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Re: Heterodontosaurid with protofeathers

Roberto Takata 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.  For 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".  So 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".  In 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.

> Lets say that we discover that mammals are more closely
> related to birds than ornithischian are, a lot of thing will be
> included within Dinosauria based on node definition - with a theropod
> and an ornithischian species as specifiers.

The definition of Dinosauria (node-based) could be amended to "the least 
inclusive clade containing _Triceratops_ and _Passer_, but not _Crocodylus_ or 
_Lacerta_ or _Testudo_."  Or something like that.  This would ensure that, if 
the clade defined by _Triceratops_ and _Passer_ becomes too expansive, then 
"Dinosauria" would cease to exist.  Under your hypothetical (and far-fetched) 
example, the entire concept of "Dinosauria" would be meaningless anyway, so the 
taxon would collapse.  

However, this amended definition would allow Dinosauria to accommodate things 
like 'lagosuchians' and pterosaurs, should either of these groups be found to 
be closer to Saurischia than to Ornithischia.