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>chris brochu wrote:
>> But the IF is irrelevant, because perforate acetabula DO diagnose
>> Dinosauria, and ankylosaurs ARE dinosaurs.  This is why character states
>> are useful for diagnoses, but lousy for definitions.
>This isn't the way things are supposed to work, Chris -- this is making
>less sense,
>not more, as we go along.

Then I apologise on behalf of the systematics community, because that's how
we're doing things.

There must be more to the story here, because what
>you've said is quite simply illogical.

No, that's more or less it.  A perforate acetabulum is currently one of
those characters diagnosing Dinosauria.  Ankylosaurs, however, don't have
this feature, despite the wealth of evidence making them dinosaurs
nevertheless.  It's no different than recognizing derived colubrid snakes
as tetrapods, despite the complete absence of an appendicular skeleton.

>"After the distribution of shared derived characters within a cladogram is
>determined, those shared derived characters which unite taxa into a
>stem-based or
>node-based taxon are used as the diagnosis of that taxon." -- from _The
>Dinosaur_, p. 105, Holtz and Brett-Surman's chapter on taxonomy and
>You say that the perforate acetabulum is a diagnostic feature for
>Dinosauria --
>that is, it's one of the apomorphies that show Dinosauria is a real group,
>and also
>one of the apomorphies that can be used to identify a member of that
>group.  You
>also say that ankylosaurs don't have this apomorphy.  Yet, ankylosaurs are
>classified as dinosaurs.  Why?

I think you're reading the quote from Holtz too literally - perhaps Tom can
add some comments.  But basically, characters diagnose a clade if, in the
most parsimonious tree for the taxa, they fall out at the root.  There is
no rule, anywhere, that says you can't have reversals further up the tree -
which is why snakes are tetrapods, despite the fact that digits diagnose a
clade including them.

I recommend reading that Rowe article I posted last week on definition and
diagnosis, as well as a few real cladistic analyses published in the
literature.    Tom's analyses of theropod relationships are a good start.

>Is or isn't the perforate acetabulum diagnostic for _all_ Dinosauria?

Depends on how you use the word "diagnostic."  It diagnoses Dinosauria, but
it doesn't diagnose all of Dinosauria.

If it isn't,
>why is it still in the list of diagnostic features?  If it is, then how
>can you be
>sure that ankylosaurs are dinosaurs, when they don't have all the diagnostic
>features for dinosaurs?

They have some, plus they have diagnostic features making them
ornithischians, characters making them thyreophorans, etc....

Again, a diagnosis is not just for the sake of identification, and
reversals are a fact of evolutionary life.

 Are you assuming ankylosaurs are dinosaurs because past
>workers said they were dinosaurs?  Or are there perhaps other features
>that are
>more useful than the acetabulum for identifying something as a dinosaur?
>To me, saying "X is a dinosaur even though X doesn't have all the diagnostic
>features of a dinosaur" is as much a contradiction as saying "6 is a prime
>number."  (A ^ ~A) --> B; in words, "from a contradiction, anything
>follows."  Your
>systematics will never make any logical sense as long as you allow casual
>violations of the laws of logic.

Another reason why computer programming is not the best analogy for


Christopher Brochu

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566