Message 2001-06-0118: Re: Crown groups

Sun, 17 Jun 2001 12:06:40 -0700 (PDT)

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Date: Sun, 17 Jun 2001 12:06:40 -0700 (PDT)
From: "Jaime A. Headden" <>
Subject: Re: Crown groups

David Marjanovic ( wrote:

<In (at least vertebrate) paleontology there is an
ever-recurring problem -- how to apply classical names like Aves
and Mammalia. One idea is to define them as crown groups, i. e.
as nodes that include all living members of the traditional

  This is not the essential definition of a crown group, which
is defined as at least two living taxa, their most recent common
ancestor, and of its descendants. It's a node-based taxon whose
specifiers are still extant. See below:

<This excludes basal members that have traditionally always been
considered members: e. g. *Archaeopteryx* and nearly all other
Mesozoic birds from Aves = (*Passer* [sparrow] + *Struthio*
[ostrich]), *Morganucodon*, *Megazostrodon*, *Sinoconodon* etc.
from Mammalia = (placentals + marsupials + monotremes),
*Proterosuchus*, *Archosaurus* etc. from Archosauria =
(*Crocodylus* + *Passer*).>

  Actually and firstly, if using the above definition, only
living taxa would be allowed in the group, largely making it
polyphyletic. Any fossil bird, mammal, archosaur would be
excluded from the crown group; thus, dinosaurs and
psuedodontorns are not archosaurians or avians! :) 
<Currently the crown-group definitions of the mentioned taxa
have priority; however, many paleontologists simply reject their
usage and often the whole crown-group concept. No consensus is
in sight.>

  Not so. All we'd need to do is stop naming groups based on
living membership. It is a good idea to name "node" groups based
on two large-content sub-groups, or three, so I have always been
in favor of utilizing {*Ornithorhynchus* + *Macropus* + *Equus*}
as the definition for Mammalia, as this group as good, strong
stability. {*Macropus* + *Equus* <- *Ornithorhynchus*} also has
strong stability, thus is Theria, a crown group only as both
internal specifiers (anchors) are still extant; using an
external specifier only makes the usage more stable, less prone
to content fluctuation or reformatting, as happened in the
groups included in Titanosauria (Dinosauria).

  Names in Aves may utilize the "ordinal" groups and their
eponyms as anchors as the refinement process of avian
sub-relationships continues. But because the arrangement is
still contested, I would hold of on defining things until such a
time as the relationships become stable (or there is a

  Elsewhere it has been mentioned that in any group whose name
stems from an included taxon, such as Ornithosuchia, the name
should have as an internal specifier (or have as the anchor) the
eponymous genus, in this case *Ornithosuchus*, so that the name
will stay with the genus. One might want to step away from this
type of taxon--name-formation, as it can become inherently
unstable. Be imaginitive, formulative in your names! :) [I don't
mean to come off as rude...]

<Should the PhyloCode have an opinion about this, and if, which

  I think a glossary defining proper definition in nomenclature
and usage should be applied to the draft if and when published.
This will allow formulators to refer to proferred concepts and
work from there. This glossary should be discussed. Recently, I
ran across a usage for the word "subequal" and got two different
definitions, but the majority was that it meant almost as equal,
if more or less. This was the first time I saw the term defined,
and had used it as [syntax = element _a_ is subequal in length
to element _b_] before that. Oh well. Time to start codifying
definitions in nomenclature to avoid other such makeshift
definitions. And though others have published on specific uses,
it would be a good idea to publish a compendium of
committee-decided definitions, even if they have [word =
definition a, definition b] format, which can be separately
cited from the Phylocode itself.

Jaime A. Headden

  Where the Wind Comes Sweeping Down the Pampas!!!!

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