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How is the Ordovician Period divided into Early, Middle, and Late epochs?

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Ordovician paleogeography — Ordovician Period The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Ordovician Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The Ordovician Period is divided into three geologic epochs based on specific time intervals and further subdivided into seven global stages defined by the International Commission on Stratigraphy.12

The division of these epochs is as follows:

Early Ordovician Epoch (485.4 million to 470 million years ago): This epoch comprises the Tremadocian (Stage 1) and Floian (Stage 2) stages.12
Middle Ordovician Epoch (470 million to 458.4 million years ago): This epoch includes the Dapingian (Stage 3) and Darriwilian (Stage 4) stages.12
Late Ordovician Epoch (458.4 million to 443.8 million years ago): This epoch consists of the Sandbian (Stage 5), Katian (Stage 6), and Hirnantian (Stage 7) stages.12

While regional faunal provincialism historically complicated global correlation, these stages were established to provide a more accurate, standardized framework for the Ordovician System.2