From Academic Kids
The Paleozoic includes six geologic periods; from oldest to youngest -- the Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian and Pennsylvanian in North America), and Permian. It extended from roughly 542 MYA to roughly 251 MYA. 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 time when the continents were beginning to be dominated by large, relatively sophisticated reptiles and relatively modern plants. 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, known as the Permian extinction. Modern practice sets the older boundary at the first appearance of a distinctive trace fossil called Phycodes pedum.
Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Rodinia and at the end of a global ice age. (See Varanger glaciation and Snowball Earth). Throughout the early Palaeozoic, 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 called Pangea, which included 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 near 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.
References and further reading
British Palaeozoic Fossils, 1975, The Natural History Museum, London.