ammonoid, any of a group of extinctcephalopods (of the phylum Mollusca), forms related to the modern pearlynautilus (Nautilus), that are frequently found asfossils in marine rocks dating from theDevonian Period (began 419 million years ago) to theCretaceous Period (ended 66 million years ago).

The ammonoids were shelled forms, and nearly all are thought to have been predacious in habit. There is evidence that these animalsconsumedzooplankton,crustaceans, and other ammonoids. The shells, which are either straight or coiled, served as protective and supportive structures as well as hydrostatic devices, enabling theanimal to compensate for varying water depths. Ammonoids are characterized and distinguished fromnautiloids by the highly crenulated and complex suture that occurs where internal partitioning walls come in contact with the outer shell wall. Ammonoids are importantindex fossils because of their wide geographic distribution in shallow marine waters, rapidevolution, and easily recognizable features.

Three groups of ammonoids succeeded one another through time, each group having a more complex suture pattern. Those with a simple suture pattern, called goniatite, flourished during thePaleozoic Era (541 million to 252 million years ago). Ammonoids characterized by a more highly folded suture, called ceratite, replaced the goniatites and were most abundant in theTriassic Period (252 million to 201 million years ago). Most ammonoid genera became extinct at the end of that period, but a few survived and evolved into manydiverse forms during the Cretaceous Period. These forms are characterized by an interwoven suture called the ammonite pattern.

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Some scientists maintain that ammonoid survival was closely tied to the availability ofplankton in Paleozoic and Mesozoic seas. They hypothesize that the sudden decline of plankton during theK–T extinction at the end of the Cretaceous brought about thedemise of the remaining ammonoid groups.