The Origin and Initial Rise of Pelagic Cephalopods in the Ordovician

Background:During the Ordovician the global diversity increased dramatically at family, genus and species levels. Partiallythe diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods areamong the top predators of todays open oceans. Their Ordovician occurrences, diversity evolution and abundance patternpotentially provides information on the evolution of the pelagic food chain.Methodology/Principal Findings:We reconstructed the cephalopod departure from originally exclusively neritic habitats intothe pelagic zone by the compilation of occurrence data in offshore paleoenvironments from the Paleobiology Database, andfrom own data, by evidence of the functional morphology, and the taphonomy of selected cephalopod faunas. The occurrencedata show, that cephalopod associations in offshore depositional settings and black shales are characterized by a specificcomposition, often dominated by orthocerids and lituitids. The siphuncle and conch form of these cephalopods indicate adominant lifestyle as pelagic, vertical migrants. The frequency distribution of conch sizes and the pattern of epibionts indicatean autochthonous origin of the majority of orthocerid and lituitid shells. The consistent concentration of these cephalopods indeep subtidal sediments, starting from the middle Tremadocian indicates the occupation of the pelagic zone early in the EarlyOrdovician and a subsequent diversification which peaked during the Darriwilian.Conclusions/Significance:The exploitation of the pelagic realm started synchronously in several independent invertebrateclades during the latest Cambrian to Middle Ordovician. The initial rise and diversification of pelagic cephalopods during theEarly and Middle Ordovician indicates the establishment of a pelagic food chain sustainable enough for the development ofa diverse fauna of large predators. The earliest pelagic cephalopods were slowly swimming vertical migrants. Theappearance and early diversification of pelagic cephalopods is interpreted as a consequence of the increased foodavailability in the open water since the latest Cambrian.