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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.Scientists know how quickly radioactive isotopes decay into other elements over thousands, millions and even billions of years.Scientists calculate ages by measuring how much of the isotope remains in the substance.Index fossils are fossils that can be used to date the rock in which they are found.
So, when looking at the history of a cliff face, it is important to read the story it tells from the bottom layer up.
The Age of Dinosaurs was so many millions of years ago that it is very difficult to date exactly.
Scientists use two kinds of dating techniques to work out the age of rocks and fossils. This considers the positions of the different rocks in sequence (in relation to each other) and the different types of fossil that are found in them.
A half-life measures the time it takes for one half of a radio isotope's atoms to break down into another element.
For instance, if an object has 50 percent of its decay product, it has been through one half-life.