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Re: Non-serpentine lacertids (was RE:WHAT'S GOING ON?)



><Had we chosen to keep birds and nonavian dinosaurs separate, we would be
>erecting a nonnatural group by arbitrarily removing those things with
>unambiguous feathers from Dinosauria.>
>
>I'm not sure what you mean here.  You said earlier:
><The names are always arbitrary - this is true for all nomenclatural systems
>ever devised.  But in the phylogenetic system, the groups are not arbitrary
>- they exist (or at least are hypothesized) and are united by common
>ancestry, whether we choose to name them or not.>
>Birds are a group:
>< Birds are still in their own group (Aves) - that this
>group is a member of Dinosauria does not change the distinctiveness of Aves,
>any less than making Aves a member of Vertebrata.>
>so why would 'removing' them be unnatural compared to 'removing' any other
>descendant group?
>


It is never natural to remove a descendent from a supraspecific taxon.
Removing birds from Dinosauria, in my view, makes as much sense as removing
ceratopsians or stegosaurs.  Ceratopsia and Stegosauria are distinct taxa,
but are internested within Dinosauria (as is Aves).

There is a difference between a group of taxa and a name applied to that
group.  The group exists regardless of our perceptions; we are limited by
our imperfect means of discerning groups, but assuming our estimates of
phylogeny actually reflect evolutionary history, the tree represents real
individuals.  By setting an upper bound, we change these ontological
individuals into classes of objects defined by some arbitrary set of
characteristics.  Since evolution does not create fixed classes, taxonomic
schemes relying on classes are not natural.

I've posted some references on this issue in the past; you should be able
to pull them up on the archive.





>Also, I'm wondering about your use of the term 'unambiguous feathers'.  Are
>you saying that feathers (not sure how 'unambiguous' is defined or used) are
>diagnostic of birds by themselves, without regard to other characters?
>Thanks!

No - that was just a quick example pulled from my head.  Groups of birds
are surely diagnosed by more than feathers, and in any case feathers are
not limited to members of Avialae.


chris


----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
1400 S. Lake Shore Drive
Chicago, IL 60605

voice: 312-665-7633
fax: 312-665-7641
electronic:  cbrochu@fmppr.fmnh.org