Re: Late K sauropods, Trachodon, teeth?

[rowe@lepomis.psych.upenn.edu (Mickey Rowe)]

sean.kerns@sdrc.com asks a bunch of questions about naming and
reconstruction. 

I think I can answer a lot of your questions by pointing out that just
because we have a name for an animal doesn't mean that we have any
idea of what the animal looked like in life, or to what other animals
it was most closely related.  When a fossil is found, it is compared
to others that are known, and if it is judged to be similar to one
that is already named, then it may be referred (i.e. given the same
name) as the previously found one.  If not, then it is given a new
name.  Regardless of how complete the fossils are, it is possible to
make mistakes going both ways here.  Individual differences between
members of the same species might cause us to give two names to the
"same" animal, or similarities between animals of different species
may cause us to lump two "different" animals under the same name.
These difficulties lead to judgement calls that people can argue over
for years.

As for naming an animal based just on its teeth...  When you find a
tooth that doesn't look like any other teeth known, it's pretty safe
to give it a name (though my impression is that this sort of practice
is much more rare than it used to be -- certainly it is for some
groups of animals).  When you find some more teeth that look like
animals that are known *only* from their teeth, however, you start to
have a real problem.  For some animals, teeth are very diagnostic.
That is, we know from more complete specimens that if two teeth look
very similar, they most likely belonged to animals we would consider
to be the same species even if we had the entire skeletons to compare.
For other animals, teeth are not very diagnostic.  You may find teeth
that are indistinguishable from those of a previously named animal,
but know from more complete finds that those teeth could have come
from animals we've already decided belong to different species.  A
case in point is hadrosaur vs. theropod teeth.  The teeth of
individual theropod species (or at least genera) are pretty
diagnostic.  Comparing newly discovered theropod teeth to older ones
can tell you pretty well what sort of animal contained the more
recently found teeth.  Hadrosaur teeth are not so good by that
criterion.  If you found some hadrosaur teeth not associated with
other parts of the animal, you would be able to say it was a
hadrosaur, but you wouldn't be able to place it in any particular
genus (and if I recall correctly you might not even be able to place
it in the proper sub-family).

In short, the answer to:

> how on earth can you possibly construct an entire animal based on a
> tooth???

is that oftentimes you just can't.  That's why, as Tom said, Leidy had
ceratopsian and hadrosaur teeth mixed together.  Certainly those
animals look very different!  We wouldn't have much trouble
discriminating ceratopsian and hadrosaurian teeth any more, but Leidy
had seen a lot fewer dinosaur fossils than have we, so he couldn't.

Another problem you have to keep in mind is that if one person finds a
tooth and names an animal based on that tooth, and another person
finds a claw from an animal of the same species and gives that same
claw another name, until and unless similar teeth and claws are found
in association (or transitively a tooth similar to the first
associated with some other bone which is similar to a bone associated
with a claw similar to the original claw -- read that a couple of
times and it should make sense :-) we won't ever know that these "two"
animals are the same.

On top of that, even if you have, for example, 80% of the bony
material from an animal, you still might not reconstruct it properly.
We feel pretty confident now that most pictures and mounts made 70
years ago were wrong, but in many cases (e.g. the T. rex which I
imagine is being/has been remounted at AMNH) our conclusions aren't
really based on new finds -- just re-examination of old ones.

Hope this helps.

-- 
Mickey Rowe     (rowe@lepomis.psych.upenn.edu)