The Paleogene period is a geological time frame that lasted from about 66 to 23 million years ago. It follows the mass extinction event that ended the Cretaceous period and is divided into three epochs: Paleocene, Eocene, and Oligocene. During this time, mammals and birds diversified significantly, filling ecological niches left vacant by the extinction of the dinosaurs. The climate during the Paleogene was generally warm, with tropical conditions prevailing in many areas. This period saw the development of modern flora and fauna, including the first appearances of many mammal groups, such as primates and whales. The Paleogene set the stage for the evolution of more complex ecosystems in the following Neogene period.