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Re: Defining Ornithischia (was Re:)



David Marjanovic (david.marjanovic@gmx.at) wrote:

> Both "definition" and "diagnosis" are in the Glossary, as follows:

"Definition":

> "A statement in words that purports to give those characters which, in
> combination, uniquely distinguish a taxon [Arts. 12, 13]."

"Diagnosis":

> "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.

This is a reference to the historical ambiguity between "definition" and
"diagnosis" which, really do seem to be synonymous. However, one does purport
to simply collect a unique group of features, but any one of those features may
be shared with a relative, while the other asks to find characters NOT found in
similar taxa. There is also a functional difference.

Interesting, then, that the one that mentions characters not found in other taxa -- thus the stricter one of the two -- is the "diagnosis", not the "definition". I submit that the Committee was somewhat confused on this point and didn't think the distinction mattered.


Phylogenetic taxonomy

Pet peeve alert.

A taxonomic definition may include a character or suite, and is in fact
suggested in the PhyloCode for purposes of trait-based definitions:
"Wing-powered flight in *Vultur gryphus*," for example. Thus, "Loss of
calcaneal spur" can be argued to be a valid point in a definition of a taxon
name, without _being_ the diagnosis, even if it were also part of it.

It can be part of a phylogenetic definition, yes (and would automatically also be part of the diagnosis of the same clade).