Unfortunately this specimen is suffering from pyrite decay, a fate that awaits nearly all fossils preserved in iron pyrites (fool’s gold). The best way to slow down the decay is to keep the specimens in a dry place, and apart from each other. It’s impossible to determine which specimen might start to decay first but when it does, that decay can spread to others. Some specimens have been sitting in collections for hundreds of years and remain fine, while others have crumbled to powder.
Find out more about ammonites on the molluscs pages.