Ammonite fossil. 19th-century illustration of a cross-section through a fossil ammonite. The calcium carbonate crystals shown in detail at lower left and lower right were found within the chambers of the ammonite. Ammonites were marine invertebrate molluscs with a spiral shell. Most were less than 20 centimetres across. They 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. This artwork, dated 1803, is from 'British Mineralogy', issued between 1802 and 1817 by British naturalist James Sowerby (1757-1822).