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tooth counts in systematics Re: Tooth Counts in Tyrannosaurs



Dinogeorge@aol.com wrote:
> 
> This is indeed strange: You have only one specimen, and it sometimes has 10
> teeth and at other times 13 teeth? Yes, I would agree that >this< kind of
> variability would make tooth count a bad character...


Not having seen the specimen, my guess is that this is a case of
asymmetry - ten on one side, thirteen on the other.  It happens in
crocs, though usually the difference is a single alveolus.

Part of my interest in "Nanotyrannus" (which I seriously think is an
immature T. rex) came from statements that Alligator does not lose
premaxillary teeth during ontogeny.  The person who said that is right -
A. mississippiensis does not lose premaxillary teeth.  But it's just
about the only living croc that does not lose premaxillary teeth during
ontogeny.

Which raises the issue - just how reliable are tooth counts in archosaur
phylogenetics?  I've been pretty careful to use characters in my own
work that I'm pretty sure are ontogenetically invariant - e.g.,
Paleosuchus starts out with four premaxillaries and stays that way
throughout life - but however variable they might be, tooth counts could
still preserve a phylogenetic signal.

This is a point the herpetologists are already way ahead of us on. 
Weins, for example, has explored methods for using scale counts in
lizards phylogenetically - scale counts vary within populations even
more than tooth counts, and yet he didn't want to just throw them out
when they could help out at some level.  The challenge is to include the
amount of variation and its nature (ontogenetic, sexually dimorphic,
interpopulational, whatever) in the characters - whether this can be
done with theropods is another matter.


chris  






-- 
----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

voice:  312-922-9410 x469
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electronic:  cbrochu@fmppr.fmnh.org