The Cretaceous Period 146 to 65 Million Years Ago The Cretaceous is usually noted for being the last portion of the Age of Dinosaurs , but that does not mean that new kinds of dinosaurs did not appear then. It is during the Cretaceous that the first ceratopsian and pachycepalosaurid dinosaurs appeared. Also during this time, we find the first fossils of many insect groups, modern mammal and bird ...