Ammonites from the Lower Chalk are very distinctive. Some, such as this one, are very robust with strong ribbing and spines. But in complete contrast, there’s another set of ammonites that don’t look like ammonites at all – they either started to uncoil or coil into a spire like a snail. It’s thought that ammonites were experiencing some stress from climate change and that led them to evolve into a whole new range of forms. It didn’t do them much good though, as they became extinct at the end of the Cretaceous period, along with the dinosaurs and two thirds of life on the planet.
Find out more about ammonites on the molluscs pages.