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In a message dated 96-09-29 23:38:20 EDT, firstname.lastname@example.org (Jonathan R.
>> You misunderstand. I am arguing against placing saurpodomorphs at
>> the base of Saurischia by noting that it would make theropods
>> secondarily carnivorous
> No, you're not. Your statement was:
> "Indeed, if sauropodomorphs are placed at the base of "Saurischia"
> between Ornithischia and Theropoda, then (most parsimoniously) the common
> dinosaurian ancestor was herbivorous and theropods were secondarily
> Which is simply not true. You have two trees where this happens,
> from my example, one in which Dinosauria becomes herbiverous (1 step) then
> Theropoda becomes carniverous (1 step), or Ornithiscia and Sauropodomorpha
> become independantly herbiverous (1 step each for 2 steps). These two trees
> are equally parsimonioius.
I wasn't counting steps here, just reversals. If you have the same number of
steps (2) but one is a reversal (as in the reacquisition of carnivory by
theropods), it is less parsimonious than if neither step is a reversal.
Besides, other trees are possible. If the common-ancestral dinosaur was
insectivorous (e.g., a small dino-bird), then uniting sauropodomorphs with
ornithischians as herbivores gives one step, and making theropods carnivores
gives one other step; but if sauropodomorphs and ornithischians are not
united, you have herbivory developing twice, carnivory in theropods once
more, giving three steps.