Britannica Sources used in this answer

How did geologists use fossils to divide the Cretaceous Period into stages?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

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.

Geologists divide the Cretaceous Period into 12 distinct stages—six for the Early Cretaceous and six for the Late Cretaceous—primarily by using fossils to establish biozones within specific type areas.1

The Role of Fossils in Geochronology

Stages are defined as packages of zones, with the base of a stage typically marked by the appearance of a specific index fossil at a designated type locality. Because no single fossil group is globally ubiquitous, geologists rely on several types of organisms to correlate strata:1

Ammonites: These are the most widely employed index fossils due to their rapid evolution and widespread distribution in marine environments.23
Other Marine Organisms: When ammonites are absent or restricted by latitude, researchers use belemnites, inoceramid bivalves, foraminiferans, echinoids, and [calpionellids].21
Microfossils: Calcareous nannofossils and radiolarians are essential for correlation between different regions due to their planktonic life habits.2

Challenges and Refinements

Defining these stages is complex because type areas often contain stratigraphic gaps or fossils with restricted geographic ranges. To address this, geologists periodically update definitions and designate alternative type areas to ensure more accurate global correlation. In some regions, such as the North American western interior, high-precision zoning is further supported by radiometric dating of bentonite beds.21