Re: Question(s) about Cladistics and PhyloCode

[Tim Williams <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:

> > A group such as Aves (birds) is _defined_ to be (for example) all 
> > descendants of the most recent common ancestor of _Archaeopteryx_ and the 
> > common sparrow;
>
> And the ancestor itself, too! Otherwise the taxon would consist of two 
> separate clades. :-)
>
> (As an aside, this would be a rather careless definition.

This is Sereno's definition, which uses a node-based definition of Aves 
anchored on _Archaeopteryx_ and _Passer_.  I much prefer this definition of 
Aves to Gauthier's, in which Aves is much less inclusive and limited to the 
crown-group.

> What if old Archie is a troodontid? 5 years ago just about everyone would 
> have gently smiled at this suggestion -- nowadays... search the archives 
> for "Buitreraptor" after taking an aspirin or two. No stability of content 
> can be expected from this definition.)

Who cares?  :-)  You can't blame evolution for our short-sighted 
definitions.  If _Buitreraptor_ falls inside the group bounded by 
_Archaeopteryx_ and _Passer_, then it's a member of Aves.  Whether one 
chooses to call _Buitreraptor_ (or even _Archaeopteryx_) a "bird" is 
subjective, given that "bird" is an imprecise term.  You can call 
_Buitreraptor_ anything you want, in the vernacular sense.  Just don't call 
him late for dinner.  (Speaking of which, I wonder if the 'La Buitrera' 
snake _Najash_ was a popular item on _Buitreraptor_'s menu...?)

> > The downside is that the content changes with phylogenetic hypothesis: for 
> > example, if a new analysis recovers tyrannosaurids closer to modern birds 
> > than to _Archaeopteryx_, then we have to say that _T. rex_ is a bird.
>
> Exactly.

Well, in this situation, we have to say that _T. rex_ is a member of Aves 
(sensu Sereno).

Cheers

Tim