Ammonite fossil, Dactylioceras sp. Fossilized spiral shell of an ammonite, an extinct squid-like cephalopod animal (sub-class:Ammonoidea) related to the living nautilus. The outer ridges of the shell are seen. Internally the shell was divided into chambers. By pumping water into and out of the chambers the animal could move down and up in the water. This sea creature lived in the outermost chamber of the shell. Many species of ammonite flourished from the Devonian period (400 million years ago) until becoming extinct at about the same time as the dinosaurs at the end of the Cretaceous period (about 65 million years ago). Jurassic Period. Germany. Fossil dimension: 4 x 4 cm.