Ordovician

The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 43.75 million years from the end of the Cambrian Period Ma (million years ago) to the start of the Silurian Period Ma.

third and final epoch of the Ordovician Period

evolutionary radiation of animal life throughout the Ordovician period