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Re: definition of dinosaur



In a message dated 96-02-22 12:11:17 EST, Thomas_R_HOLTZ@umail.umd.edu (th81)
writes:

>Padian and May (1993) formally defined "Ornithischia" as Triceratops and and
>all taxa closer to Triceratops than to Saurischia.

Since Saurischia is the same thing as Dinosauria, because ornithischians
descended from certain primitive saurischians, this definition is vacant.
There are _no_ taxa closer to _Triceratops_ than to Saurischia, once you
realize that _Triceratops_ descended from/is nested within
Saurischia/Dinosauria. Other people may have different opinions, of course;
but I've examined all the characters I can find that supposedly unite and
distinguish Saurischia from Ornithischia, and they're all questionable:
either plesiomorphies, or apomorphies shared with some prosauropod taxa, or
just plain ill-defined.