Ceratites ammonite fossil. Cut and polished specimen of a Ceratites ammonite fossil. Ammonites were marine invertebrate molluscs that formed a spiral shell. The lines on the shell mark chambers added as the ammonite grew. It lived in the newest and largest chamber. Shells ranged in width from under 1 centimetre to over 1 metre. Ammonites first appear in the fossil record around 400 million years ago and became extinct at the end of the Cretaceous period 65 million years ago. The Ceratites ammonites lived during the Triassic Period. This specimen is 147 million years old. It was found in Madagascar.