The Cretaceous Periods was a time in geological history that lasts from about 144 million years ago to about 65 million years ago. Many huge dinosaurs and flying reptiles lived in the Cretaceous Period. Placental mammals first appeared at this time. At the end of the Cretaceous Period, the dinosaurs became extinct and the period marked the end of the age of dinosaurs what is known as the Great Extinction. Mammals and plants started to evolve in different forms.