Pyritized Ammonite

Here is a beautiful Jurassic ammonite preserved in iron pyrite, commonly known as fool's gold. Almost hard to believe that this came from a squid-like animal with a shell (instead of a snail -- though I have plenty of those fossil shells as well). Ammonite fossils make great index specimens for dating rock formations, and have been found in every continent of the world. Sadly, they went extinct in the late Cretaceous, after what was likely a catastrophic bolide impact, alongside the dinosaurs.

http://www.thefossilforum.com/index.php?/gallery/image/28933-pyritized-ammonite/