The Paleogene is a geologic period that spans from about 66 million years ago to 23 million years ago. It is part of the Cenozoic Era and is divided into three epochs: the Paleocene, Eocene, and Oligocene.
The Paleocene epoch began after the mass extinction event that marked the end of the Cretaceous period. During this epoch, the Earth's climate was relatively warm and the continents continued to drift towards their current positions. Mammals began to diversify and fill the ecological niches left vacant by the extinction of the dinosaurs.
The Eocene epoch is known for its warm climate and the presence of vast tropical rainforests. The early part of the Eocene was particularly warm, with the poles being ice-free. This epoch saw a significant diversification of mammals and the emergence of modern groups such as primates and hoofed animals.
The Oligocene epoch saw a cooling trend, leading to the development of a more temperate climate. This cooling had significant effects on the Earth's flora and fauna, with the spread of grasslands and the decline of forests. Many modern mammal groups continued to evolve during this time.