One of the main geological eras of Earth, lasting from 542 to 251 million years ago. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.
This span of time saw the appearance and development of all of the major groups of organisms (phyla and most of the classes) of Earth life. The early Paleozoic was dominated by invertebrates, the middle Paleozoic by primitive fish, and the late Paleozoic by amphibians and reptiles. The period ended with the largest mass extinction in the history of Earth, and was followed by the Mesozoic Era.