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Re: Quick cladistics question



Jonathon Woolf wrote:
>(warning: ignorant question ahead) 
        If you had any idea how many of these I've asked...

>Is this actually the way the Dinosauria is defined, cladistically?
        I'm hoping Dr. Holtz will clear this up for us (no hinting here...).
        
>That looks like a crown-based definition (I think that's the right term),
>which makes no sense for a group like Ornithischia which exists only as
>fossils.
        You're *very* close. It is a node based definition. You are correct,
it isn't a crown clade because one of the anchor taxa is not currently
alive. Before anyone asks, refer to the posts about a week ago on the types
of definitions in Phylogenetic Taxonomy.

>I would have thought that Dinosauria was defined as the first
>animal to have the identifying synapomorphies for Dinosauria (whatever
>those are) and all its descendants.   
        This is an apomorphy based definition. For obvious reasons
(convergence, what constitutes the "first occurance" of a trait, etc.) these
are not looked upon kindly by some folks. Most clades, at least on the
Sauropsid limb of the tree of life, tend to be defined on stem or node based
terms.
----------------------------------------------------------------------------
      Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
            Web Page:  http://faraday.clas.virginia.edu/~jrw6f