The Mesozoic and Paleozoic ERAS are two of the three major divisions of geologic time. The Mesozoic Era, which began about 252 million years ago and ended about 66 million years ago, is divided into the Triassic, Jurassic, and Cretaceous Periods. The Paleozoic Era, which began about 541 million years ago and ended about 252 million years ago, is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.