[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]


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 
not more, as we go along.  There must be more to the story here, because what
you've said is quite simply illogical.

"After the distribution of shared derived characters within a cladogram is
determined, those shared derived characters which unite taxa into a stem-based 
node-based taxon are used as the diagnosis of that taxon." -- from _The Complete
Dinosaur_, p. 105, Holtz and Brett-Surman's chapter on taxonomy and systematics

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 
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 still
classified as dinosaurs.  Why?

Is or isn't the perforate acetabulum diagnostic for _all_ Dinosauria?  If it 
why is it still in the list of diagnostic features?  If it is, then how can you 
sure that ankylosaurs are dinosaurs, when they don't have all the diagnostic
features for dinosaurs?  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."  
systematics will never make any logical sense as long as you allow casual
violations of the laws of logic.

-- Jon W.