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Re: correct this "definition"



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/