Ammonite- The spiral form of ammonite, a cephalopod mollusk, was achieved by primitive forms of cephalopods by the Silurian period of the Lower Paleozoic era. It evolved into its own subclass called ammonoidea probably from the nautiloids. Its shell is called involute, because the last turn completely surrounds or enfolds all the other turns. Ammonites were highly successful during the height of their expansion. Ammonites probably traveled about in the same manner described for orthoceras. They completely disappeared at the end of the Cretaceous period, about 60 million years ago. In the end, they developed strange curling and twisting habits. Ammonites are an important guide fossil for dating paleontological zones of the Mesozoic era during their relatively short burst of fame. Ammonites are beautiful fossils, because they are decorated with ridges, turbercules and spines which form intricate spiral patterns. Ammonites also started out as straight-shelled animals. Ammonite fossils can be pyritized or covered with iridescent mother-of-pearl.