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Re: 11th specimen of Archaeopteryx



It's not a perfect analogy, but think of this in terms of how the term
'planet' was defined a few years ago.

If you asked an astronomer in 2005 whether or not Eris was a planet,
the answer would be that it depended on what you meant when you said
"planet," which had no formal definition outside convention. This is
equivalent to "avian" now. Is *Archaeopteryx* an avian? Impossible to
say because nobody can agree what that word even means.

Someday the Phylocode will define the word for us. Will everyone agree
to follow that definition? Probably not, just as not all astronomers
accept the IAU definition of planet. But at least it's something
concrete.

The next step will be figuring out whether or not *Archaeopteryx*
meets the parameters of the definition or not. Depending on where the
boundary is set, this may be controversial for decades if not
centuries, given shifting phylogenetic positions.

But, that's all about "avian," not "bird". There will never be a
universal definition of "bird," outside the dictionary definition,
though those often hinge on the scientific definition. This is from
the OED online:

Bird:
"Any feathered vertebrate animal: a member of the second class (Aves)
of the great Vertebrate group, the species of which are most nearly
allied to the Reptiles, but distinguished by their warm blood,
feathers, and adaptation of the fore limbs as wings, with which most
species fly in the air."

Is *Archaeopteryx* a bird in this definition? It seems to define the
"Class Aves" as any vertebrate with the following characteristics:
1. endothermic (probably to some degree)
2. feathers (check)
3. wings (check)

So, according to the OED definition *Archaeopteryx* is a bird and no
phylogenetic shifts can change that. It is also a member of the "Class
Aves" but, not necessarily of the clade Aves. Note that under this
definition, *Microraptor* and presumably all aviremigians are also
"birds", because it says that "wings" are not necessarily used in
flight.

But, again, this is a layman's definition of a colloquial term, not
something scientific.

Matt


On Wed, Nov 2, 2011 at 11:51 AM, Mike Keesey <keesey@gmail.com> wrote:
> 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/
>