The Paleozoic and Cenozoic Eras are two of the three major divisions of geologic time. The Paleozoic ERA, which began about 541 million years ago, is characterized by the emergence of life in the oceans and the colonization of land by plants and animals. The Cenozoic ERA, which began about 66 million years ago, is marked by the extinction of the dinosaurs and the rise of mammals. Both eras are divided into smaller units of time, such as epochs and periods.