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Re: Quick cladistics question

Jeff Poling wrote:
>    Here's a cladistics question I do not recall ever being addressed:
>    Dinosauria is defined as the most recent common ancestor of _Triceratops_
> and modern birds.

(warning: ignorant question ahead) 

Is this actually the way the Dinosauria is defined, cladistically?  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.  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.   

-- JSW