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How is the Cretaceous Period divided into the Early and Late Cretaceous epochs?

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

The Cretaceous Period is divided into two rock series, Lower and Upper, which correspond to the Early Cretaceous Epoch and the Late Cretaceous Epoch, respectively. These divisions are defined by the following time intervals:1

Early Cretaceous Epoch: 145 million to 100.5 million years ago.1
Late Cretaceous Epoch: 100.5 million to 66 million years ago.1

Both epochs are further subdivided into six distinct ages, which were historically named by geologists in the 19th century based on rock stages found in specific type areas across Europe. The stages for the Early Cretaceous are the Berriasian, Valanginian, Hauterivian, Barremian, Aptian, and Albian. The stages for the Late Cretaceous are the Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian.1

Geologists define these stages by the base of the earliest biozone at a boundary type area, often utilizing fossils such as ammonites and inoceramid bivalves for correlation.12