The Mesozoic era is the geological era that ran from the end of the Paleozoic era, about 251 million years ago, to the beginning of the Cenozoic era, about 65 million years ago. It is commonly thought of as the age of reptiles and is the era in which the dinosaurs flourished. The Mesozoic also saw the appearance of the first mammals and birds. It is generally subdivided into three periods: Triassic, Jurassic, and Cretaceous.