TROODON, FABROSAURUS AND TOOTH TAXA IN GENERAL

[Tetanurae@aol.com]

George Olshevsky wrote:
<<Absolutely. It's always up to the individual researcher. Some dinosaurs
have
been described from a single tooth, others from a single bone. Of course,
there's no guarantee that these species will stand the test of time, but some
do. For example, _Troodon formosus_ is based on a single tooth described in
1856, and it's regarded as a valid species (and is even a senior synonym of
several other dinosaur species, one of which is also based solely on a single
tooth--from a different place in the jaw).>>

George brings up a point that I have been thinking about quite a bit
recently.  Are tooth taxa really, truely diagnostic?  This runs a very fine
line, and the conclusion I have come to is a very cautious no.

Let me explain this a little more in depth though.  As much as the mammal
paleontologists would like you to believe, dinosaur teeth are very
distinctive and can hold great diagnostic power.  There is however a limit to
this, and I think that isoltaed tooth taxa have crossed it like _Troodon_,
_Galtonia_ and _Fabrosaurus_.

Let me give an example.  The late Cretaceous rocks of western North America
are periodically littered with small assymetrical theropod teeth that have
large posterior denticles and constricted roots.  One of these teeth was
named _Troodn formosus_ in 1856.  Within the next century and a half a lot
more material was collected and named _Stenonychosaurus inequalis_ and
_Pectinodon bakkeri_, but has since been lumped into _Troodon formosus_.

Is this the right thing to do?  No.  Although the teeth associated with the
skeleton named _Stenonychosaurus inequalis_ are similar to those of both
_Troodon formosus_ and _Pectinodon bakkeri_, we cannot be 100% that they come
from the same animal.  In fact it is quite possible, even probable that there
was more than one animal running around North America with the _Troodon_
tooth-type.

Thus, _Troodon_, _Troodon formosus_ and the Troodontidae should be regarded
as _nomina dubia_ because there is no way to be certain that ANY further
material can be associated with the names.  The skeletal material should be
regarded as belonging to _Stenonychosaurus inequalis_ (as originally named)
and the family should be called Saurornithoididae.

Case in point.  _Fabrosaurus australis_ was named in the 60's based on a
dentary with teeth.  Somewhat later, a lot of skeletal material was
discovered and discribed by Thulborn as belonging to _Fabrosaurus australis_.
 Later yet, Galton erected the genus and species _Lesothosaurus diagnosticus_
to house the superb skeletal material described by Thulborn because he, for
some reason, thought that _Fabrosaurus_ and the new material were somehow
physically distinct.  Thulborn and Sereno have (independantly) recently shown
this to be in error, as the type of _Fabrosaurus australis_ and
_Lesothosaurus diagnosticus_ are quite similar.  Thulborn and Sereno come to
quite different conclusions however.  It is Thulbron's position that all the
material reffered to _Lesothosaurus diagnosticus_ and fabrosaurid indet. yet
described from South Africa belongs to _Fabrosaurus australis_.  It is
Sereno's position that basal ornithischian teeth are entirely plesiomorphic
and not diagnostic enough, so _Fabrosaurus australis_ must be a _nomen
dubium_.

They are both wrong.  basal ornithischian teeth are in fact quite diagnostic,
and all indications seem to be that the _Fabrosaurus australis_ material came
from an animal quite similar to the skeleton described as _Lesothosaurus
diagnosticus_.  BUT, Sereno as well as Santa Luca indicate that there is a
second basal ornithischian from the same formation (not _Heterodontosaurus_)
that was nearly twice as large as the _Lesothosaurus diagnosticus_ material
that seems to form a clade with the _Lesothosaurus diagnosticus_ material.

So, does the type of _Fabrosaurus australis_ belong to the smaller
_Lesothosaurus diagnosticus_ animal or the bigger animal?  I don't know.
 Neither does anyone else, and we can't know unless there was more material
in the type of _Fabrosaurus_ (or a time machine).

Thus, _Fabrosaurus australis_, _Fabrosaurus_ and Fabrosauridae should be
regarded as _nomina dubia_.

There are many other superb, and distinctive tooth taxa named out there such
as _Pekinosaurus_ and _Tecovosaurus_.  But let's get this clear.  Distinctive
does not equal diagnostic.  How sure can you be that there was absolutely ONE
species that had said tooth type in the formation?  You can't, unfortunately.

Don't get me wrong, I'd really like to see what _Tecovosaurus_ looked like,
it's teeth look strangely thyreophyran, but if I ever found a skeleton that
had the _Tecovosaurus_ tooth-type, the best I would do would be to call it
_cf Tecovosaurus_.

In any case, I hope I didn't bore anyone :-)

Peter Buchholz
Tetanurae@aol.com

"No words, no words , no words....  They should have sent a poet....  I had
no idea, I had no idea, I had no idea, I had no idea......"