The Mesozoic Era Period is divided into three periods: the Jurassic, the Triassic, and the Cretaceous. The Jurassic Period lasted from 199.6 million to 145.5 million years ago and was characterized by the emergence of dinosaurs and the evolution of birds. The Triassic Period lasted from 251.9 million to 199.6 million years ago and saw the emergence of the first mammals and the evolution of modern conifers. The Cretaceous Period lasted from 145.5 million to 66 million years ago and saw the extinction of the dinosaurs.