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Tooth-Based Taxa



  Okay, perhaps I have a little less at stake than others on
this topic, as I primarily indulge in animals without teeth :)
-- but needless to say, I felt I could offer my opinion on the
topic of basing taxa on teeth alone. I previously posted on this
subject, but that was briefly commented on to work in progress.
So, here goes a little deeper indulgence in one aspect of my
interests.

  There are a lot of dinosaurian taxa that have been named
_soley_ on the basis of a tooth. Or a collection of teeth. The
list is quite long, so I'll not belabor this list and post with
an attempt to discuss them all.

  For some taxa, the form of a tooth is strongly dependant on
it's position; this varies to a great deal in ornithischians and
some theropods, especially the troodontids and
"hypsilophodontians". In the latter, the progression mesial to
distal is from a typical, basal ziphodont (or blade-shaped)
tooth, to a marginally phyllodont (or leaf-shaped) tooth, and
ending at a low-crowned strongly phyllodont tooth. In some
lineages, the form has varied to include the rhomoid teeth of
iguanodonts, and the chisel-like teeth of heterodontosaurs,
ceratopsians, and Agilisaurus. Typically, the phyllodonty arises
out of a angled row of denticles that are oriented equally basal
and distally on a crown; this changes caudally, as the tooth
becomes phyllodont, in that the denticles are radially arranged
until they become arraying in a single, vertical direction. I
have funny little -dont words for all these, to clarify the
terminology used for non-mammalian teeth. Needless to say, teeth
are strongly variant to position and even which jaw they are
located in. There is often a robust vertical ridge in
prosauropod, non-peg-toothed sauropod, and most ornithischian
teeth, which is associated with herbivory, and this can even
loose definition in caudal ornithischian teeth. It's loss has
been used as an autapomorphy in some hadrosaurs, it's
acquisition in others, and generally has a previously
undocumented variability in Ornithischia as a whole (more on
this when I get around to playing with the material more).

  In theropods, marked heterodonty has been noted in
troodontids, and to a less similar degree, tyrannosaurids and
dromaeosaurids. In fact, it was this degree that led Matthew and
Brown to refer *Dromaeosaurus* to the Deinodontidae
(=Tyrannosauridae): The D-shaped premax teeth [properly, the
caudal inflection of the mesial carinae] anteriorly and the
"normal" teeth caudally. In troodontids, teeth vary from the
premaxillary, throughout the dentary, and probably throughout
the maxillary, tooth rows. In the maxillary row, anyway, the
disinction is a lot less, as most of the teeth are largish and
only the caudal-most change in shape (slightly).

  These being extreme examples, it should be noted that the
conical form of the "caniniform" or conodont dentition of some
predators (spinosaurine spinosaurids are a good example) and
ornithischians like heterodontosaurids and basal
pachycephalosaurians, are yet another example of how teeth can
vary, and finding a tooth with similar form may not be
immediately applied to a known taxon. It became fashionable in
the 1800's and early 1900's to name taxa on the sole basis of
teeth, suggesting their distinction from known teeth qualified
their taxonomic status. I disagree.

  In the case of *Troodon*, the type is a premax or anterior
maxillary tooth, probably the former, based on comparison with
other troodontids. Unfortunately, this cannot be verified. No
premaxilla from the Judith River, Lance, Milk River, or Two
Medicine has yet to be found showing this morphology of tooth.
Nor a maxillary; though one maxilla has been found, it is devoid
of teeth or even roots. Thus, for the time being, though Currie
(1987) is in favor of formal nomenclature and application of the
types of *Pectinodon*, *Stenonychosaurus*, and
*Polyodontosaurus* to objective junior synonyms, their
relationship cannot be proven. As such, the name *Troodon*
should be termed a nomen dubium and restricted to the type until
such time as a premaxilla and maxilla attached to a specimen
that can demonstrate sympatry with the other "troodon" genera.
They are probably all the same animal, but you may never know,
and there could be as many as four different species of
troodontid applied to the name *Troodon* presently.

  Similar to *Astrodon johnsoni*: it's a tooth that can be
suggested as a titanosauriform, but there's nothing that says
it's the same thing as anything described from the Antlers, Glen
Rose, Trinity Sandstone, Arundel fascies (Potomac), etc.. Best
left as a nomen dubium, as with most tooth-based taxa, until
actual affinity can be determined.

  Same for (not comprehensive) *Paronychodon/Euronychodon*,
*Prodeinodon,* *Phyllodon,* *Asiatosaurus,* *Siamosaurus,* ...

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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