::Now you are asking for long term evolutionary trends across several major taxa. If I where to venture a guess, I'd say that herbivory in tetrapods is mainly found in groups that constitute the "major fauna" of any given period. Today the "major fauna" are birds and placental mammals. In the Mesozoicum, "major fauna" would be dinosaurs, in the Paleozoicum it would be various Labyrinthodonts and anapsid reptiles. Now, this is a hunch, at best it is "original research", and as such can't be put in Wikipedia.