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New Nature paper on Tyrannosaur Locomotion



Yes, Tim Williams found me out...

In this week's Nature:
JOHN R. HUTCHINSON and MARIANO GARCIA (2002).  _Tyrannosaurus_ was not a
fast runner.  Nature 415: 1018 - 1021

and the News & Views:
ANDREW A. BIEWENER (2002). Walking with tyrannosaurs.  Nature 415: 972-973.

In brief, Hutchinson & Garcia model various hindlimb paramaters (limb
lengths, segment weights, muscle fibre lengths, and more) from various
specimens (MOR 555, a _T. rex_; a small tyrannosaur from the Field Museum; a
_Coelophysis_; a modern alligator; a modern chicken).  They attempted to
calculate (for various body positions) the value T, the percent of the body
mass represented by leg muscles per leg required to allow the animal to run.
"Running" in this case was set at Froude number (Fr) = 2.5, based on
observations of modern animals and on biomechanical theory.

(The Froude number is a dimensionless descriptor of speed.  For their model
they calculate Fr  = (velocity)^2/(hip height x g).  An ostrich running at
12 m/s is doing Fr = 16; in biomechanical theory, Fr = 1 represents the
walk/run transition).

Most of their estimates for the adult _T. rex_ found values of T which were
unreasonably high (each leg's muscle mass being 30-50% of the organism!) for
nearly all the different postures for the Big Guy they tried.  The other
specimens they looked at had lower values: T = 20 for the young leggy
tyrannosaur, down to T =  4.7 or so for the chicken.

The basic conclusion: big _T. rex_ specimens did not have sufficient muscle
mass to be able to produce a fast run.  However, they point at that at a
fast walk (presumably close to but not exceeding Fr = 1), MOR 555 would
still be moving at 5 m/s (that is, 11 mph).

A side note: other big theropods and big non-theropod dinosaurs would be
suffering from the same set backs (and arguably worse with broad stumpy
feet), so this does not invalidate _T. rex_ as a _Triceratops_ or
_Anatotitan_ chaser.

Checking this against the recent Day et al. (2002) Middle Jurassic trackway,
their critter (estimated to be 1.93 m tall and moving at an estimated
maximum speed of 8 m/s) seems to be moving at a Froude number of 3.8.  This
is faster than the walk-run transition (theoretically at Fr = 1).  Now an
1.93 m hip height theropod is big: comparable to _Dryptosaurus_ or a smidge
under the UUVP 6000 specimen of _Allosaurus_: Greg Paul estimates that
latter specimen at 1.32 tonnes.  So *if* the size and speed estimates of the
MJ theropod are correct, then one tonne theropods could run, at least
slowly.

Very, very interesting stuff.  It will be even more interesting to see this
model tested against other, smaller theropods and other dinosaurs.  I would
be interested in seeing how other young tyrannosaurs and ornithomimosaurs
fare in this model.

                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