Question: Can you tell how old a fossil is by looking at it?

Duncan Murdock answered on 24 Mar 2010:

Sometimes, yes. Certain kinds of fossils are only found in rocks of a certain age. For example, you don’t find dinosaurs or ammonites in rocks younger than 65 million years. But most of the time we have to date the rock they’re found in. We can do this in two ways – the first is to look at the sequence of rocks stacked on top of each other (the ones at the bottom must be older than the ones higher up). Then sometimes you find minerals that contain radioactive elements, by looking to how much they have decayed we can work out how old the minerals (so alsio the rocks they are in) are. By combining these two techniques (known as relative and absolute dating) we can put a date in most rocks. Once you know which fossils occur when, you can the use fossils to independently date the rocks, this is actually how most dating is done because we’ve been studying rock and fossils for so long most of the work has already been done. People looking for oil and gas use this method of dating to find rocks the right age to contain what they’re looking for.