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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:

  "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.>

  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.

  Phylogenetic taxonomy distinguishes definitions based on the mathematical
form of its terminology, which is a departure from the rather fuzzy use from
before its intruduction so many decades ago.

  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.

  Cheers,

Jaime A. Headden
http://bitestuff.blogspot.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


       
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