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Re: [Re: Reptilia]

On 7 Dec 1998 archosaur@usa.net wrote:

> > The cladistic definition was mentioned before, but I'll mention it again:
> > the most recent common ancestor of chelonians (turtles), _Sphenodon_
> > (tuatara), squamates (lizards and snakes), and crocodylians, plus all of
> > its descendants.
> The only problem I have with the cladistic definition is that it does not tell
> one what the diagnostic features are.  All it says is the most recent common
> ancestor.

The definition is not concerned with features. That's up to the diagnosis.

Phylogeny-based definitions are far more stable than feature-based
definitions. What if the diapsid skull condition were found to have
evolved twice? Diapsida would become a polyphyletic taxon. By the
phylogenetic definition, however, it remains a valid taxon (although its
membership may change) no matter what.

> So what constitutes being the common ancestor?

Being the most recent critter which gave rise to all of the anchor taxa,
natch. How can you tell if a particular organism is the MRCA? You can't.
But, if an animal were found to have all of the synapomorphies of
Reptilia, but none of the synapomorphies of any reptilian subclades, and
occured before any other reptiles, there'd be a possibility. 

--T. Mike Keesey                                   <tkeese1@gl.umbc.edu>
THE DINOSAURICON                http://www.gl.umbc.edu/~tkeese1/dinosaur
(shameless plug) vote at http://www.coolsiteoftheday.com/csoty/98reference.html