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Re: Re: definition of dinosaur



>Mickey Rowe said (02/22/96; 6:27p):
>
>>You have to remember to make a distinction between *definition* of a
>>taxon and *diagnosis* of a taxon.  
>
>OK, I've hit this problem before.
>
>I think I have it now:
>
>     1.  A diagnosis, in any taxonomic scheme, tells you what is actually
>         there.

A diagnosis is a list of the characters that, er, characterize the taxon.
However, do to the nature of evolution, there are almost always SOME members
of that taxon that do not demonstrate all of those characters due to
vestigialization, new adaptations, reversals, etc.  (For example, four limbs
with digits is part of the diagnosis of Tetrapoda, despite the fact that
amphisbaeneans, snakes, whales, etc.  do not show this) (For another
example, an amniotic egg is part of the diagnosis for Amniota, although
therian mammals have since modified that reproductive system).

>     2.  A cladistic definition does not.

A cladistically defined taxon is one in which the definition is based soley
on ancestry (being evolutionists, I think we all accept common descent with
modification as our starting point).  Additional discoveries will almost
always show that our diagnosis (characters) and composition (list of taxa)
of a given taxon are incorrect, but there will always exist a clade (even
one species) which fits that defintion.

>     3.  Cladistic definitions are better than diagnosis-based 
>definitions.

See above.


Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084