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RE: How herbivory developed among dinos




On Mon, 5 Jun 2000, Thomas R. Holtz, Jr. wrote:

>   From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> philidor11
> 
> 
>   Some time ago there were mentions on the list that various people were
> working on this topic.  Has anything been published?  Is the current
> hypothesis that there was a single radiation, apparently starting in South
> America, or did herbivory more likely develop among different groups in
> different places?
>   Thanks!
> 
> 
>   It looks as if herbivory twice early in Dinosauria (and many more times,
> independantly, later on within advanced maniraptoriforms, esp. birds).
> Ornithischia and Sauropodomorpha seem to have evolved the condition
> convergently, as all the numerical phylogenetic analyses to date place
> sauropodomorphs closer to theropods than to ornithischians.  (In an
> alternative, appealing, but as yet unsupported scenario, Ornithischia and
> Sauropodomorpha form a clade, Phytodinosauria, which would support a single
> origin of herbivory).

It is interesting to note that finding sauropomorphs were closer to
theropods than to ornithischians has promted everyone to assume that
herbivory evolved twice. However the interpretation
where herbivory evolved once at the base of the Dinosauria and theropods
reversed to the ancestral, pre-dinosaurian condition is equally
parsimonious (2 steps). The leaf shaped anterior teeth of the so-called 
basal theropod, Eoraptor, are intriguing under this interpretation (a
holdover from an herbivorous ancestor?). I think intuitively
that that scenario has seemed less likely so it has been ignored.
Putting aside the above hypothesis, I think there is growing
evidence that Herrerasaurids
are basal saurischians outside of the theropod-sauropodomorph clade and
this would then indicate that the basal dinosaur was herbivorous after
all.

cheers

Adam Yates