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Larson's "Variation and Sexual Dimorphism"



I just got my copy of the "Tyrannosaurus rex - The Tyrant King" yesterday
and have been thoroughly enjoying reading it.  It was better than I thought
it was going to be (especially the first chapter - wow, what an extensive,
detailed list!!).  I have only begun to read it thoroughly, but what I have
gone through so far is really good.  Well, I guess I was a bit dissapointed
with the CD-ROM.  The pictures of the disarticulated skull are really good,
but small.  CD's are cheap, make the pictures HUGE.

I am stuck on Larson's chapter on Variation and Sexual Dimorphism.  Maybe I
am just misreading what he has to say, but I find his arguments for both
"Nanotyrannus" and Tyrannosaurus "x" unconvincing.  Does he believe that a
juvenile Tyrannosaurus would look exactly like a small adult with little
change?  Does he not believe that CMNH 7541 and BMR P2002.4.1 are juvenile
"somethings"?  Currie's 2003 paper did shoot down some of Carr's
characteristics that show CMNH 7541 to be a juvenile, but ended with it had
too many teeth to be Tyrannosaurus.  (As an aside, has anybody out there
actually seen the skull of CMNH 7541?  How many maxillary teeth does it
have?  14?  15?  Could the ambiguity simply mean that one of the tooth
positions was in the process of being reabsorbed?)  Is tooth numbers really
a strong enough argument on its own?  There is a lot of variation in tooth
numbers in other species that are known from multiple specimens (Allosaurus,
Coelophysis).  I remember a conversation with Hans Deiter-Sues where he said
something to the effect that dinosaurs were not mammals.  Reptiles do not
have dentition that is uniform.  There is frequently a range in teeth for
them.  SDSM 12047 (one of the T. "x" specimens) even has 11 teeth in one
maxilla and 12 in the other.

He also returns to some of the old arguments for "Nanotyrannus" that I
thought were well put down, such as the lachrymal horn (which I thought was
plaster, and well established as plaster).

With Tyrannosaurus "x", he has four characteristics that he claims separates
it from T. rex.  The first is the shape of the 2nd dentary tooth.  Did I
misread what it said in the book?  None of the specimens that he says are
"x" even preserve the teeth?  Not a very strong argument, or am I missing
something?

The second is the shorter skull of the "x" specimens.  With so many of the
skulls of others found incomplete, squashed, or disarticulated, can we be
sure of this?  Ralph Molnar once told me he was impressed with the variation
in lengths of the bones in the skulls available, yet they all ended up being
about the same length.  Only one of the skulls he lists as being "x" seems
to be smaller than others.  One in fact (MOR 008) is the largest skull ever
found.

The third separator is tooth count, but it is only off by one (see above).
I would have appreciated a chart to show the tooth count of the specimen's
he claims are "x", rather that the reference to them.  I wish I could count
them myself, but that is not possible.  I cannot find my own reference to
numbers of teeth and he does not provide one.

The fourth difference is the size of the lateral pneumatic lachrymal
foramen, which he says is smaller in "x".  Even he says that this difference
may be individual and not statistically significant.

His conclusion is that "it is likely that a second North American Latest
Cretaceous species of Tyrannosaurus exists" seems not well supported.

After reading this, how many participants were swayed one way or another at
the conference in either or both cases?  Man, I wish I had been able to go
to this conference.

[By the way, I promise to thoroughly read chapter 20 soon, it is just at the
end of the book, so it will take me a while....]

Darryl Jones  <dinoguy@sympatico.ca>

For information on tyrannosaurids and
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visit my web page at:

http://www3.sympatico.ca/dinoguy/