Goniatite ammonites. Computer illustration of goniatid (order Goniatitida) ammonites underwater. 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. Goniatids lived from the Middle Devonian period (around 390 million years ago) until the end of the Permian period, around 139 million years later.