Message 2001-06-0050: Re: subscribers

Wed, 02 May 2001 11:24:33 -0700

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Date: Wed, 02 May 2001 11:24:33 -0700
From: chris brochu <>
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

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


Christopher A. Brochu
Assistant Professor
Department of Geoscience
University of Iowa
Iowa City, IA 52242
319-353-1808 phone
319-335-1821 fax


Feedback to <> is welcome!