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Date: Sun, 17 Jun 2001 14:24:03 -0700 (PDT)
From: "Jaime A. Headden" <qilongia@yahoo.com>
To: david.marjanovic@gmx.at, phylocode@ouvaxa.cats.ohiou.edu
Subject: Re: Crown groups (long)
David Marjanovic (david.marjanovic@gmx.at) wrote:
<Should I have used {} instead of () ?>
You can use any enclosing objects you wish :). I just have a
tendency to use swung brackets "{}" instead of parentheses.
Square and corner brackets work, too ... [] and <>. Just be
consistent in usage or you will get confusing.
<"(*Crocodylus* + *Passer*)" was intended to mean "the most
recent common ancestor of *C.* and *P.* and all its
descendants".>
Understood. I wrote that based on what you'd earlier said
suggesting a crown group meant _only_ the living descendants, a
group which is inherently polyphyletic unless it is so recent
that it has _no_ extinct forms from which to draw upon...
I wrote:
<<...All we'd need to do is stop naming groups based on living
membership.>>
<Which means abandoning the crown group concept? ~:-|>
It doesn't help anything, just a useful set of taxa that we
can confortable draw data from, instead of ill-preserved
fossils, in many cases. You can use the taxa, but the concept is
not helpful, as it suggests the utility of the group is only by
its living membership. Pinnipeds (not to suggest they are
monophyletic) have an interesting fossil record, as do extant
sloths, and to suggest they should be limited to extant forms
would be in ignorance of the fact that, however convergent,
there are several taxa that so much more complicate the
phylogeny, even complete, than extant forms can contemporize.
*Bradypus* and *Choloepus* are not even in the same
_superfamily_ in most phylogenies ... otariids and odobenids and
phocids might as well all be superfamilies considering the
described fossil taxa that proceed the modern forms [wait, they
are...].
<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.>
The PhyloCode makes no record of the possibility of using an
exclusive specifier in a node-based taxon, thus making the clade
a node and stem, together. Art. 11 suggests utility for
specifiers, but does not specify flexibility, but rigidity. I'm
all for that. As Mike Keesey has suggested here, it may be
appropriate for formulate taxa as per concepts of the original
formulation, so that a group will have a certain content, only
under certain phylogenies. This actually stabilizes unusual
groupings = (Ornithomimoidea Sereno, 1999 {*Mononykus* Perle et
al., 1993b + *Ornithomimus* Marsh, 1886 <- *Passer* Lin. 18??} )
So that Sereno's name only applies if the two inclusive
specifiers are mutual sister groups of a clade that does not
include birds as a descendant, which is the formulation.
<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.)>
By eponym, one is using the name of a genus as the basis for
the clade name in such a fashion that there is only a slight
modification to the name in reference to it's "group" status, as
in adding a suffix. Not using part of the name's vernacular with
further terms. *Passeriformes* has a stem modifier, so does.
Unless there was an "Avesuchus" to worry about, "bird-crocs"
does not count.
=====
Jaime A. Headden
Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
Where the Wind Comes Sweeping Down the Pampas!!!!
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