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> Jon, I know you better than to think that you don't understand the concepts
> of ancestral states and secondary reversals!

True.  :-)  I think you also know me well enough to know that sometimes
my questions aren't as foolish as they sound.  Y'see, what you and
Buchholz and Holtz have all done here is confirm that I'm right, even as
you're trying to show that I'm wrong.  There's more to the story then
the simple, clean picture they're trying to jam down my throat.  There
_are_ other ways of telling that an animal is a dinosaur, even if it
doesn't have a perforate acetabulum.  So saying that the diagnosis of
Dinosauria consists of an inturned femoral head
and a perforate acetabulum is, at the very least, incomplete.  You say
it's diagnostic for _basal_ dinosaurs?  Fine with me.  You say its
_absence_ in a fossil that otherwise looks like a dinosaur is diagnostic
of Ankylosauria?  Also fine.  Anything along those lines is fine with
me, as long as you remember the qualifiers.  But please, don't try to
tell me it's diagnostic for _all_ members of Dinosauria when not all
members of Dinosauria have it.  

(Basing a diagnosis on just two characteristics doesn't seem very safe
to me, anyway.  If an animal could lose one of the diagnostic traits by
evolution, it could lose both of them.  Or you could have another
evolutionary line acquire one or both by convergence.  Anyone who doubts
this is possible may want to examine the case of the Nimravidae, a
subgroup of mammalian Carnivora whose members share several major
adaptations with felids.  Mammal paleontologists have wrangled for years
over whether nimravids are felids or not.  Current opinion is that they
are not, that all those traits are examples of convergence.  Which is
probably why Michael Benton lists no fewer than eight diagnostic traits
for Dinosauria in chapter 16 of _The Complete Dinosaur_.)

> With primitive dinosaurs
> known (_Eoraptor_, _Pisanosaurus_, etc.), we know that a diagnostic
> characteristic of _primitive_ dinosaurs is a perforate acetabulum.  Thus,
> any descendant of those basal dinosaurs is, by definition, a dinosaur.

So we have a second diagnostic procedure here: anything that can be
diagnosed as belonging to a subgroup of Dinosauria also belongs to
Dinosauria.  Reasonable.

-- Jon W.