Amphibians arose in the Carboniferous period with some favoring a terrestrial environment while others remained aquatic but in both case growing to tremendous sizes. As the Earth became dryer during the Permian period large amphibians struggled in favor of animals with hard shelled eggs, such as Dimetrodon, that didn't require an aquatic environment. But amphibians made a resurgence in the wetter Triassic.
—David Foss