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Date: Wed, 02 May 2001 11:24:33 -0700
From: chris brochu <cbrochu@blue.weeg.uiowa.edu>
To: phylocode@ouvaxa.cats.ohiou.edu
Subject: Re: subscribers
> Why not just call them lophotrochozoans, and let them remain an informal >taxon, at the very least until we can demonstrate whether or not it is based >on symplesiomorphies rather than synapomorphies. OK, I should probably keep my mouth shut (or my fingers still), but I'm addicted to making an ass of myself, so here are some thoughts. One of the purposes behind phylogenetic nomenclature (or any kind of nomenclature) is precision. It does us little good if the names we use have fluctuating meanings. Informal names will always have imprecise meanings. The classic example, of course, is "Dinosauria" - one common objection to formally defining Dinosauria in a way that might include birds is that a standard, colloquial, informal meaning already exists for "dinosaur." Any survey of children's books on dinosaurs or kits of toy "dinosaurs" should dispel that myth rather quickly. "Dinosaur" can mean "nonavian dinosaur," "big extinct reptile," "big extinct animal," or "any extinct organism." I have seen plesiosaurs, mammoths, Dimetrodon, sabertooth cats, and even trilobites listed as "dinosaurs." If the informal meaning is so precise, why is it so hard to find a consistent lower bound for the group in the media? Another example (closer to my heart) is "crocodile." I am always being asked how old the oldest "crocodile" is. The answer is, "it depends on what you mean by 'crocodile.'" Do you mean crown-group crocodylian? Member of the "genus" Crocodylus? A crocodylid? Mesoeucrocodylian? Crocodyliform? Crocodylomorph? All of these have, at one time or another, approximated the informal term "crocodile," and all of these have very different temporal, geographic, and morphological properties as currently understood. But when I say Crocodylus, or crocodylian, or mesoeucrocodylian, my meaning suddenly becomes much more precise. I could go on with "plant," "bug," or "rat." I am aware that the contents and properties of formally-defined taxon names will fluctuate over time. This is not the same as having an imprecise meaning - the meaning of a taxon name will remain precise, even if our understanding of its membership, diagnosis, or any other property changes. chris ------------------------ Christopher A. Brochu Assistant Professor Department of Geoscience University of Iowa Iowa City, IA 52242 christopher-brochu@uiowa.edu 319-353-1808 phone 319-335-1821 fax