An herbivore is any animal that eats plants. There have been herbivores for a long time, but not as long as carnivores. The first animals that did not feed on other animals were photosynthesizers and chemosynthesizers, but not actual herbivores. Herbivores truly evolved when blue-green algae grew on the ocean floor and provided a means of energy gain without killing other animals or needing to stay near volcanic vents. When the Cambrian Explosion took place, herbivores and carnivores took on many new forms such as trilobites, echinoderms, halwaxiids, anomalocaridids, worms, etc. The herbivores took on forms such as worms, arthropods, and a few crustaceans and fed on the algae. When fish evolved during the Silurian, millipedes took to the land and fed on Cooksonia, so that they would not have to return to water for periods of time. Many fish were carnivorous, except for filter-feeders like Pteraspis, and eventually fish crawled onto land in the form of amphibians and started life there. In the late Carboniferous, amniotes appeared in the form of small reptiles. These amniotes were anapsids, synapsids, and diapsids. While diapsids and synapsids were primarily carnivorous, the anapsids were herbivores. Pareiasaurs, Procolophonids, Acleistorhinds, and turtles fed on land and sea vegetation and grew hard plates called scutes to protect them. At this time the herbivores took the opportunity to become the largest land animals and herbivores would remain the largest land animals until the early Paleocene. When the Permian-Triassic Extinction wiped out all of the large animals, the diapsids took over. Anapsids would never reach sizes larger than Archelon of the Cretaceous. As the diapsids diversified into dinosaurs, crocodiles, lepidosaurs, etc., the synapsids became small, carnivorous animals. Plateosaurus, Pisanosaurus, and Antetonitrus were herbivores of the Triassic. As the carnivores continued to evolve, so did the herbivores. Even a few crocodiles were herbivorous, like Lotosaurus, Aetosaurus, and Simosuchus. The herbivorous dinosaurs were Ornithopods, Sauropods, Thyreophorans, and Ceratopsians, and the Sauropods were the largest herbivores to ever walk the planet. Amphicoelias was 200 ft long and was the largest animal and herbivore to live on Earth, but it suffered from the side effects of large size: the larger an animal is, the more susceptible it is to die from environmental changes. If the environment produces less food one year, the climate heats or cools, or a new animal is introduced into the environment, the largest animals are most likely to die. When the Cretaceous-Paleogene Extinction killed off the large diapsids, the synapsids took over. They became the largest land and sea herbivores and carnivores on the planet. Land herbivores were horses, elephants, rhinos, hippos, pigs, cows, sheep, deer, etc. and herbivores still beat carnivores in size on land. In the water, Basilosaurus was the largest animal and was carnivorous, but it died due to climactic cooling. When the Homo sapiens took over, they became omnivorous and killed off the large herbivores and carnivores using complex hunting techniques and tools.