Re: Pan-clades, good or bad?

["Jaime A. Headden" <qilongia@yahoo.com>]

My short answer on the use and utility of pan-stems, as de Queiroz and
Gauthier use them:

Pan-stems have a highly functional use in that they are definitions for
clades that include ALL possible members, extant or otherwise, that belong
on the stem to the crown-clade. This makes them rather important in some
cases, as they become the most useful taxa in a system of living
organisms. De Queiroz, Gaffney, and Gauthier has a publication in press
ascribing this theory to turtles with some rather ... upsetting ...
philosophies endorsed, so when that paper is published, we can all see
what is in fact hitting the final print (maybe Ragnarok will NOT occur).

Pan-stems were argued for a long time back. For all extant Tetrapoda,
pan-stems were used for the most inclusive clades including the largest
crown-groups exclusive of one another, as in say traditional groups of
"amphibians," "mammals," "turtles," "lacertilians," "crocs," and "birds."
Each of these gets a pan-stem, usually applied to the traditional name for
the crown: Panlissamphibia, Panmammalia, Pantestudines/Pantestudinata,
Panlacertilia, Pancrocodylia, and Panaves. We've already seen the last in
print in the Ostrom Symposium volume from Yale a few years back. EACH
grouping of pan-stems gets it's own pan-stem: Panarchosauria, Panreptilia,
Pandiapsida, Panamniota, and Pantetrapoda. In some cases, the alternate
arrangements get their own potential pan-stems, such as a turtle+mammal
clade, or a "turtle as reptile" clade, or even a turtle+lacertilian clade.
De Queiroz et al. will be offering multiple pan-stems for the various
possible placements of some living turtles to one another. Now imagine
doing that with snakes ... or birds.

  The MAJOR problem I have with pan-stems is that these are very important
historical points in the history of things DYING and SURVIVING (we honor
what lives today with names, and ignore the various fossils species
outside these relatively few nodes -- they are usually IN the stem, or in
another sister or more inclusive stem, under this philosophy, as is
generally applied in the node-stem triplet application to cladistics).
They have _little_ utility in recognizing diversification, new features or
populations, or essentially record any information about the speciation
and the arrangement of species or populations save those that live today.

  There is a warning in de Queiroz et al. that I hope will be fixed before
it hits critical mass and triggers Ragnarok [yes, yes, it's an extreme]
(and it's the major reason I dislike the quick application of the
PhyloCode): depending on getting the name defined first, it is possible to
define a new, younger name that will immediate gain precedence over any
older, synonym those authors chose not to define. De Queiroz et al.. do
just this in arguing that if the content proves identical between the
pan-stem and the included next most-inclusive node or stem, the pan-stem
should have priority ... it will, after all, be defined first and makes
"more" sense as it is THE most inclusive stem of that included clade.
PhyloCode offers that if someone defines a name with a definition that is
in conflict with historical usage, such as the point brought up between
Mickey and myself recently regarding Hesperornithes and
Hesperornithiformes, then history _be damned_, as well as constancy
through the literature. Out goes history and precedent, because PhyloCode
will seek to "reset" priority. Those who disagree with PhyloCode will
continue to use systems and nomenclature that others following the Code
will not, and neither would be technically _wrong_. The pan-stem
application above that of another stem (say, "Pansynapsida" has priority
over "Synapsida" if someone publishes the definition first for the same
content) is the best example of this.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


        
                
__________________________________
Do you Yahoo!?
Friends.  Fun.  Try the all-new Yahoo! Messenger.
http://messenger.yahoo.com/