The beginning of the Tertiary period is marked by the extinction event that killed off the dinosaurs.
The Tertiary period is a geological time period that started 65 million years ago and lasted until 2.588 million years ago. It encompasses five different epochs, beginning with the Paleocene epoch, then the Eocene epoch, the Oligocene epoch, the Miocene epoch and the Pliocene epoch.
Paleocene
The first epoch of the Tertiary period is the Paleocene epoch, which lasted from roughly 65 million years ago until 56 million years ago. What distinguishes the beginning of the Paleocene epoch from the end of the Cretaceous period, which immediately precedes it, is the extinction event that wiped out the dinosaurs and many other forms of life on the planet, which is referred to as the K-T boundary.
Eocene
The Eocene epoch came directly after the Paleocene epoch, beginning roughly 56 million years ago and lasting until 34 million years ago. The event that marks the beginning of the Eocene epoch is the first signs of modern mammals in the fossil record, which are the mammalian ancestors of the modern human being.
Oligocene
The next epoch in the Tertiary period is the Oligocene epoch, which began about 34 million years ago and lasted until about 23 million years ago. The beginning of the Oligocene epoch is identified by a large extinction, probably from another impact event, followed by a significant change in the ecosystems of many parts of the world.
Miocene
The Miocene epoch is the fourth of the Tertiary period and immediately follows the Oligocene epoch. It began roughly 23 million years ago and lasted until 5.332 million years ago. Unlike most changes in epoch there is no single event to mark the transition from the Oligocene epoch to the Miocene epoch. Rather, there was a general cooling of the climate and a number of ice ages, which had a significant impact on various ecosystems.
Pliocene
The Pliocene epoch is the final epoch of the Tertiary period, and it lasted from about 5.332 million years ago until 2.588 million years ago, though until 2009 the Pliocene epoch was considered to have lasted until 1.805 million years ago. Like the Miocene epoch, there is no single geographical event to mark the beginning of the Pliocene epoch. Rather, further cooling from that already experienced in the Miocene epoch is what defines the boundary of the two.
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