- of, relating to, or denoting the era between the Precambrian eon and the Mesozoic era. Formerly called primary.
The Paleozoic lasted from about 570 million to 245 million years ago, its end being marked by mass extinctions. The Lower Paleozoic sub-era comprises the Cambrian, Ordovician, and Silurian periods, and the Upper Paleozoic sub-era comprises the Devonian, Carboniferous, and Permian periods. The era began with the first invertebrates with hard external skeletons, notably trilobites, and ended with the rise to dominance of the reptiles