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Paleozoic. One of four (sometimes more) Geologic Eras. The division of time into Eras dates to the 18th Century. anyone know who defined the eras and when?. The eras are the highest division of the Geologic Time Scale. Palaeozoic is an accepted alternate spelling for Paleozoic.

The Paleozoic includes six Geologic Periods. From oldest to youngest -- Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian and Pennsylvanian in North America), and Permian. It extended from roughly 545 million years before present to roughly 245 million years. It follows the PreCambrian Era and is followed by the Mesozoic Era.

The Paleozoic covers the time from the first appearance of abundant, hard shelled fossils to the period when the continents were first dominated by large, relatively sophisticated reptiles and relatively modern plant types. The lower (oldest) boundary was classically set at the first appearance of creatures known as trilobites and archeocyathids. The upper (youngest) boundary is set at a major extinction event 300 million years later. Modern practice sets the lower boundary at the first appearance of a distinctive feeding trace fossil called Phycodes pedum.

Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Rodinia, and the end of a global ice age. See Varanger glaciation and Snowball Earth. Throughout the Early Paleozoic, the earth's landmass was broken up into a substantial number of relatively small continents. Toward the end of the Era, the continents gathered together into a supercontinent Pangea including most of the Earth's land area.

At the start of the Era, life was confined to bacteria, algae, sponges and a variety of somewhat enigmatic forms known collectively as the Ediacarian fauna. A large number of body plans appeared nearly simultaneously at the start of the Era -- a phenomenon known as the Cambrian Explosion. There is some evidence that simple life may already have invaded the land at the start of the Paleozoic, but substantial plants and animals did not take to the land until the Silurian and did not thrive until the Devonian. Although primitive vertebrates are known from very close to the start of the Paleozoic, animal forms were dominated by invertebrates until the mid-Paleozoic. Fish populations exploded in the Devonian. During the late Paleozoic, great forests of primitive plants thrived on land forming the great coal beds of Europe and Eastern North America. By the end of the Era, the first large sophisticated reptiles and the first modern plants (conifers) had developed.