The Tertiary Period falls within the Cenozoic era. The period itself lasted over 63 million years and contained five separate epochs: the Paleocene, the Eocene, the Oligocene, the Miocene and the Pliocene. Mammals thrived during this period after the extinction of the dinosaurs in the Cretaceous Period. Categorizing some of these animals by epoch can be helpful in remembering where they emerged within the period.

Paleocene Epoch

About 65-55 million years ago, the Paleocene Epoch saw the formation of the ice caps and the development of a cooler climate. With the dinosaurs now extinct, mammals began to grow in diversity and population. Early primates appeared during this time period along with rodents and the ancestors of those present-day herbivores that have hoofs.

Eocene Epoch

During the Eocene Epoch, about 55-34 million years ago, Europe and North America diverged and the oldest fossils of present-day mammals appeared. Such mammals included the ancestors of elephants as well as bats and whales. The first horse, Eohippus, also entered the stage. Known as the "Dawn Horse," it weighed only 30 to 40 lbs. and likely had a short lifespan of only 4 to 6 years.

Oligocene Epoch

Approximately 34-23 million years ago, the Oligocene Epoch encompassed many geological events. While Antarctica and Australia separated, India and Asia collided and thrust the Himalayas into being. Incidentally, man's best friend arrived before man himself. The earliest forms of pigs, cats and dogs are thought to have arrived during the Oligocene. Additionally, sea levels were relatively low and the first toothed whales appeared.

Miocene Epoch

Occurring around 23-5 million years ago, the climate warmed during the Miocene Epoch. During this window of time, Africa thrust into Europe, causing the formation of the Alps. Primates and other such creatures emerged in greater number. Horses continued to develop and early camels and rhinos came on the scene.

Pliocene Epoch

Between 5 and 1.8 million years ago, the Pilocene Epoch marked an important time for humans. As South America and North America converged at Panama and Africa boxed out the Mediterranean Sea, the geography of the world began to strongly resemble our current-day geography. As the earth dried and cooled, grasslands spread and grazing mammals diversified. It's thought that, during this time, the ape lines and human lines parted