Britannica
Sources used in this answer
Why are ammonites useful for dating and correlating Jurassic rock layers?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
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.
Ammonites are highly effective index fossils for dating and correlating Jurassic rock layers due to several key biological and evolutionary characteristics:
- Rapid Evolution: Ammonites underwent rapid evolutionary changes, which allows geologists to divide the Jurassic Period into precise, fine-scale time intervals known as biozones.12
- Distinctive Morphology: Their shells feature elaborate, complex suture lines where internal partitions meet the outer wall. These unique patterns make individual species easily identifiable, which is essential for accurate classification and stratigraphic sequencing.23
- Abundance and Preservation: As common marine organisms with hard, durable shells, ammonites are frequently preserved in the rock record, providing a reliable data source for researchers.42
While they are primary tools for developing relative time scales, their utility is occasionally limited by their geographic distribution. Many Jurassic ammonite species were restricted to specific regions, meaning that while regional zones are well-established, correlating these zones globally can sometimes be challenging.51