More on the Timescale

shell fossil

Geologists have further subdivided the timescale into “Epochs” and “Stages”. For example, the mass extinction that saw the end of the dinosaurs is recorded in rocks of the Maastrichtian Age, at the end of the Late Cretaceous Period, within the Mesozoic Era of the Phanerozoic Eon.

More simply a geologist might just refer to the Maastrichtian Age of the Late Cretaceous.

The boundaries of main time units have been placed at major changes in the history of life, for example:

The Mesozoic Era ended at a time of major change in the fossil record, caused by a mass extinction.

An even larger mass extinction at the end of the Permian Period of the Paleozoic Era saw the demise of over 90 percent of all living marine species!

The Phanerozoic Eon began 542 million years ago, at the beginning of the Cambrian Period, and spans all the time since the emergence of organic life with hard skeletal remains.