The Tertiary period is before the Cretaceous and after the Quaternary and lasted from 5.3 to 65.5 and is part of the Cenozoic era. The extinction event at the close of the Cretaceous Period wiped out the dinosaurs, large reptiles, and many other species. This left room for new animals to develop.

The mammals became the dominant animals. In fact, the Cenozoic Era is often called the Age of Mammals. Most of the main groups of mammals were present by the Eocene Epoch. With the dinosaurs and other large reptiles gone mammals grew in size, numbers and diversity. They filled ecological niches in the sea on land and in the air.

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