Characterization of nautiloid orders Ellesmerocerida, Oncocerida, Tarphycerida, Discosorida and Ascocerida: new superorder Multiceratoidea

摘要

Multiple scars of shell muscles in the body chamber are characterized in five Palaeozoic nautiloid orders: Ellesmerocerida, Oncocerida, Tarphycerida, Discosorida and Ascocerida. The number of the scars is highly variable, from about 5 pairs in tarphycerids to about 60 pairs in ascocerids. The shell muscles that were attached to the scars in ellesmerocerids, oncocerids, dicosorids and ascocerids are here interpreted as the longitudinal mantle muscles and not as the head/funnel muscles. The interpretation of the multiple scars in tarphycerids is still unclear. In all five orders, the connecting ring structure is of the Nautilus type. The outer spherulitic-prismatic layer of the connecting ring is much thicker than that of the layer in Nautilus. It is highly porous in ellesmerocerids, oncocerids and tarphycerids and similar to that in Nautilus, but less porous and more dense in discosorids and ascocerids. The inner fibrous-organic layer of the connecting ring in all five orders is not preserved, but it has probably been considerably thinner than that in Nautilus. In some oncocerid taxa, the siphuncle contains actinosiphonate deposits of complicated structure. The ontogenetic growth of these deposits is reconstructed. On the basis of the presence of multiple muscle scars and the Nautilus type of connecting ring, the ellesmerocerids, oncocerids, discosorids and ascocerids are included in the new superorder Multiceratoidea. It is still unclear whether tarphycerids also belong to this superorder.