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Date: Sun, 17 Jun 2001 22:35:28 +0200
From: David Marjanovic <david.marjanovic@gmx.at>
To: PhyloCode mailing list <phylocode@ouvaxa.cats.ohiou.edu>
Subject: Re: Crown groups (long)
> David Marjanovic (david.marjanovic@gmx.at) 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
> groups.>
>
> 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.
That's what I meant... a whole node[-based clade] of course includes the
most recent common ancestor and all its descendants!
> 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! :)
Should I have used {} instead of () ?
"(*Crocodylus* + *Passer*)" was intended to mean "the most recent common
ancestor of *C.* and *P.* and all its descendants".
> <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.
Which means abandoning the crown group concept? ~:-|
> It is a good idea to name "node" groups based
> on two large-content sub-groups, or three,
sure
> [...] {*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).
Is "<- *Ornithorhynchus*" a qualifying clause, so you speak of a node, or an
external specifier, so you speak of a stem?
In the latter case, this definition doesn't define Theria, because Theria
only has (as far as I know) a (node-based) crown group definition that
equals {Marsupialia + Placentalia} in contents.
> 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
> concensus).
BTW, I think I should have mentioned that the crown group {*Struthio* +
*Passer*} is called Neornithes by people who don't like to call it Aves
(they use Aves for {*Archaeopteryx* + *Passer*}). Anyone interested in the
different definitions and contents should look at
http://dinosauricon.com/taxa/mammalia.html,
http://dinosauricon.com/taxa/synapsida.html,
http://dinosauricon.com/taxa/archosauromorpha.html,
http://dinosauricon.com/taxa/avialae.html and
http://dinosauricon.com/taxa/neornithes.html, where the crown-group
definitions for Mammalia and Archosauria and the non-crown-group definition
for Aves is favored. Archosauriformes in
http://dinosauricon.com/taxa/archosauromorpha.html is called Archosauria by
people who don't like crown groups.
> 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.
(That the crown group Archosauria excludes *Archosaurus* is probably not so
much of a problem because, as far as I know, it was named long after
Archosauria and is poorly known, so Archosauria is not based on it. As far
as I know, *Archosaurus* was named because it was [regarded as] the oldest
member of Archosauria sensu lato, so *Archosaurus* is not an eponymous genus
of anything.)
> 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! :)
Sure, but this won't solve problems like Aves or Mammalia... (Avesuchia and
Avemetatarsalia, see http://dinosauricon.com/taxa/archosauromorpha.html, are
good ideas, in my opinion.)
> <Should the PhyloCode have an opinion about this, and if, which
> one?>
>
> 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. [...]
> 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.
Good idea!
Just how thick will the companion volume have to be? :.-(