Forming Fossils

Time Periods

What are the Paleozoic, Mesozoic, and Cenozoic eras?

The divisions between the eras on the geologic time scale represent major changes on Earth. The division between the Pre-Cambrian and Paleozoic, about 543 million years ago, represents an increase of life on Earth. The division between the Paleozoic and Mesozoic represents a major decrease in plant and animal species (called an extinction) about 250 million years ago. It is also called the “Permian Extinction,” or the “Great Dying,” in which up to 90 percent of all species died out. The division between the Mesozoic and Cenozoic, about 65 million years ago, also represents a major extinction of plant and animal species, including the dinosaurs. This extinction was not as extensive as the Great Dying: only about 50 percent of all species died out at this time.


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