Ammonites

Ammonites were cephalopods, relatives of today's nautilus, octopus and squid. They are extinct now, but used to be among the most common organisms living in the Triassic seas. There are many examples of their fossilised remains, ranging from perfectly 3-dimensional to completely flattened. The rapid morphological changes in many groups make them an important tool in rock dating. Many species in fact only lived for a few thousand years.