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Re: correct this "definition"



Is there any point in naming the "final common
ancestor" in the more important clades? Perhaps this
might be useful as teaching tool; the name-form could
be chosen so as to make it obvious that "this
'organism' is one of the hypothetical founders". The
name could carry other information, such as (e.g.)
best-guess age, taxonomic relationship to the
"Universal Ancestor" or clade status relative to
living descendants.

Heh. If this idea is found to be useful, I vote for
something memorable and easy to type; like "Bob" or...

Don

--- "T. Michael Keesey" <keesey@gmail.com> wrote:

> On Wed, Apr 16, 2008 at 10:39 AM, 
> <hammeris1@bellsouth.net> wrote:
> > Dougal Dixon writes in his encyclopedia "Complete
> Book of Dino's":
> >
> >  Dinosauria:
> >  The ruling reptiles are characterized by:
> >  * the number of bones in the skull
> >  * the presence of a flange on the upper arm bone
> that held powerful muscles
> >  * three or fewer finger bones in the fourth
> finger
> >  * Three or more vertebrae fixed to the hip bones
> >  * a hole rather than a socket in the hip for the
> leg bone
> >  * a small ball-like head on the thigh bone
> >  * a strong joint between the foot bones and the
> bones of the hind leg
> >
> >         Note he has this on his dinosaur
> classification / tree page,
> >  but aren't the "ruling reptiles" the archosaurs? 
>  Anyway, if some of
> >  this is contrary to the current thought on what
> makes a dino a dino,
> >  please point it out.  This came out in 2006 and
> is the most current
> >  dino book I've picked up out of the 20 or so I
> have.
> 
> I think this is a point that bears repeating: this
> is a diagnosis, not
> a definition. What makes a dinosaur a dinosaur is it
> being descended
> from (or identical to) the final common ancestor of
> _Iguanodon_,
> _Megalosaurus_, and _Hylaeosaurus_ (or something
> along those lines--as
> previously mentioned, there are a few minor
> variations on this
> definition). Diagnostic character states are
> something we use to
> figure out whether or not a given organism is
> descended from that
> ancestor. They help us to apply the definition, but
> they are not part
> of the definition.
> 
> For example, look at the character state "three or
> more vertebrae
> fixed to the hip bones". As Tom Holtz pointed out,
> some Late Triassic
> forms do not have this character state. Does this
> mean that they are
> not dinosaurs? No, because there is evidence that
> supports them being
> descended from the final common ancestor of
> _Iguanodon_,
> _Megalosaurus_, and _Hylaeosaurus_. That character
> state is probably
> not diagnostic for _Dinosauria_, although it may be
> diagnostic for
> certain major subclades. If we based the definition
> on that character
> state, it might indicate a polyphletic group--not
> good.
> 
> Even the character states that are diagnostic are
> only diagnostic for
> the time being. Chances are that they appeared at
> different times
> before the dinosaurian ancestor. We could find a new
> specimen that has
> all of these character states except for the loss of
> the postfrontal
> bone, or a specimen that has none of these character
> states except
> that is has three phalanges in the fourth manual
> digit. These
> specimens would probably not be dinosaurian (unless
> there was some
> kind of reversal), but would feature certain
> formerly diagnostic
> character states.
> 
> To sum up: the diagnosis may fluctuate as new data
> comes to light, but
> the definition remains the same.
> -- 
> T. Michael Keesey
> Director of Technology
> Exopolis, Inc.
> 2894 Rowena Avenue Ste. B
> Los Angeles, California 90039
> http://exopolis.com/
> --
> http://3lbmonkeybrain.blogspot.com/
>