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More tyrant Q & A's

Here's more on tyrannosaur questions.

Dwight Stewart writes:
>How strong is the evidence for dimorphism in Tyrannosaurus rex?

Actually, there is pretty good evidence for dimorphism: there do seem to be
gracile morphs and robust morphs.  The question is whether the dimorphism is
sexual or taxonomic.

Truett Garner summarized the evidence very nicely.

On top of that, there is the question of cranial ornamentation and how that
matches up with the limb and ischia (and chevron) dimorphism.  If, for
example, all the really ornate skulls go with the gracile morph, that would
be pretty strong evidence for sexual dimorphism.

>The book indicated that Dr.
>Bakker isn't sold on dimorphism
>       & (in fact) believes that the more robust specimens represent a
>different species.

Bakker is famous for being a rather extreme "splitter" taxonomically: that
is, in a given assemblage of fossils, he considers the actual in-species or
in-genus variation to be very small, so that the assemblage should be broken
up into many species.  Other people vary on how much variation they accept
within a given species: some might consider all the specimens from one
species, other people divide them in two, and so forth.

>       As an admittedly remote example, extant species vary more within
>themselves that the total
>       variation among known Tyrannosaurus specimens.

NOT a remote example, and very germane to the subject.  Some modern species
DO vary widely in terms of their morphology.  This is a problem in taxonomy
(modern and paleo-): where do we draw species boundaries.

Dwight also wrote:
>To me this would tend to
>indicate a potential
>       for "social interaction" among Tyrannosaurs that might fit more of a
>hyiena model
>       than an African lion model.

I don't see how that would follow, especially as how hyenas (or spotted
hyenas, at least) have a fairly peculiar social organization, at least (if
not more) complex than lions.

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