Index fossils are fossils that can be used to date the rock in which they are found.The best examples are fossils of animals or plants that lived for a very short period of time and were found in a lot of places.The second method is called absolute dating and is done by analysing the amount of radioactive decay in the minerals of the rocks.Scientists find out the age of a dinosaur fossil by dating not only the rocks in which it lies, but those below and above it.
Suppose a dinosaur fossil has been found in the beds of an ancient delta (the mouth of a river leading to the sea).
A fossil will always be younger than fossils in the beds beneath it and this is called the principle of superposition.
In an undisturbed sequence of rocks, such as in a cliff face, it is easy to get a rough idea of the ages of the individual strata – the oldest lies at the bottom and the youngest lies at the top.
Sometimes, scientists already know the age of the fossil because fossils of the same species have been found elsewhere and it has been possible to establish accurately from those when the dinosaur lived.
Geologists call this the principle of lateral continuity.
The sediment of this area was laid down after ammonite A appeared 199 million years ago, and before ammonite B became extinct 195 million years ago.