although herbivory (reliance on diet entirely of plants) was long thought to be a mesozoic phenomenon, evidence of it is found as soon as the fossils which could show it. within less than 20 million years after the first land plants evolved, plants were being consumed by arthropods. herbivory among four-limbed terrestrial vertebrates, the tetrapods developed in the late carboniferous (307 - 299 million years ago). early tetrapods were large amphibious piscivores. while amphibians continued to feed on fish and insects, some reptiles began exploring two new food types: the tetrapods (carnivory) and plants (herbivory).