Paleozoic period:
What we observe is that herbivory is a great idea. The Pelycosaur Edaphosaurus is reasonably common in the Late Paleozoic but it is not like Volkswagens on a city street. By the early Permian, however, immediate descendants of the herbivorous pelycosaurs, the early dicynodonts, are just all over the place. There are masses of them. We can finally talk about an ecosystem that looks like a modern ecosystem with herds of tetrapod herbivores in place and carnivores to feed upon them, a classic energy flow pattern working its way up from the primary producers to the herbivores to the carnivores. The ecosystem starts to look modern.
The group that initially radiated was in the synapsid lineage, our ancestors, descendants of the pelycosaurs, and included a whole bunch of herbivores called dicynodonts. (Most dicynodont herbivores were so ugly that only a mother could love them. I think you had to be ugly to be a dicynodont, such as this character with all of its facial horns, or somebody like Moschops or Lystrosaurus--this is one of the more complimentary reconstructions of Lystrosaurus that I have seen. Dicynodonts ranged from creatures the size of an average dog up to things like Lystrosaurus that were large pig sized. The group was entirely herbivorous and occurred in herds. We can speculate about their social behavior and all the rest of that but that is not the focus of where I am headed. Their presence in turn underwrote a radiation of synapsid carnivores, the cynodonts
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