Fossils in rocks are studied to measure geological time. Some fossils are more important as their characteristics define precise geological stages. Here, for the Hauterivian, the defining biological marker is the Acanthodiscus ammonite. Why are fossils studied?
Several methods exist to measure geological time and one of them is the study of different fossils that are found in the rock. The presence or absence of one or another type of fossil enables me to understand the succession of events that make up the Earth’s history. However, some fossils are more important than others as they precisely define specific periods: these are called index fossils.
In the case of the Hauterivian stage, it is the Acanthodiscus ammonite that serves as the defining biological marker. So, when I discover this ammonite in another rock, even several hundreds or thousands of kilometres away, I can deduce that they date from the same epoch.
The fossil must meet 3 criteria to be considered as an index fossil:
- the species must have existed for a short period in terms of geological time; for example, the Acanthodiscus only lived for about 500,000 years,
- the fossil must be present in different parts of the globe,
- it must be sufficiently abundant for geologists to find it.