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Re: Heterodontosaurid with protofeathers



On Wed, Mar 18, 2009 at 11:43 PM, Tim Williams <tijawi@yahoo.com> wrote:
>
>> Well, the dramatic content expansion is not a peril associated
>> exclusively to apomorphy-based definitions. Any phylogeny-based
>> definition: apomorphy, branch or node are subjected to such events.
>
> Branch- and node-based definitions can be defined such that they preclude 
> these kind of situations from arising. ÂFor example, a node-based definition 
> for a taxon could be framed such as "the least inclusive clade containing A 
> and B, but not C, D or E". ÂSo if C or D or E fall inside a clade that has A 
> and B as internal specifiers, then the taxon is no longer viable.
>
> For stem-based definitions you could say "The most inclusive clade including 
> A but not B, or W or X or Y or Z". ÂIn this case the taxon would persist, 
> but the multiple external specifiers helps limit the content of the taxon if 
> a huge shift in topology were to occur.

Actually, the apomorphy-based definition of _Avifilopluma_ is
self-destructing (or, to be more precise, auto-synonymizing).
"'Avifilopluma' is defined as the clade stemming from the first
*panavian* with feathers homologous (synapomorphic) with those of Aves
(Vultur gryphus Linnaeus 1758). 'Feathers' here refers to
hollow-based, filamentous, epidermal appendages produced by
follicles." So if the trait extend beyond _Panaves_ (=pan-Aves, i.e.,
the avian total group), then it becomes a junior synonym of that name.
Of course, that doesn't affect the present situation, but it shows
that such possibilities are not limited to node- and branch-based
definitions.

I'm not sure I see a problem that needs fixing here, though. What else
would the name "Avifilopluma" refer to? It doesn't make any sense to
give that particular name a node- or branch-based definition. And if
you don't have a use for the name ... don't use it!
-- 
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies
Glendale, California