Palaeontologists have found that in each layer of rock, there is often a unique fossil which is not found in any other layer – these special fossils are called Zone Fossils.

Once the zone fossils have been matched with particular layers other fossils found in the same layer must all be the same age. Of course this can only give us a relative date for fossils, but by being able to put fossils in the right place in a sequence can help us enormously.

Zone fossils have to meet some strict requirements if they are to be useful;

• They should evolve very quickly

• They should have been able to spread widely across the world, usually in the sea.

• Their changes should be very easy to spot.

Some of the best examples of zone fossils are Ammonites.

These were ocean-living creatures that are found in rocks all over the world. They are especially common along the beaches of the Yorkshire coast. They have a coiled shell inside which are small chambers that were once filled with air. Between each chamber there is a wall and where this meets the outer shell there is often a wiggly line called a suture line. This line is different for every single species and makes it very easy to spot the difference between two species.

Ammonites are really good as zone fossils because they evolved very quickly throughout the same period that the dinosaurs ruled – the Jurassic. Here are just some of the ammonite zones that can be found in rocks in Yorkshire.