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Regarding Phytodinosauria, Saurischia, and so forth
I remember a time not so long ago when Dinosauria -- defined as Saurischia +
Ornithischia -- excluded such taxa as _Herrerasaurus_, _Staurikosaurus_, and
lagosuchians as stem-group genera below the Saurischia-Ornithischia
divergence. But as work progressed, even the most die-hard cladists finally
agreed that, yes, _Herrerasaurus_ should be included within _Saurischia_, at
the base of _Theropoda_. The new "eodinosaur" became _Eoraptor_. Now the word
is out that _Eoraptor_, too, belongs in Saurischia below _Herrerasaurus_ on
the branch leading toward Theropoda + Aves.
It is just a matter of time, I think, before the cladists realize how derived
even lagosuchians were with respect to the as-yet-hypothetical dinosaurian
common ancestor, and the lagosuchians take their proper place on the dinosaur
cladogram, below _Eoraptor_ but above the Saurischia-Ornithischia divergence
on the branch toward Theropoda + Aves. (The image of rats in a maze,
exploring each incorrect path before obtaining the cheese, comes to mind.)
It is also merely a matter of time before the cladists realize that
Ornithischia itself does not fit anywhere on the branch from the common
dinosaurian ancestor toward Theropoda, but instead belongs above Sauropoda
and Prosauropoda on the other branch. That's the branch Bakker called
Phytodinosauria. Saurischia will then be understood as a useful paraphyletic
group: the clade Dinosauria minus the clade Ornithischia.
Segnosaurs, with their fully tetradactyl prosauropod-like feet, which cannot
possibly be derived from any tridactyl theropod foot with a near-vestigial,
retroverted digit I, will eventually be moved back into Phytodinosauria where
they belong. The theropod "synapomorphies" enumerated in the _Erlikosaurus_
skull, for example, will then be correctly understood as homoplasies.
Likewise for the humongous tridactyl segnosaur forelimbs, derivable without a
heck of a lot of trouble from prosauropod-like forelimbs.
Remember--you heard it here first (unless you've been reading Bakker and Paul
and have made your own commonsense deductions).