The Cenozoic Era is easy to define: it's the stretch of geologic time that kicked off with the Cretaceous/Tertiary Extinction, 65 million years ago, and continues down to the present day. Informally, the Cenozoic Era is often referred to as the "age of mammals," since it was only after the dinosaurs went extinct that mammals had the chance to evolve into various open ecological niches and dominate terrestrial life on the planet. This characterization is somewhat unfair, however, since (non-dinosaur) reptiles, birds and fish also thrived during the Cenozoic!
Somewhat confusingly, the Cenozoic Era is divided into various "periods" and "epochs," and scientists don't always use the same terminology when describing their discoveries. (This situation stands in stark contrast to the preceding Mesozoic Era, which is neatly divided into the Triassic, Jurassic and Cretaceous periods.) Here's an overview of the subdivisions of the Cenozoic Era; just click on the appropriate links to see more in-depth articles about the geography, climate and prehistoric life of that period or epoch.
The Periods and Epochs of the Cenozoic Era
The Paleogene period (65-23 million years ago) was the age when the mammals began their rise to dominance. The Paleogene comprises three separate epochs:
The Paleocene epoch (65-56 million years ago) was fairly quiet in evolutionary terms. This is when the tiny mammals that survived the K/T Extinction first tasted their newfound freedom and began to tentatively explore new ecological niches.
The Eocene epoch (56-34 million years ago) was the longest epoch of the Cenozoic Era. The Eocene witnessed a vast profusion of mammalian forms; this was when the first even- and odd-toed ungulates appeared on the planet, as well as the first recognizable primates.
The Oligocene epoch (34-23 million years ago) is notable for its change in climate from the preceding Eocene, which opened up even more ecological niches for mammals. This was the epoch when certain mammals (and even some birds) began to evolve to respectable sizes.
The Neogene period (23-2.6 million years ago) witnessed the continuing evolution of mammals and other forms of life, many of them to enormous sizes. The Neogene comprises two epochs:
The Miocene epoch (23-5 million years ago) takes up the lion's share of the Neogene. Most of the mammals, birds and other animals that lived during this time would have been vaguely recognizable to human eyes, though often considerably bigger or stranger.
The Pliocene epoch (5-2.6 million years ago), often confused with the ensuing Pleistocene, was the time when many mammals migrated into the territories that they continue to inhabit during the present day. Horses, primates, elephants, and other animal types continued to make evolutionary progress.
The Quaternary period (2.6 million years ago to the present) is, so far, the shortest of all the earth's geologic periods. The Quaternary comprises two even shorter epochs:
The Pleistocene epoch (2.6 million-12,000 years ago) is famous for its large megafauna mammals, such as the Woolly Mammoth and the Saber-Toothed Tiger, that died off at the end of the last Ice Age (thanks partly to predation by the earliest humans).
The Holocene epoch (10,000 years ago-present) comprises pretty much all of modern human history. Unfortunately, this is also the epoch when many mammals, and other forms of life, have gone extinct due to the ecological changes wrought by human civilization.