The 12 periods of life from the first hard shelled animals over 500 millionyearsago. Paleozoic Era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian. Mesozoic: Triassic, Jurassic, Cretaceous. Cenozoic: Paleogene, Neogene, Quaternary. Based on Chemical Elements by Mayoketchup.