Ammonoid - An order of extinct cephalopods, characterized by an external shell that is symmetrical, chambered and coiled.
A group of extinct cephalopod molluscs with coiled shells. They lived in the seas during the Mesozoic Era. Their shells are divided into chambers. The final chamber of the shell was relatively large and long and held the soft body of the animal, which was probably rather like a squid or octopus with tentacles and a beak. The chambers are separated by walls of shell known as septa, often with highly folded margins where they connect with the outer shell layers ( sutures). Each septum was perforated by a tube (the siphuncle) which joins together all of the chambers of the shell from its earliest growth stage (the tiny, egg-like protoconch) to the final septum. This tube has a special surface which allowed the animal to regulate the quantity of gas (mainly nitrogen) and water in each chamber and therefore move up or down in the sea, rather like a submarine. We interpret the characteristics of ammonoids by comparing them with nautiloids which are still alive today.
Extinct relatives of cephalopods (squid, octopi, and chambered nautiluses), these mollusks had coiled shells and are found in the fossil record of the Cretaceous period.