Paleozoic
Paleozoic, earliest era of the Phanerozoic Eon, comprising the Lower Paleozoic (570–400 million years ago), containing the Cambrian, Ordovician, and Silurian periods; and the Upper Paleozoic (400–225 million years ago), containing the Devonian, Carboniferous (Mississippian and Pennsylvanian), and Permian periods.
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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Olympic Mountains to Palermo