Here’s a little bit of a change up for the AquaNerd Picture of the Week. Instead of showing off the typical aquarium livestock or gorgeous reef tank, we’re deviating from the norm and showing off an interesting ammonite fossil seen at the London Museum of Natural History. Ammonites are an extinct group animals that are from the class Cephalopoda. They are related to octopuses, squid, and cuttlefish, but as you can tell they possessed a very hard shell like the chambered nautilus. Despite the shell, scientists believe these are more closely related to the strictly soft bodied cephalopods instead of the nautiloids. Ammonites are of particular interest to paleobiologists as they are known as index fossils and are often used to date rock layers and other animals found in the same layer.