Here is an Upper Cretaceous ammonite with tubercles (swellings) on its ribs. Some of these ammonites had shells of up to 40cm in diameter. Like many of the ammonites in the Lower Chalk, this specimen has a very eccentric form. It has been suggested that ammonites were already feeling stress from extreme climate change (the Chalk formed in a greenhouse climate with very high temperatures, sea levels and carbon dioxide in the atmosphere). The ammonites responded by experimenting with all sorts of forms, but it didn’t help as they became extinct at the end of the Cretaceous period.
Find out more about ammonites on the molluscs pages.