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Re: Tyrannosaurus ?



At 09:47 AM 3/25/99 -0800, Adam Hockers wrote:
> I have heard that because of the great differences in male and female
Tyrannosaurus Rex that it may be two different speices altogether.  T. Rex
and T. X.  Could this be true or is it a myth.

Could it be true?  Possibly.  However, the details of variation within _T.
rex_ specimens (incidentally, the "r" in "rex" should always be lowercase)
have not been well worked out.  There are many different aspects of
tyrannosaur skulls (rugose knobs on the postorbitals, connection between
lacrimals and postorbitals, tooth numbers, etc.) which vary among the known
specimens, and even from right to left sides of individuals.  Some
possibilities to explain these include:  sexual dimorphism; two or more
species being present; one species with individual variations; one species
with changes over time (on the hundreds of thousands of year scale).  The
first steps to test this (identify *what* is variable, which variations are
on which specimens, and where the specimens fit stratigraphically) has not
been published yet.  Because we don't have the first step yet, we have no
strong reason to accept any particular explanation.

>Another question concerning T. Rex is its top speed.  I have seen in some
older books that T. Rex was very slow (3-5 mph), but in some other book T.
Rex is said to have been rather fast (30-35 mph).  Where did these
completely different speeds come from?

Well, top speeds for living animals are exceedingly hard to come by
(biologists have measured the top speeds for only a few species of living
wild animals), so determining the top speeds of extinct animals is a LOT
more difficult.

But, basically, the old tradition went like: _T. rex_ is big; big animals
move slowly; _T. rex_ moved slowly.  The newer tradition went something
like: _T. rex_ has numerous features associated with fast running (long
lower limb elements, specialized feet, big long ilium, etc.); animals with
these features move very fast; _T. rex_ moved very fast.

However, there are complications here (for example, questions have been
asked about how likely a _T. rex_ could withstand a fall at top speed).
Nevertheless, almost all paleontologists who work on theropods now agree
that _T. rex_ and other tyrannosaurids were faster than any other large
meat-eating dinosaurs, and that smaller tyrannosaurs (at least) were
probably among the fastest dinosaurs, period (comparable to ornithomimids of
the same size).  However, actual numerical values for these speeds are a
problem.

Hope this helps.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661