ZOOLOGY A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site ammonite A member of an extinct group (sub-class Ammonoidea) of marine cephalopods with a chambered shell. Ammonites were largely pelagic (dwellers of the open ocean) and typically spiral-shelled, of diameter 0.01–2 meters (0.4in–6.6ft), though helical shells have been found . They first appeared about 415 million years ago in the Paleozoic era as small straight-shelled creatures called bacrites, then quickly evolved into a variety of shapes and sizes. During their evolution, the ammonites faced three catastrophic events that eventually led to their extinction. The first crisis occurred during the Permian period (about 250 million years ago), when nine-tenths of all ammonite species were wiped out. The survivors recovered, diversified, and went on to flourish throughout the Triassic, but at the end of this period (206 million years ago) they faced near extinction, when all but one species died out. During the subsequent Jurassic period, the number of ammonite species rose once more. The final catastrophe occurred at the end of the Cretaceous period when the ammonites, along with the dinosaurs, and many other groups of organisms became extinct. Their closest living relative is the Nautilus. Related category    • ZOOLOGY Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP