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Re: more cladistics

On Thursday , 6 September 1995, Tom Holtz wrote:

>>           Yes, the position of a taxon on a tree determines its status.
>>What determines the position of a taxon on a tree? SHARED DERIVED
>>CHARACTERS that are inherited via common ancestory.  You said so, yourself.

>This is a point a lot of people miss.  In cladistic taxonomy, the DEFINITION
>of a taxon is based on position: nothing more, and nothing less.  If, with
>further discovery, a derived character previously used to *diagnose* (but
>not define) a taxon is found to have a different distribution (e.g., if
>feathers are found on Coelophysis), the definition of the taxon DOES NOT
>CHANGE.  In the example above, the taxon Avialae will retain the same
>defintion regardless of the discovery of feathers on Coelophysis.  The
>character diagnosis will change, but the defintion will remain the same.

That's the beauty of cladistic taxonomy.  It allows you to argue about
characters and their distribution, not the definition of taxa - the thing which
people who don't confer to it seem to get so worked up about half the time :o)

Steve Salisbury
Vertebrate Palaeontology Laboratory
School of Biological Sciences
University of NSW, Australia