The Cenozoic Era is known as the "Age of Mammals." This era is broken down into time periods according to the life forms that lived during that time. In the Paleogene Period, rodents, small horses were common and rhinoceroses and elephants first appeared. By the end, dogs, cats, and pigs became common mammals. The Neogene Period gave rise to early primates, which included early humans.
The final period of the Cenozoic Era was the Quaternary Period. This was the time when lions, saber-toothed tigers, cave bears, giant deer, woolly rhinoceroses, and woolly mammoths became the dominant species of their time.
The extinction of many groups of giant mammals has occurred during this time period, which ranges from 65 million years ago to the present day. The result of these extinctions allowed smaller groups of mammals to diversify and prosper. The most significant extinction was that of the dinosaurs. Besides a few flightless birds that were still classified as dinosaurs, the era of the dinosaurs had come to an end. As a result, without dinosaurs, plant life was able to proliferate. Nearly every plant known to man in the present day had originated in the Cenozoic Era.
In North America, forests covered much of the continent during the early part of the era. However, as the climate cooled off, the forests died and gave way to open land that led to the formation of the Great Plains of the Midwestern United States.