Re: Ceratosauria vs. Neotheropoda?

[Tim Williams <tijawi@yahoo.com>]

Mike Taylor <mike@indexdata.com> wrote:

> Oh, and of course option #3: define the groups as
> paraphyletic using a
> formal phylogenetic definition.  I know a lot of
> people have religious
> objections to this, but I'm not one of them.  For
> example, I'd be
> perfectly happy with Proasauropoda = (Sauropodomorpha -
> Sauropoda)
> where Sauropodomorpha = (Saltasaurus not Allosaurus) and
> Sauropoda =
> (Saltasaurus not Melanorosaurus).


In the right hands, this seems like a sensible idea.  Paul (2008) forcefully 
argued for paraphyletic groups to be recognized as formal taxonomic categories 
- in this case, a paraphyletic Iguanodontidae that would include _Iguanodon_ 
and other non-hadrosauroid hadrosauriforms.  


However, in the wrong hands, paraphyletic groupings could spell big trouble.  
For example, I can foresee the BANDits resurrecting the Thecodontia as a 
paraphyletic group that includes the (alleged) ancestors of birds, and claiming 
the validity of Thecodontia is justified under the rules of phylogenetic 
taxonomy.  


I know Thecodontia is an extreme example; but I can see paraphyletic groups 
being misused and abused by people who (whether by accident or design) equate 
evolutionary 'grades' with clades, and come up with all sorts of spurious 
phylogenetic hypotheses and conclusions as a result.  Even Paul (2008) is 
somewhat guilty of this, in comparing the evolution of 'iguanodonts' (a grade) 
with that of hadrosaurids (a clade), as if the two groups represent separate 
halves of a whole (Hadrosauriformes), rather than one being a subset of the 
other.


In short, paraphyletic groups could so easily fall prey to typological 
thinking.  Although cumbersome, terms like "non-hadrosaurid hadrosauriforms" 
and "non-sauropod sauropodomorphs" convey the notion that these are grades in 
evolution.  Paul argued that the practice of avoiding paraphyletic groups was 
discriminatory, because families such as Iguanodontidae were being penalized 
simply because they happened to give rise to a "terminal" taxon 
(Hadrosauridae).  However, this is only a big deal if you're hooked on 
rank-based classification.  By contrast, modern taxonomy (enshrined by 
PhyloCode) is moving away from ranks, and discarding the typological baggage 
that comes with it.


Cheers

Tim