Re: definition of diapsida via "Feduccia's delusion"

["T. Mike Keesey" <tmk@dinosauricon.com>]

On Sat, 1 Dec 2001, Rob Schenck wrote:

> Susan E Evans in The Early HIsotry and relationships of the Diapsida,
> in Benton's Phylogeny and Clasification Blah blah blah says
>
>  "Four derived character states...can be used to diagnose diapsids:
> upper temporal fenestra; suborbital fenestra...lower temporal
> fenestra; cervical vertebrae longer than mid dorsals." she then notes
> that afifth character state of locked tibio-astragalar joint might be
> used, but doesnt use it in her study.She also says that the two major
> groups within diapsida are Araeoscelidia and neodiapsida.  I will not
> pretend i know what and araeoscledian is.
>
> i hope this is helpful, and ya, i just got this book a week or two ago
> so i keep refering to it.  I am confused tho, how is definition of a
> group different than diagnosis of a group?

What you gave is a diagnosis. A definition would be something like "the
most recent common ancestor of _Araeoscelis_, _Lacerta_, and _Crocodylus_,
plus all of its descendants". If the above characters were found to be
primitive for this clade but not present in the clade's outgroup, then
they would be considered diagnostic of the clade.

In phylogenetic taxonomy, the definition remains stable, while the
diagnosis and content may change as our understanding of the phylogeny
improves.

> Does rank apply to super species classifications in the phylogenetic
> system?

No. There have been attempts, but a simple illustration will show that
absolute ranks do not mix with a system that uses only monophyletic taxa:

ancestral species--descendant species A--x
                `--descendant species B---->further descendants

If this clade is given the rank of, say, class, then descendant species A
must represent an order, a family, a tribe, and a genus -- 4 ranks (not
even including suborder, superfamily, etc.) for what is only a species.
Furthermore, it is impossible to assign the ancestral species to any
monophyletic order, family, tribe, or genus.

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