Plectronoceratoidea
Plectronoceratoidea is a superorder of primitive nautiloids from the Late Cambrian and Early Ordovician that include the ancestors of subsequent cephalopod orders. Included within are the exclusively Cambrian Plectronocerida, Protactinocerida, and Yanhecentida, and the Cambro-Ordovician Ellesmerocerida. The Plectronocerida is the earliest group of the four and gave rise to the other three. Of these four orders, only the Ellesmerocerida crossed (barely) into the Ordovician, with two genera, "Ectenolites" and "Clarkoceras". The Ellesmerocerida in turn gave rise to the superorders Endoceratoidea, Actinoceratoidea, Discosoratoidea, Orthoceratoidea, and Nautilitoidea. The Ellesmerocerida, Endoceratoidea, Actinoceratoidea, and Discosoratoidea are restricted to the Paleozoic. The Orthoceratoidea , which extend at least into the Triassic, gave rise to the Ammonoidea which became extinct by the end of the Cretaceous. Nautilitoidea exist living today as the Nautilidae, represented by "Nautilus" and "Allonautilus". Plectronoceratoids are generally small to tiny forms with orthoconic or endogastric shells, a few being exogastric, with proportionally large ventral siphuncles that contain numerous diaphragms. Orders are determined principally by differences in siphuncle detail.