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

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

The Jurassic Period is divided into three distinct epochs, which are sometimes referred to as the Lias, Dogger, and Malm. These epochs are further subdivided into a total of 11 stages, typically established using index fossils such as ammonites, bivalves, and protozoans.1

Epochs and Stages

Early Jurassic (201.3 million to 174.1 million years ago): Comprises the Hettangian, Sinemurian, Pliensbachian, and Toarcian stages.1
Middle Jurassic (174.1 million to 163.5 million years ago): Comprises the Aalenian, Bajocian, Bathonian, and Callovian stages.1
Late Jurassic (163.5 million to 145 million years ago): Comprises the Oxfordian, Kimmeridgian, and Tithonian stages.1

While these divisions are standard, some researchers note controversy regarding the exact placement and dating of stage boundaries, largely because many Jurassic ammonites had limited geographic distributions, complicating global correlations.1