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Series: Geologic Time Periods in the Paleozoic Era

During the Paleozoic Era (541 to 252 million years ago), fish diversified and marine organisms were very abundant. In North America, the Paleozoic is characterized by multiple advances and retreats of shallow seas and repeated continental collisions that formed the Appalachian Mountains. Common Paleozoic fossils include trilobites and cephalopods such as squid, as well as insects and ferns. The greatest mass extinction in Earth's history ended this era.