BackgroundDuring the Ordovician the global diversity increased dramatically at family, genus and species levels. Partially the diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods are among the top predators of todays open oceans. Their Ordovician occurrences, diversity evolution and abundance pattern potentially provides information on the evolution of the pelagic food chain.Methodology/Principal FindingsWe reconstructed the cephalopod departure from originally exclusively neritic habitats into the pelagic zone by the compilation of occurrence data in offshore paleoenvironments from the Paleobiology Database, and from own data, by evidence of the functional morphology, and the taphonomy of selected cephalopod faunas. The occurrence data show, that cephalopod associations in offshore depositional settings and black shales are characterized by a specific composition, often dominated by orthocerids and lituitids. The siphuncle and conch form of these cephalopods indicate a dominant lifestyle as pelagic, vertical migrants. The frequency distribution of conch sizes and the pattern of epibionts indicate an autochthonous origin of the majority of orthocerid and lituitid shells. The consistent concentration of these cephalopods in deep subtidal sediments, starting from the middle Tremadocian indicates the occupation of the pelagic zone early in the Early Ordovician and a subsequent diversification which peaked during the Darriwilian.Conclusions/SignificanceThe exploitation of the pelagic realm started synchronously in several independent invertebrate clades during the latest Cambrian to Middle Ordovician. The initial rise and diversification of pelagic cephalopods during the Early and Middle Ordovician indicates the establishment of a pelagic food chain sustainable enough for the development of a diverse fauna of large predators. The earliest pelagic cephalopods were slowly swimming vertical migrants. The appearance and early diversification of pelagic cephalopods is interpreted as a consequence of the increased food availability in the open water since the latest Cambrian.

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Numerous plectronocerid nautiloids appear in the Upper Cambrian of China. We have restudied their siphuncular structure, first described some 20 years ago. The siphuncle is characterized by: (1) long and holochoanitic septal necks dorsally but short and recurved necks laterally and ventrally; (2) strongly expanded connecting rings laterally; (3) two calcified layers in each connecting ring, outer spherulitic-prismatic and inner compact, the latter perforated by numerous pore canals; and (4) highly oblique siphuncular segments. The strongly expanded lateral sides of the connecting rings, together with the highly oblique course of the siphuncular segments, considerably enlarged the surface area of the connecting rings in each chamber, thereby increasing the transport capacity of cameral liquid. Thus, from their first appearance, plectronocerid nautiloids had developed a siphuncle for the replacement of cameral liquid with gases, and this system had a better and a more sophisticated design than that seen in stratigraphically younger nautiloids. However, their small orthoconic or slightly cyrtoconic shells were not well adapted for jet-powered swimming.

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