The Paleozoic Era. . Geologic periods in Paleozoic record 7% of Earth's history Cambrian, Ordovician, Silurian, Devonian Carboniferous (Miss., Penn.) Permian Boundaries in the major divisions originally defined by changes in the fossil recordStart of the Paleozoic used to be defined by appearance of first easily visible fossils; later lowered to include small shelly fauna; then lowered further to first appearance of complex branching trace fossils. .