any of a group of extinct cephalopods (of the phylum Mollusca), forms related to the modern pearly nautilus, that are frequently found as fossils in marine rocks from the Devonian Period (416 million to 359 million years ago) to the Cretaceous Period (146 million to 66 million years ago). The ammonoids were shelled forms, many predacious in habit. Ammonoid shells, which are either straight or coiled, served as protective and supportive structures as well as hydrostatic devices, enabling the animal to compensate for varying water depths. Ammonoids are characterized and distinguished from nautiloids by the highly crenulated and complex suture that occurs where internal partitioning walls come in contact with the outer shell wall. Ammonoids are important index fossils (index fossil) because of their wide geographic distribution in shallow marine waters, rapid evolution, and easily recognizable features. Three groups of ammonoids succeeded one another through time, each group having a more complex suture pattern. Ammonoids with a simple suture pattern, called goniatite, flourished during the Paleozoic Era (542 million to 251 million years ago). Ammonoids characterized by a more highly folded suture, called ceratite, replaced the goniatites and were most abundant in the Triassic Period (251 million to 200 million years ago). Most ammonoid genera became extinct at the end of that period, but a few survived and evolved into many diverse forms during the Cretaceous Period (146 million to 66 million years ago). These forms are characterized by an interwoven suture called the ammonite pattern.