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Re: The Two Principal Dinosaur Clades Defined

In a message dated 95-12-11 13:55:29 EST, Thomas_R_HOLTZ@umail.umd.edu (th81)

>Although Dinogeorge, Greg Paul, and Bob Bakker will disagree (as they
>should: the "consensus" should always be challenged), all recently published
>analysis of dinosaurian systematics with an explicit data matrix and so on
>has found that the Sauropodomorpha is the sister taxon to Theropoda within a
>monophyletic Saurischia.
>However, as Bakker and Galton pointed out in 1974, and many since, there are
>a LOT of potential synapmorphies between "prosauropods" and ornithischians.

The problem here is not that Sauropodomorpha is a sister taxon to Theropoda;
it _is_, as the basal clade of Phytodinosauria. That's why cladistic analyses
keep turning up the sister-group relationship of Sauropodomorpha and
Theropoda. The problem is to recognize Ornithischia as a subclade or sister
group of Sauropodomorpha with a more recent common ancestor than
Sauropodomorpha + Theropodomorpha. Using the 17 characters for Dinosauria
given in the _Marasuchus_ paper (Sereno & Arcucci 1994, I think--or was it
the other one in the same JVP issue?), Sauropodomorpha and Ornithischia are
virtually indistinguishable.