Re: Defining Ornithischia (was Re:)

[David Marjanovic <david.marjanovic@gmx.at>]

> 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).