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On why I think that Hutchinson & Garcia *might* be correct...



Okay, most of the reason has to do with the fact it is a good paper itself,
and a good initial model.  Of course it can (and should) be refined; of
course additional specimens are needed; of course more taxa living &
extinction should be added.  This is agreed by everyone.

For my own (phylogenetic) take on this:  I agree 100% with Greg Paul and Per
Christiansen and Phil Currie and others that tyrannosaurs show the hindlimb
adaptations of cursorial organisms; if anyone questions this, go back to JVP
14, issue 4 and read my paper!

However, the adapations in question are Tyrannosaurid adaptations, not
_Tyrannosaurus_ adaptations.  At the level of _T. rex_ they are all
symplesiomorphies.  They evolved in creatures of considerably smaller body
size than the Big Guy.

One of the main insights of biomechanics is that anatomical scaling doesn't
always mean performance scaling; animals in different size regimes do not
always operate proportionately.  Anatomy can try and scale all it want, but
the physical factors involved (gravity, inertia, etc.) is not obliged to
scale precisely with it.

For example, a domestic cat can make vertical leaps much larger (in
proportion to their bodies) than can a tiger or lion or even a leopard
(which is a damn good leaper, incidentally).  A hypothetical 1 tonne big cat
(even if built allometrically along felid lines) would likely face
biomechanical obstacles that a 100 kg big cat doesn't.

Now, IF _T. rex_ were found to have the traditional cursorial adaptations
(iliac convergence, elongate distal limb elements, arctometatarsi) evolving
AFTER it had split from its closest relatives, I'd think that there might be
reason to think it was a specialized runner.  That does not seem to be the
case.

As it stands the phylogenetic and traditional data suggest:
  * that it was the descendant of smaller, specialized runner
  * that it very well might have been a faster mover than other large bodied
dinosaurs
  * that it could kick the butt of any big mammal (okay, the data don't
really say that, but sometimes I have to throw in my own visceral (rather
than cerebral) reactions... :-).

Incidentally, many apologies for misrepresenting part of the H&G analysis.
They were modeling a _T. rex_ moving with a G (ration of ground reaction
force to body weight) of 2.5, not a Fr of 2.5.  For comparsion, they mention
that an ostrich moving at 12 m/s (an Fr = 16) has a G of 2.7; therefore,
their model is testing a _T. rex_ moving at the upper end of the faster end
of the proposed estimates: a good all out run. (This is in the "Determining
forward velocity" section of the paper).

Take care, everyone.

                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