Message 2001-06-0122: Re: Crown groups

Sun, 17 Jun 2001 20:43:14 -0500

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Date: Sun, 17 Jun 2001 20:43:14 -0500
From: kritosaurus <>
To: David Marjanovic <>
Subject: Re: Crown groups

On crown group nomenclature:

    In my opinion, yes, crown clades are difficult to deal with. The
PhyloCode makes two statements which are relevant: Recommendation 10A
(pointed out by David) advocates minimal disruption of current use, while
recommendation 11A advocates respecting the historical sense of the term.
Naturally, the first use of most taxa EXCLUDES fossil forms, and this is
closest to a crown-clade application. Is this the historical sense? Does
excluding fossil forms disrupt the activities of most systematists (who are,
for the most part, neontologists)?

    One point, which is not much of a revelation to most systematists, is
that the conservative and typological nature of the Linnaean system has
resulted in the "pigeonholing" non-crown-clade taxa into crown clades. This
is partly, if not wholly, the result of the rank system; if Archaeopteryx is
not a member of (crown) class Aves, we must erect a new class for it, or
expand Aves to accommodate it. Thus, we should not be too quick to place
much emphasis on the historical inclusion of fossils in established
groups... there really was nowhere else for them to go.

    Without a crown clade definition, one is reduced to determining which
taxa have the "necessary and sufficient" morphology to be in the group,
essentially a character-based, typological argument thinly veiled as
phylogenetic definition. This point has been made by Dr. Gauthier in
reference to Aves. The criteria used may in fact be historical, but they
have also changed over time, especially with the discovery of new forms. Of
course, there is the further complication of transitional forms, and the
need to "draw a line" in a continuous spectrum.

    This is an extension of the lumper/splitter dichotomy to the extreme;
indeed, much of the conflict I have seen in the conversion of traditional
taxa has been of this type (which "basal" forms should be members?). Once
made, this distinction may then become trivial when the specifiers fail to
encompass the "type" one is attempting to capture (either through changing
ideas of morphology, phylogeny, or discovery of new forms).

    I was once informed by a theropod dinosaur savant that, if flying
dinosaurs outside of the Archaeopteryx + Passer clade were ever discovered,
it would  be deemed necessary to redefine Aves to include them. I doubt the
ICPN, when formed, will be amenable to spending its time reviewing every
such case.

    On the other hand, a crown clade definition has EXACTLY the same
problem... the "necessary and sufficient" criterion is simply
non-morphological, being extant status. No matter how carefully defined, a
standard node-based definition may potentially fail to include all extant
members (this time, thought, new discovery and misinterpretation of the
character may be deemed less likely). There are other ways to define crown
clades, and I have yet to explore these in sufficient detail.

    So, with what is in the Code now, is there a clear way to go? No. Should
there be? Maybe. Will we all agree to one way? Not likely. Will agreeing to
one method turn off a segment of the systematic community? Yes.

    David is entirely correct, "setting priority to zero" will make things
difficult (at least one worker, while public decrying priority, has
nonetheless rushed papers into print to secure it). I prefer trying to get
ideas into print before Implementation, so that perhaps workers in each
field can reach a consensus. I'm not sure that will work. I am not looking
forward to the rush of papers, myself. I believe most groups have a number
of touchy nomenclatural issues about which half of the affected parties will
feel unhappy with whatever nomenclature results. Compromise would be nice,
but I cant say I'm entirely optimistic about it. I spend my time trying to
find compromises for ornithischians.


Jonathan R. Wagner
9617 Great Hills Trail #1414
Austin, TX 78759

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