The first (542–251 Ma ago) of the three eras of the Phanerozoic. The Cambrian, Ordovician, and Silurian periods together form the Lower Palaeozoic sub-era; the Devonian, Carboniferous, and Permian the Upper Palaeozoic sub-era. During the Palaeozoic, two major orogenies occurred: the Caledonian during the Lower Palaeozoic, and the Hercynian in late Palaeozoic times. The faunas of the Palaeozoic are noted for the presence of many invertebrate organisms, including trilobites (Trilobita), graptolites (Graptolithina), brachiopods (Brachiopoda), cephalopods (Cephalopoda), and corals. By the end of the era, amphibians and reptiles were major components of various communities and giant tree-ferns, horsetails, and cycads gave rise to extensive forests.
http://www.geologypage.com/2013/10/paleozoic-era.html Palaeozoic Era.