Preceded by the Archeozoic and Proterozoic, the Paleozoic is the third era in the Earth geological history. It began about 600 million years ago and ended about 220 million years ago. The Paleozoic is divided into seven geological periods: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. From fish to reptiles, the Paleozoic is characterized by the consolidation of animal life as sea and land animals throve in warm moist weather, with marsh and swampy territory. During this era, the oldest mountain system, such as the Appalachians were formed.