Re: Dinosaur Taxonomy (was Re: Re: dinosaur flatware)

[pharrinj@PLU.edu]

On Thu, 11 Jan 1996 Dinogeorge@aol.com wrote:

> In a message dated 96-01-11 17:22:23 EST, you write:
> 
> >Tom Holtz sent me a near-flame when I suggested that herrerasaurs were 
> >paraphyletic.  He directed me to studies showing staurikosaurs and 
> >herrerasaurs to constitute a monophyletic group.  
> 
> All the known herrerasaurians fit into a monophyletic group; but what about
> the common ancestor of Herrerasauria and Theropoda? Whichever clade you put
> it in becomes paraphyletic--so I guess a cladist wouldn't put it in either
> (and would, I suppose, leave it a paraspecies all by itself). I classify the
> common ancestor in Herrerasauria, which makes it paraphyletic.

But how can you classify the (hypothetical) common ancestor of 
Herrerasauria and Theropoda in either group?  If you use a typological 
classification, you cannot place it in the Herrerasauria, for instance, 
because it necessarily lacks the defining characteristics of that group.  

If, on the other hand, you use a descent-based classification, the common 
ancestor cannot be included in the Herrerasauria because it is not 
descended from the common ancestor of the known herrerasaurs.

The same arguments hold for placing the common ancestor in the Theropoda 
as well.

Thus, in any grouping of two taxa, there must actually be three taxa:  
the two in question, plus the species ancestral to both, a species with 
the defining characteristics of the higher-level taxon (whether 
typological or descent-based) but lacking those of either lower-level taxon.

Hey, I don't make the rules (and I don't necessarily like them), but it 
looks like that's the way it's gotta be.  Evolution is a messy business.

     Nick Pharris
     Pacific Lutheran University
     Tacoma, WA 98447
     PharriNJ@plu.edu