Message 2001-02-0037: Addendum 1b: Clade names from apomorphy names

Wed, 07 Feb 2001 18:11:26 -0600 (CST)

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Date: Wed, 07 Feb 2001 18:11:26 -0600 (CST)
From: "Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>
Subject: Addendum 1b: Clade names from apomorphy names


        Recognizing the popular perception that the previous proposal
(Recommendation 11.8C) *discourages* the use of node- and stem-based
definitions in context, which it was certainly not meant to do, I would like
to withdraw it and replace it with a new version. This version was rewritten
with consideration to the productive comments of all of the participants in
this discussion, and I hope it will be found more amenable. Note that the
examples have been re-written as well.

        Jonathan R. Wagner


Draft 2 of item (Recommendation 11.8C?) for PhyloCode, Article 11

Recommendation 11.8C: It is suggested that, for the sake of logic,
consideration be given to the apomorphy-based class of definition when
converting or otherwise defining clade names derived from apomorphy names.
However, in doing so, the historical and morphological aspects of the name
should be considered. In cases where the apomorphy is not easily defined or
recognized, or is merely a general trait of the group, an apomorphy-based
definition may be less desirable. In cases where the apomorphy constitutes
strong evidence for the monophyly of a group, or is considered a key point
in the decision to formally name the group, an apomorphy-based definition
may be more appropriate. Failure to use an apomorphy-based definition for a
clade named for an apomorphy under any circumstances does not constitute
grounds for rejection or suppression of the name and/ or definition.
	Example 1: Often, genus-rank names under other Codes are based on
distinctive characteristics of the organism. These taxa are named in
accordance with the concept of the obligatory binomial, not necessarily to
recognize the distinctness of any substantial group of species. Although the
name is often formulated in reference to features of the nomenclatural type
(type species), or morphology shared by most or all of the member species,
it is not always the reason for which the group was named. Therefore, the
above should not be construed as a mandate for wholesale conversion of
generic epithets under apomorphy-based definitions.

	Example 2: It could be argued that _Mammalia_ might be best defined using
the specifier "mammary glands." However, this trait has been of little use
in tracing the history of the group, due to the rarity of preserved soft
tissues in fossils. Most traditional definitions/ diagnoses of the clade, at
least in the context of the evolution of the group, have centered on
osteological characters. Recent attempts to impose "crown-clade," other
node-based and osteological apomorphy-based definitions serve to illustrate
that a mammary-gland specified definition is probably not useful to the
systematic community.

	Example 3: The name _Ankylopollexia_ Sereno 1986 refers to the ankylosed
pollex of some ornithischian dinosaurs, a trait which had been considered
restricted to only a very few species. However, its recognition outside of
that group, its functional significance, and the paradoxical interpretation
that this group gave rise to a clade of entirely thumbless animals, probably
all contributed to the decision to name the group. In this context, an
apomorphy-based definition might be most appropriate for the converted name.

	Example 4: _Arctometatarsalia_ Holtz 1994 was redefined by the original
author (Holtz 1996) because of perceived problems with the apomorphy-based
class of phylogenetic definitions. However, given that the original
"concept" of the taxon was centered on the apomorphic configuration of the
pes, which was a common characteristic of a newly hypothesized group of
theropod dinosaurs, it might be more appropriate to retain the original
definition. On the other hand, the group was well supported by other
evidence in the original study, and further study has shown that the nominal
character occurs homoplastically in several other closely related clades. It
therefore might be appropriate, or at least equally justified, to use
another definitional format for the clade.

     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi


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