This is a heavily pyritized Pleuroceras ammonite fossil collected near Forcheim, Germany. It's Jurassic, Pliensbachian stage or approximately 185 million years old. You can feel the heft of the iron pyrite in the specimen when you hold it in your hand.
Pyrite or “Fools Gold” is an iron sulphide that occasionally - under unique geochemical conditions - covers or replaces prehistoric creatures and plants, transforming them into incredible fossils with a gold-like lustre.
Pyritized fossils tell us a lot about the past environments of our planet. Research indicates that prehistoric animals that become pyritized, such as these ammonites, were rapidly buried under ocean sediments that were low in organic matter. In this case there would not be a lot of decaying material present. Another important condition was anaerobic seawater – the water was low in dissolved oxygen. One of the final conditions for pyritization to occur is to have large numbers of sulphate reducing bacteria (they live in oxygen deficient water) and a high concentration of reactive iron. The bacteria change the sulphates into sulphides which can then diffuse with the iron into the trilobite or other organisms forming our spectacular fossils.
Ammoniteswere predatory mollusks that resembled a squid with a shell. These cephalopods had eyes, tentacles, and spiral shells. They are more closely related to a living octopus, though the shells resemble that of a nautilus. Ammonites appeared in the fossil record about 240 million years. The last lineages disappeared 65 million years ago at the end of the Cretaceous.