The Eras are the next largest interval units into which the Geologic Time is divided and represented on the chart. Eras encompass major intervals of Time and are defined based on the fossil life-forms found in the rock layers, and the Law of Superposition.

The Hadean, Archean, and Proterozoic Eons do not have recognized Eras.

There are three Geologic Eras currently identified. The Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. See illustration at right.

Each of the names of the Eras reflects the relative stage in the development of life. Paleozoic means old life, Mesozoic means middle life, and Cenozoic means new life.

The Paleozoic Era

The Paleozoic Era is the oldest of the three Eras and dates from 540 Million to 248 Million Years Ago. During the Paleozoic Era multicelled living things acquired hard body parts, bones, vertebral columns, mandibles, and teeth. Common in the Paleozoic Era were trilobites, crinoids, brachiopods, fish, insects, amphibians, and early reptiles.

The Mesozoic Era

The Mesozoic Era extended from 248 Million to 65 Million Years Ago. The Mesozoic Era was important for the fossil remains of the dinosaurs and other reptiles that lived. However, the Mesozoic Era landscape was also occupied by insects, early mammals, plants such as conifers and ferns, fish, and finally flowering plants and early birds.

The Cenozoic Era

The Cenozoic Era began 65 Million Years Ago with the extinction of the dinosaurs and continues into the Present. The extinction of the dinosaurs at the end of the Mesozoic Era opened up vast new habitats and environments for early mammals and birds to adapt to and occupy.