THE GEOLOGIC TIME SCALE

The history of the Earth is broken up into a hierarchical set of divisions for describing geologic time. As increasingly smaller units of time, the generally accepted divisions are Eon, Era, Period, Epoch, and Age. In the time scale shown below, two levels of this hierarchy are represented.

The Phanerozoic Eon is shown along the left side of this figure and represents the time during which the majority of macroscopic (larger than single celled) organisms, algae, fungi, plants and animals, lived. When first proposed as a division of geologic time, the beginning of the Phanerozoic (approximately 540 million years ago) was thought to coincide with the beginning of life. In reality, this eon coincides with the appearance of animals that evolved external skeletons, like shells, and the somewhat later animals that formed internal skeletons, such as the bones of vertebrates.

The Precambrian Eon is all time older than approximately 540 million years ago.

The Phanerozoic is divided into three major divisions: the Cenozoic (recent life), the Mesozoic (middle life), and the Paleozoic (ancient life) Eras. These divisions reflect major changes in the composition of ancient faunas (groups of animals), each era being recognized by its domination by a particular group of animals. The Cenozoic has sometimes been called the "Age of Mammals", the Mesozoic the "Age of Dinosaurs" and the Paleozoic the "Age of Fishes". This is an overly simplified view, but one which has some value for the beginner. Other groups of animals, besides the dinosaurs, lived during the Mesozoic. In addition to the dinosaurs, animals such as mammals, turtles, crocodiles, frogs, salamanders, and countless varieties of insects also lived on land.

Additionally, there were many kinds of plants living in the past that no longer live today, and many ancient floras (groups of plants) went through great changes too, but not always at the same time that the animal groups changed.

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