T. rex_ has the most reinforced skull of
all.  The teeth of _Gig._ are very similar to those of
_Carcharodontosaurus_, being very standard steak-knife teeth found in most
toothed carnivorous archosauriforms.  _T. rex_, on the other hand... er,
jaw, has very thick teeth ("knife-edged bananas" gives an idea of size and
shape).  As I have written elsewhere, this probably has a LOT to do with
different modes of feeding in the two forms.

(Incidentally, although _T. rex_ skulls are more heavily reinforced, they
are far from solid: many of the skull bones have hollow chambers in them, as
do those of other maniraptoriform groups).

Now, as for total length: this measure is not biologically very meaningful
in animals with long tails (certainly less meaningful than mass, against
which many physilogical and biomechanical factors must scale).  Furthermore,
the complete tail is unknown in _Giganotosaurus_ at present, and the
complete tail of a _T. rex_ is yet to be published (there might be one
preserved with "Sue", but it isn't described yet). Thus, total length
measures for these giant theropods are huge guesses: they could be off by a
meter and a half, depending on the proportions of the tail vertebrae.

The old 48-50' long _T. rex_ many of us grew up with is a myth: it was based
on the overly elongated tail of the AMNH mount.  Although the tail of that
mount has been corrected to a more proper coelurosaurian count, the details
are still inaccurate (based on comparison with other  tyrannosaurid taxa).
Tyrannosaur tails are very similar to ornithomimosaur tails in many details
(chevron shapes and patterns, centrum proportions, etc.).

So, what was the biggest theropod?  Extraordinarily difficult to say.  Given
the currently described material, the known fossils of _Giganotosaurus_ are
probably from individuals larger than those that produced the known fossils
of _T. rex_.  However, we are dealing with very small sample sizes, so these
differences may not be statistically significant.

Lurking in the wings are other giant contenders, notably _Spinosaurus_ (some
incomplete specimens of which indicate very large individuals rivalling the
above taxa in mass).

Hope this helps.

P.S.  Before ANYONE posts about the Rigby _T. rex_ aka "Imperator" as
evidence of tyrannosaurs exceeding _Giganotosaurus_ in size, PLEASE PLEASE
PLEASE PLEASE read the list archives for the last couple of weeks (and back
to earlier posts from past years on the subject).

                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.inform.umd.edu/SCHOLAR/programs/elt.html
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796