Ammonoids are a major group of extinct shelled cephalopods (the group containing octopuses and squid) that lived from the Devonian (~380 million years ago) to the end of the Cretaceous (~65 million years ago). While most modern cephalopods have an internal shell or no shell at all, ammonoids had chambered shells subdivided by complex folded walls, or septa.
The term “ammonite” is often used to refer to ammonoids as a whole. However, it actually refers to a smaller sub-group of ammonoids, distinguished by extremely complex septa, and restricted to the Jurassic and Cretaceous Periods.
