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An era in geology is a time of several hundred million years. It describes a long series of rock strata which geologists decide should be given a name. An example is the Mesozoic era, when dinosaurs lived on the Earth.
An era is made up of periods, and several eras make up an eon. The Phanerozoic eon started about 541 million years ago (mya). It is made up of the Paleozoic era (from the Greek for "oldest life"), the Mesozoic era ("middle life") and the Cainozoic era ("latest life").