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Re: Archaeopteryx



----- Original Message ----- 
From: "T. Michael Keesey" <mightyodinn@yahoo.com>
To: "Mailing List - Dinosaur" <dinosaur@usc.edu>
Sent: Monday, June 16, 2003 12:04 PM
Subject: Re: Archaeopteryx


>>
> > > Aves is defined as Archaeopteryx + Neornithes. So whatever will be
found
> > > by further examination of fossils of Archaeopteryx,
> > > Archaeopteryx will always stay inside Aves, always being a bird.
>
> So replace "bird" with "avian".
>
> > Am I the only person who finds this to be a bit tautological?
>
> If you have a better definition for _Aves_, let us know.
> (Incidentally, some researchers do use a different definition, a crown
group
> equivalent to _Neornithes_.)

I can't really offer a definition, only a description and a list of included
taxa.  With higher taxa, I don't think that its possible to define them
without reference to the latter.  But then, that's probably agreeing with
what you are saying.


> I think it would only be tautological if there were some other component
to the
> definition of _Aves_, but the only assumption it really rests on is that
> _Archaeopteryx_ and _Neornithes_ share a common ancestor.

Well, without wanting to be facetious,  all organisms are believed to share
a common ancestor.  The taxon thus becomes defined by what you leave out,
rather than what you include.

At the moment, the _Archaeopteryx_ plus Neornithes definition is acceptable
because it doesn't include any animals that most people would never accept
to be birds.  But what if you found, in some future study, that
_Archaeopteryx_ is a highly convergent form derived from more basal therapod
stock, so that to maintain this definition you would need to accept, say,
ceratosaurs as birds.  This might make a few people choke on their
cornflakes.  I suspect there would then be a push to redefine Aves so that
it remains basically similar to its current conceptual status - i.e.
includes mostly the same members that it does today.

You could extend the argument to the ridiculous to illustrate the point - if
_Archaeopteryx_ was a highly convergent basal archosaur, Aves would include
crocodiles.  Or a highly convergent lepidosauromorph - Aves would then
include snakes.  Again, it is unlikely, in this hypothetical situation, that
the definition stated above would be retained.  Instead, it would be
redefined in such a way that the overall concept of Aves remained reasonably
stable.

The logical conclusion is that, even under the practice of cladistic
nomenclature, the _definition_ used for a higher taxon is not actually the
ultimate conceptual definition of the group.  Rather, the group is defined
(in the 'hearts and minds' of scientists and lay persons alike) in terms of
the balance of included taxa, and the phenotypic description.  The cladistic
'definition' is simply a convenient standard of delineating the taxonomic
boundaries of the group with reference to a recent, commonly accepted
phylogenetic theory.  However, if the pertinent conclusions of that
particular phylogenetic theory change significantly, then the group will be
redefined.  This fulfills the interests of common sense and taxonomic
stability.  My impression of the phylogenetic systematics of dinosaurs is
that this is happening all the time.

One is then left to wonder if, even in the days of modern phylogenetic
systematics, the essence of major taxa have changed significantly since
their original Linnean definitions.

Don't agree?  Ask yourself whether, in the case of _Archeopteryx_ being
found to be an outgroup to the Saurischia, you'd accept Aves > Saurischia.
If you would, you deserve credit for being consistent in your approach, but
I would suggest that you've got the cart well and truly in front of the
ungulate.

Cheers
Colin

P.S. After just re-reading this before sending, I just want to clarify that
the 'you' in the above paragraph is rhetorical rather than being directed
towards Mike!