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Re: TETRAPODS, PHYLOGENETIC TAXONOMY, AND CLEAR DEFINITIONS



At 08:37 PM 7/27/98 -0400, Johnathon wrote:

>(Basing a diagnosis on just two characteristics doesn't seem very safe
>to me, anyway.)

A) No one here said that these are ALL the diagnostic characters for
Dinosauria.  You never asked for that list!  We were giving examples of how
a system works.  Should you be interested in the best (currently) published
diagnosis for Dinosauria, you may wish to find:

Novas, F.E. 1996.  Dinosaur monophyly.  Journal of Vertebrate Paleontology
16:723-741.

(Acutally, you need Sereno & Novas. 1994. JVP 13:451-476 to complete the
data matrix Novas discusses).

Can't get these papers?  Maybe someone on the list could xerox them for you.
However, if you are going to want detailed technical information on this
subject, you are going to have to make an effort to do the library research.

In the past people have criticized myself and others for being too technical
on the list, so I've been trying to keep a happy medium (basically a 100-200
(freshman to sophmore) undergraduate college course level).  As such, I may
not mention every last detail about some subjects, nor should I.  If I asked
you what project you're working on, I wouldn't expect you to post the entire
source code for your latest project!

If you are really serious in a graduate school level discussion of
systematics, diagnoses and their discovery, definitions and their
construction, and so forth, I or Brochu or others could post some books
(probably available feom amazon.com) that you could read up on.

B) You are correct in saying a diagnosis of just two characters might not be
very secure.  (Of course, it isn't "based" on two characters: it is
discovered to be two characters).

On the one hand, when there are lots and lots of characters unique to one
group and not found in its closest relatives, then you can be relatively
secure about the membership of a newly discovered form within that clade.
However, big diagnoses could also be statements of ignorance: the reason
that we have a long diagnosis may be because the forms of intermediate
morphology haven't been discovered.

To use a dinosaur example, if we knew just _Coelophysis_ and _Archaeopteryx_
among Mesozoic theropods, the _Archaeopteryx_-later bird clade would have a
humongous diagnosis.  However, as allosaurs, Ornitholestes, troodontids,
oviraptorosaurs, dromaeosaurids, etc. would be discovered, we'd find that
some features which were once considered diagnostic to birds had a broader
distribution.  Thus, the list of diagnostic avian features would get shorter.

>So we have a second diagnostic procedure here: anything that can be
>diagnosed as belonging to a subgroup of Dinosauria also belongs to
>Dinosauria.  Reasonable.

[*Banging my head agaist the computer in frustration*] - This is a
definition, not a diagnosis.  That was the point of what I, and Brochu, and
Bucholz, and Wagner, and others were trying to get across.  There is a
difference between the "diagnosis" (a description of character states,
regardless of the taxonomic system used) and the "definition" (under
phylogenetic taxonomy, a statement of common ancestry).  Since you've
already demonstrated you have a copy of _The Complete Dinosaur_, check out
pp. 103-105 (i.e., the section labelled "Definition and Diagnosis").

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661