RE: Greg Paul's new (or newly named) iguanodonts

[Tim Williams <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:


> Now this is interesting -- a phylogenetic definition for the name of a
> paraphyletic taxon! That's the first time such an idea is presented
> seriously.

Well, some fella named "Mike Taylor" suggested it (though not yet in print) ...

http://dml.cmnh.org/2007Sep/msg00027.html

... and he was serious.  :-)

BTW, I have no conscientious or ideological objections to naming paraphyletic 
taxa, in principle.  But I suspect that in practice these paraphyletic groups 
could become more trouble than what they're worth.  By definition, paraphyletic 
groups could only be 'diagnosed' by characters they *don't* have, rather than 
characters they do have.  But the distinction could be lost by those who are 
unfamiliar with phylogenetic nomenclature.  Thus, these paraphyletic groups 
could be mistaken as clades, and used (and abused) as such in 
phylogenetic/evolutionary discussions.  They would be not only misleading, but 
potentially dangerous too.  

For example, the paraphyletic group "Thecodontia" might be revived in 
discussions on bird origins, especially by the tiny minority who advocate a 
non-dinosaurian origin of birds (and who, as it is, tend to play fast and loose 
with phylogenetic concepts).  We could see a paper containing sentences like 
"Birds (Aves) probably evolved from Thecodontia, given that primitive birds 
share the following characters with thecodonts..."  Naming paraphyletic groups, 
no matter how practical-minded and honorable the motive behind it, could 
further confuse the distinction between derived/apomorphic and 
primitive/plesiomorphic characters.  I think the current practice of using 
(admittedly cumbersome) terms such as "thecodont-grade archosauromorph" or 
"non-dinosaurian archosauromorph" helps ensure that groups like "Thecodontia" 
continue to be *excluded* from phylogenetic discussions.  

Cheers

Tim


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