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Re: Defining Ornithischia (was Re:)

Again, I must disagree.  Any statement that defines one thing in terms
of others is a definition.  That's what the word means.  You can't
just hijack the word and make it mean what you want it to mean -- the
kind of definition that you happen to prefer.

The ICZN is ambiguous about this. "Definition" is not in the index of the ICZN (http://www.iczn.org/iczn/includes/page.jsp?nfv=&booksection=index), while "diagnosis of taxon" is (in Recommendations 13A and 13B and in the Glossary), but "definition" also occurs, clearly in the sense of "diagnosis", in Art. 13.1.1 (To be available, every new name published after 1930 must [...] be accompanied by a description or definition that states in words characters that are purported to differentiate the taxon [...]) and also in 12.1, 12.2.1, 12.2.6, 13.4 and 13.5. The mentioned Recommendations use "diagnosis" in that sense, however, and 13A even explains "diagnosis" as "that is to say, a summary of the characters that differentiate the new nominal taxon from related or similar taxa".

Both "definition" and "diagnosis" are in the Glossary, as follows:
"A statement in words that purports to give those characters which, in combination, uniquely distinguish a taxon [Arts. 12, 13]."
"A statement in words that purports to give those characters which differentiate the taxon from other taxa with which it is likely to be confused."
They seem to be treated as synonyms, then.

BTW, I haven't managed to find a statement on whether definitions/diagnoses can be changed or not. There is a lot in the ICZN that is implied rather than made explicit...

So?  Witness all the changes in the phylogenetic definition of
Theropoda as chronicled at:

That doesn't stop any of them from being definitions.

OK... they are definitions, but none of them is valid because the PhyloCode is not yet in effect. So, in a sense, they aren't really definitions, only suggestions for definitions. :-)