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Re: Thoughts on Systematics and IntraSpecific Variations



At 01:10 AM 4/15/99 -0400, Allan Edels wrote:

>    It seems to me, that the definition of characters to be used in a
>cladistic analysis is the potentially source of disagreement in the results
>of any given cladogram.

Yes, universally agreed.

>If there is no standard for characters that can be
>agreed upon, then the results can be questioned.   (This has been one the
>most confusing aspect of cladistics for me  -  who says what characters are
>valid, and what does "slightly longer" mean in a femur? [If there are no
>standards?]).

There are exactly the same standards used in any sort of descriptive
science: is the state in fact observable?  Can an independant worker recover
the same information?  And PLEASE do not mistake character description for
cladistics: traditional systematics used much the same types of character
descriptions (go back and read them and you'll see what I mean), and
cladistic methodology can and does employ other data as well: molecules
first and foremost, but also behavior, etc. (of course, most of these types
of data are not availble to paleontologists).

>    It one thing to say that the squamosal is robust, it's another thing to
>accurately quantify how robust that really is, versus other squamosals.

Yes, this has been the greatest failing of most cladistic (and traditional)
studies of dinosaurs (and eutherian mammals, and beetles, and
actinopterygian fish, and etc., and etc., and etc.).  Josh Smith and Chris
Brochu previously pointed out, quite correctly, the problematic of such
definitions.  Some of us who have previously employed said poorly described
characters are trying to better describe them in current works (i.e.,
explicit numerical descriptors, for example).  Additionally, other
techniques are being tried: in this sort of case, running the data set with
proportionality and "robusticity" characters (and absence characters)
included, and comparing the results to running the data set with these types
excluded (something I did in my disseratation, actually).

>I
>think we need to start looking at the work Ralph Chapman (and others, of
>course) are doing in morphometrics.  It is possible that this work may
>enable us to accurately describe a squamosal, or an orbital fenestra based
>on a comparative range of sizes, robustness, or gracile-ness, or a
>compendium of shapes (e.g. - REAL definitions of 'circular', 'oval',
>'elongated oval', etc) - that paelontologists can agree on, and use as valid
>characters in any analysis.

I agree whole-heartedly: we should describe our characters as carefully as
possible.  However, papers cost money to publish (yes, the dirty little
secret of Academia: you pay to publish your papers, not the other way
around), so for space reasons most analyses don't have the space for the long 

>    This work could also help address the problem of valid ranges of sizes
>and shapes for variations within species.  (The only work I know of that
>started looking at similar problems was by Tim White, working with Don
>Johanson with "Lucy" [ _Australopithecus afarensis_ ]).

Oh, there are plenty of others (most issues of JVP have a paper or two on
such things).  Ralph Chapman himself has published some of these (for
example, his contribution with Dave Weishampel on _Plateosaurus_ in the 1990
Currie & Carpenter Dinosaur Systematics volume).  Dodson published classic
papers on _Protoceratops_ and on lambeosaurines to help sort of
morphometrics within and between species.  Dave Smith had his recent paper
on shape variation of various bones in _Allosaurus_ (in JVP a year ago).

By the way (and you're going to hate this), but _A. afarensis_ is very
likely an junior synonym, and many paleoanthropologists are now calling it
_Praenthropus africanus_.  (Bernard Wood and a colleague had a paper in a
recent issue of _Science_ where this was alluded to, although the topic was
the definition of the genus _Homo_: I've been aware of the taxonomic debate
over the "_afarensis_" taxonomy for a while, but haven't bothered to xerox
all the relevant articles).  And don't go blaming cladistics for this one:
these debates have almost all been within the context of traditional Linnean
taxonomic nomenclature.  (Although Bernard Wood himself is one of the
leading workers in cladistic analyses of hominin primates).

>I'm probably wrong
>here, but I'd like to hear what other work was being in this area.

See above.  Ralph (who knows more about this topic than just about anyone)
could probably come up with a good list of appropriate studies.

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