Mystery #4Tier 2-3fossils / deep time

How can scientists say a fossil is millions of years old?

A scientist points at a fossil and says, quite calmly, that it is one hundred and fifty million years old. Nobody was standing there with a calendar. Nobody wrote the date down. So how on Earth can anyone know the age of something that ancient?

The Grand Canyon seen from the rim, its walls stacked with hundreds of coloured rock layers laid down one on top of another — Photo: Grand Canyon National Park Service · Public domain · via Wikimedia Commons

Your guess

What's your hunch? How might you tell the age of a fossil?

There's no single magic word to type, have a go in your own words, then reveal the answer to see how close you got.

Make a guess to unlock the hints and the answer.