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tooth counts in systematics

chris brochu wrote:
> 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.

        BINGO.  That is the situation.  It is the only example that I 
cited.  However, it is not the only example that I have seen.  So, I 
guess we could safely say that it happens in crocs and it happens in 

        Yeah, but I think even the systematists would probably agree that 
maxillary and mandibular counts are bad characters given intraSPECIMEN 
variation in the state...

> 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.
        Given the above, I will echo something I just sent to Chris 
concerning this:

        On the one hand I hope that premax count at least works for 
theropods, because I thought we might finally have a character there that 
didn't suck.

        and on the other hand I hope that it doesn't work, because then I 
can be vindicated in my visceral feeling that cladistics sucks and needs 
improvement at the very basic level of character selection.  

        Hmmm...maybe back to the drawing board.

Josh Smith
University of Pennsylvania
Department of Earth and Environmental Science
471 Hayden Hall
240 South 33rd Street
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