Shell outside the body
Ammonites were a group of marine mollusks where the current squid also belongs to. Nowdays squids have their shell inside but the Ammonites had their shell outside their body, just like snails.
The ammonites are around 112 million years old and completely fossilized. These specimens are sawed and polished. This makes the different rooms nicely visible. The two halves together form a pair and give a good idea of how the ammonite slowely grew by expanding its shell with new rooms.
Ammonites from Madagascar
The most beautiful ammonites are found in Madagascar and may have a diameter up to 25 cm. These ammonites belong to the Cleoniceras family. In some hollow chambers calcite crystals started to grow. These are actually crystal geodes inside the fossil. Ammonites got, just like the dinosaurs, extinct about 65 million years ago.