An introduction to Paleozoic This era comprises two significant events in animals' life history. Suddenly, at the beginning of this era animals divided to different groups and the animals' kingdom appeared within a couple of million years. At the end of this era, the biggest extinction of history occurred and about 90% of various marine animals disappeared. The reasons of these two events are not clear, and this subject needs more research. At middle Paleozoic, animals, plants and fungi increased and insects commenced to fly. Paleozoic is more than half of Phanerozoic. Paleozoic takes about 300 million years. Six main continents were available within this period of time. Each of these parts belongs to certain sections of present continents. For instance, the present west margin of North America has been along equator bearing an eastern-western direction in early Paleozoic. Whereas, Africa was in the South Pole. Paleozoic continents include huge mountains in the margins and regression of shallow seas occurred. Calcareous sediments are good evidence of periodical continental seas. Some of the Paleozoic rocks have connective significance, for example most of the calcareous rocks of this period are important in making buildings and industrial activities. Coaly sediments of Western Europe and Eastern America were formed in Paleozoic.