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On Wed, 23 Feb 2000, T. Mike Keesey wrote:

> Jaime Headden (I think) noted that _Pteranodon_ has a "pygostyle" a little
> while back on this list. I also learned today while browsing around Dave
> Peters' Pterosaur Homepage that _Drepanosaurus_ (a prolacertiform) has an
> odd claw-like structure at the end of its tail that is probably fused
> vertebrae (_Drepanosaurus_ has a lot of odd skeletal structures, actually
> ...)
> So the fusing of vertebrae at the end of the tail seems to have happened
> independently in at least four lineages of archosauromorph, and may have
> had different purposes in each (_Pteranodon_'s certainly did not support
> feathers; _Drepanosaurus_' looks like something to grab with; who knows
> what Nomingis' was for?).
> Which of these structures should properly be called pygostyles?

I don't know if you were asking a rhetorical question, or if you were
soliciting an answer.  Since I don't know the answer, I'll ask a question
Why can't they all be called pygostyles?

> Pygostylia is a subclade of Aves...

Since I wasn't aware of this either, I can only assume that this clade
doesn't have a character-based definition, but rather is defined in a more
stable fashion either by node or by stem.

In any case, I don't see any conflict.  Arctometatarsalia is (now) both a
condition and a clade, and they are mutually exclusive concepts (well,
sort of...not all arctometatarsalian ankles belong to dinos that are part 
of the clade, and not all members of the clade must by necessity have
*expressed* arctometatarsalian ankles).
Why cannot the same dichotomy be applied to pygostyles?