Next -- pyritised ammonites. The one I have is cut in an unexpected way -- not a horizontal cross-section, but vertical.



Fossilised Ammonites (or what most people simply call Ammonites) are fossils (duh) of ammonites -- marine invertebrate animals that lived from Jurassic well into the Cretaceous era and then went extinct. Now their fossils are used to date  the rock layer in which they are found.


This particular specimen has its shell replaced by pyrite. Pyrite is one of the several minerals that can form an Ammonite fossil.