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 Evolution and Development of Cephalopod Chambers and Their Shape

The Ammonoidea is a group of extinct cephalopods ideal to study evolution through deep time. The evolution of the planispiral shell and complexly folded septa in ammonoids has been thought to have increased the functional surface area of the chambers permitting enhanced metabolic functions such as: chamber emptying, rate of mineralization and increased growth rates throughout ontogeny. Using nano-computed tomography and synchrotron radiation based micro-computed tomography, we present the first study of ontogenetic changes in surface area to volume ratios in the phragmocone chambers of several phylogenetically distant ammonoids and extant cephalopods. Contrary to the initial hypothesis, ammonoids do not possess a persistently high relative chamber surface area. Instead, the functional surface area of the chambers is higher in earliest ontogeny when compared to Spirula spirula. The higher the functional surface area the quicker the potential emptying rate of the chamber; quicker chamber emptying rates would theoretically permit faster growth. This is supported by the persistently higher siphuncular surface area to chamber volume ratio we collected for the ammonite Amauroceras sp. compared to either S. spirula or nautilids. We demonstrate that the curvature of the surface of the chamber increases with greater septal complexity increasing the potential refilling rates. We further show a unique relationship between ammonoid chamber shape and size that does not exist in S. spirula or nautilids. This view of chamber function also has implications for the evolution of the internal shell of coleoids, relating this event to the decoupling of soft-body growth and shell growth

A new early Smithian ammonoid fauna from the Salt Range (Pakistan)

Recent extensive investigations in the Salt Range (Pakistan) yielded abundant, well-preserved ammonoid faunas of earliest to latest Smithian age that provided the basis for a major revision of Smithian ammonoid taxonomy and for the establishment of a high-resolution biostratigraphic sequence. Here, an additional new ammonoid fauna of typical early Smithian affinity from the uppermost part of the Ceratite Sandstone of the Nammal Gorge section is described. The new fauna, termed Euflemingites cirratus beds, is bracketed between the underlying early Smithian 1CFlemingites flemingianus beds 1D, here renamed Clypeoceras superbum beds, and the overlying middle Smithian Brayardites compressus beds. Comparison with a recently published high-resolution biochronological scheme for the Smithian of the NIM (northern Indian Margin) based on the Salt Range, Spiti (Himachal Pradesh, northern India) and Tulong (South Tibet) basins shows that the Euflemigites cirratus fauna correlates with the Dieneroceras beds from Spiti based on the common occurrence of the ammonoid species Kraffticeras pseudoplanulatum. The trans-panthalassic biogeographical distribution of Euflemingites cirratus allows correlating the new ammonoid fauna with part of the Meekoceras gracilitatis ammonoid zone of western USA. Three new species (Kashmirites weisserti, Arctoceras schalteggeri and Vercherites wyleri) are described