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Re: Archaeopteryx



> Colin McHenry wrote:
> 
> 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.

Pretty much.

> > 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.

Not facetious at all -- that is the one underlying assumption of phylogenetic
taxonomy. If you accept this, I don't see how any phylogenetic definition can
be tautological.

> The taxon thus becomes defined by what you leave out,
> rather than what you include.

It can be either way. A node-based clade is defined as all descendants of the
last common ancestor of a set of internal specifiers, and a stem-based clade is
defined as all descendants of the first ancestor of the internal specifier(s)
which is not also an ancestor of the external specifier(s).
 
> 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.

It isn't scientific to have "touchy-feely" taxa. "Oh, I feel like this should
be in _Aves_. No, this should not be in _Aves_." That's completely subjective.
The idea behind phylogenetic taxonomy is to have rigorous definitions which can
be objectively applied to a hypothesized tree.

Personally, I really don't think we have to worry about ceratosaurs being
avians. But if dromaeosaurids, troodontids, oviraptorosaurs, therizinosaurs,
etc. fall in Clade(_Archaeopteryx_ + _Neornithes_), then, yes, they should be
considered avians.

> 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.

This is exactly the old sort of archetypal thinking that PT is supposed to free
us from.

Emotional attachment to older usages is something I think we all share. The
question is whether we really think it's the proper way to go about things. A
system ruled by opinion and authority, or by precedence and explicit formulae?
We can have the latter, but only if we're able to see a clade in terms of its
definition, and not our preconceptions.

=====
=====> T. Michael Keesey <keesey@bigfoot.com>
=====> The Dinosauricon <http://dinosauricon.com>
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