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Re: Defining Ornithischia 2

David Marjanovic writes:
 > > could it be that a clade could turn out to be paraphyletic?
 > The definition of "clade" is "an ancestor and all of its
 > descendants". You can see why "clade", "monophylum", and
 > "monophyletic taxon" are synonyms.

... and why "monophyletic clade" is a tautology (and one of my pet

Anthony Docimo writes:
 > so....we might call [Ornithischia, for example] a clade, until such
 > time as there's evidence casting doubt on that status....and then
 > we'd call it a taxon rather than a clade.

Yes, if you're working with a "definition" or ornithischia that
consists of a list of members.  Then if you find that one had an
origin independent of the others then, yes, you would have shown that
your "Ornithischia" is not a clade.  But it's simpler in most cases to
start with a definition of the taxon a being a clade -- for example,
Ornithischia = everything more closely related to Triceratops than to
Tyrannosaurus -- and then ask the question, are these (taxa, genera,
species, specimens) part of this clade?  So if we found that
Stegosaurus is in fact a highly derived Tyrannosaurid theropod, we
wouldn't say "... and therefore Ornithischia is not a clade" but
"... and therefore Stegosaurus is not an ornithischian".

It's dealing the same question either way: it's just a matter of
transforming the question into a more tractable form.

[David, I know you know this :-)  That last bit is for Anthony.]

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor    <mike@indexdata.com>    http://www.miketaylor.org.uk
)_v__/\  Interchicken -- Say it with domesticated edible avians!