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Re: side-note -- Re: 11th specimen of Archaeopteryx



On Wed, Nov 2, 2011 at 1:21 AM, Brian Hathaway <hammeris1@att.net> wrote:
> Soooo ... it is NOT a bird, then?
> I know, I know, no consensus.
> Maybe it should always be a transistional creature -
> dinosaur/bird - a dird!

There's two issues here: the issue of nomenclature, and the issue of phylogeny.

On nomenclature, there is no standard definition of "bird". It's a
vernacular word that is unambiguous for living taxa, but has a fuzzy
boundary when you start looking outside the crown group. "Bird" is
often equated with the formal name _Aves_, which, as a formal name,
cannot have a fuzzy boundary. But, again, there is no standard
definition. The zoological code of nomenclature (ICZN) does not
mandate definitions for taxa ranked above the family group. The
phylogenetic code (PhyloCode) mandates definitions for all taxa, but
it is not active yet.

_Aves_ has been used for the following groups:
1) The crown group (the last common ancestor of all living birds, and
all descendants of that ancestor).
2) The _Archaeopteryx_ node (the last common ancestor of Archaeopteryx
and living birds, and all descendants of that ancestor).
3) A branch-based clade excluding dromaeosaurids (the first taxonomic
unit[s] ancestral to living birds which was/were not also ancestral to
dromaeosaurids and all descendants thereof).
4) A branch-based clade excluding dromaeosaurids, troodontids,
oviraptorids, and other groups traditionally not considered avian.
5) The clade of feathered dinosaurs (the first taxonomic unit[s]
possessing feathers homologous with those of living birds, and all
descendants thereof).
6) The total group (the first taxonomic unit[s] ancestral to living
birds which is/are not also ancestral to other extant taxa [e.g.,
crocodylians], and all descendants thereof).

#1 is called _Neornithes_ by many people. It is likely to be the
definition under the PhyloCode, because the code recommends that
neontological names be used for crown groups. This serves to limit the
number of unjustified inferences that people make about members of the
stem group (the total group minus the crown group).

#2 is commonly used, but, it's unstable, because we're not sure if
_Archaeopteryx_ is closer to living birds or to dromaeosaurids (or
just outside the bird-dromaeosaurid clade). It's such a primitive form
that this question may never be fully resolved.

#3 is usually called _Avialae_ (although that name has also been
applied to the clade of winged dinosaurs with powered flight), and
even people who use definition #1 for _Aves_ sometimes still use
"bird" to refer to this clade instead.

#4 is the same as #3 in most phylogenies (or nearly so), but this
definition was explicitly crafted to preserved the traditional usage
of _Aves_ among paleontologists. Notably it is agnostic as to whether
_Archaeopteryx_ is a member.

#5 is difficult to apply, and is also called _Avipinna_ anyway.

#6 is not popular at all, as it would make ceratopsids, titanosaurs,
etc. avians. This clade has been called by many other names:
_Avemetatarsalia_, _Ornithosuchia_ (abandoned), _Ornithodira
(incorrectly), _Panaves_. Under the PhyloCode it will likely be called
"_Pan-Aves_". (But if it is given a different name, the informal name
"pan-Aves" will still be possible.)

On phylogeny, as I mentioned before, the position of _Archaeopteryx_
is unstable due to how close it is to an ancestral form.

So once we discover the precise phylogeny and get everyone to agree on
the nomenclature, we'll have a concrete answer! ;)

-- 
T. Michael Keesey
http://tmkeesey.net/