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RE: More on "Arbitrary" Paleontology

At 10:31 PM 6/6/99 -0400, Tom Mitchell wrote:

>Tom Mitchell:
>I can't wait.  So I hope you won't mind if I pose a naive question about
>monophyly.  What do you make of the period in the history of paleontology in
>which dinosaurs were not regarded as a single, coherent animal group, but
>two separate groups that are distantly related? (I've run across this in
>Ostrom and Wilford.  Also, "The Dinosauria" suggests that "the term dinosaur
>has a significant arbitrary component"). I gather that everyone (everyone?)
>now believes that this problem has been left behind, and that dinosaurs had
>a common ancestor.

Or, to be more precise, that all dinosaurs had a common ancestor which was
also a dinosaur.  (Man, it has been years since I used that formulaic
phrase: too long have I been writing and speaking in PT... :-)

>How firm is this consensus?

Extraordinarily.  (Of course, with Phylogenetic Taxonomy, the ancestor of
_Triceratops_ and Neornithes is by definition a dinosaur, so it is
essentially a moot point.  Nevertheless, it would take some amazingly
bizarre and spectacular specimens to demonstrate that (for example)
Crocodylia or Parasuchia or Pterosauria or such were closer to Ornithischia
(for example) than to Saurischia.)

>What is at stake in it?

Not too much, actually.  The folks who revived the idea of dinosaurian
monophyly in the 1970s (Galton, Bakker, Bonaparte) have sufficient enough
other contributions to the field so that their careers are not bound up in
its validity, if that is the sort of question you are asking.

In terms of its scientific importance, it does help clarify that the
dinosaurian radiation represents a single historical event, which is
interesting.  However, if it did turn out that ornithischians and
saurischians were separated by traditionally non-dinosaurian forms, then
there would be the equally interesting observation that the primitive
"dinosaur" shape (c. 1 m long upright biped with parasaggital gait,
digitigrade stance, enlarged deltapectoral crest, asymmetrical hand, etc.)
TWICE led to a major adaptive radiation.

>What sort of evidence would count for or against monophyly?

Anatomical evidence.

>What effect does cladistics have on this debate?

Well, put that sentence into the past tense...  The basic data were
presented in a non- (or proto-) cladistic context.  Cladistics helped
clarify the distribution of these features, though.  To put it into a
hypothetical future case: discovery that some traditionally non-dinosaurian
forms lie within the clade Saurischia + Ornithischia would need to be
demonstrated to explain the data better than the current consensus: at
present, a cladistic analysis would be the best means of demonstrating such
a claim.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661