Cenozoic Era. The Cenozoic Era began 65 million years ago with an asteroid impact that killed off a majority of the dinosaurs and ends at the present day.The Cenozoic is commonly divided into three periods: 1  Paleogene (65.5 to 23.03 million years ago).2  Neogene (23.03 to 2.6 million years ago).enozoic Era. The Cenozoic Era began 65 million years ago with an asteroid impact that killed off a majority of the dinosaurs and ends at the present day.