Re: Quick cladistics question

[Adam Yates <zooamy@zoo.latrobe.edu.au>]

On Mon, 31 Mar 1997 Dinogeorge@aol.com wrote:

> 
> << And one other one other question:  how is it decided when one definition
>  has precedence over the other?  As part of my example, you mentioned that
>  under those conditions Dinosauria would become synonomous with Archosauria,
>  and Archosauria would be sunk.  I thought Archosauria had been named first?
> >>
> 
> No, Dinosauria was named first, in 1842; Archosauria was named in 1869.
> Priority by date of publication is the usual way of deciding precedence, but
> there are no formal rules above the family level, and actually anything can
> go. For example, Opisthocoelia and Goniopoda both have publication priority
> over Sauropoda and Theropoda, respectively, but you know which names are
> universally accepted.
> 

This leads to an interesting point. Padian and May (1993) advocate that 
within the Phylogenetic Taxonomic System, priority be given to the first 
name and phylogenetic definition applied to a clade. Sounds fine on the
 surface. Nevertheless there are some difficulties. As an example 
Gauthier employed the taxon "Temnospondyli" for the stem group of the 
modern Amphibia (ie all tetrapods closer to modern amphibians than to 
amniotes). This is all fine, given that most workers thought that 
modern amphibians were derived from dissorophoid temnospondyls. However it 
now seems that there is 
a very real possibility that this phylogeny is "up the 
creek". In short the temnospondyls (as we know them)  may not be a part of 
the tetrapod crown clade at all. Modern amphibians may instead be derived 
from within the assemblage known as the Lepospondyli. This leads to the 
crazy situation of having to kick out ALL traditional "temnospondyls" 
(Eryops, Cacops, Paracyclotosaurus etc.) out of the Temnospondyli and to 
then label another group "temnospondyls" that has NEVER previously been 
included in the taxon.  An analogous situation would be that if the 
dinosauria had been a stem based taxon including all those forms more 
closely related to birds than to crocodiles and it was then discovereed 
that Walker had been right and that Aves was derived from within the 
Sphenosuchia. Then Terrestrisuchus, Dibothrosuchus, Sphenosuchus etc. 
would become dinosaurs while Apatosaurus, Tyrrannosaurus, Triceratops, etc. 
would not be dinosaurs.
Cleary in such extreme situations the old phylogenetic definition should 
be dropped in favour of one that is less likely to upset the content of 
the taxon. For this reason I would suggest that the Temnospondyli be 
defined as Eryops and all taxa closer to it than to Pantylus (a well 
known and well described lepospondyl). BTW de Queiroz and Gauthier never 
actually verbally defined these taxa, the definitions were implicit in 
their classification. Does this still count as a formal definition? If it 
didn't it could solve some problems.

Adam Yates