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. Partially the diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods are among the top predators of today's open oceans. Their Ordovician occurrences, diversity evolution and abundance pattern potentially provides information on the evolution of the pelagic food chain. METHODOLOGY/PRINCIPAL FINDINGS: We 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/SIGNIFICANCE: The 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

The myodocope ostracod Entomozoe from the early Silurian of Severnaya Zemlya, Russian Arctic

The myodocope ostracod Entomozoe aff. Entomozoe tuberosa (Jones 1861) has been identified from the Silurian Telychian Stage, Llandovery Series of Severnaya Zemlya, Russia. Entomozoe was previously known only from Scotland (E. tuberosa), Greenland (E. aff. E. tuberosa) and South China (E. cf. E. tuberosa). The new find signifies that Entomozoe has biostratigraphical and palaeogeographical significance: all occurrences are from Upper Llandovery sediments of tropical/subtropical palaeolatitudes. It represents a rare and early species link between the 'Baltic-British' Silurian ostracod faunal province and;'ostracod assemblages of the Russian Eurasian Arctic, and supports the notion that the North Kara Terrane and Laurentia were once palaeogeographically close. The palaeoenvironmental setting of the Russian material is consistent with the idea that this Early Silurian myodocope was probably benthonic