Silurian oncocerid Octamerella (Cephalopoda) from Gotland, Sweden

Abstract

An actinosiphonate deposit with radial lamellae occurs in the siphuncle of several genera classified within the Oncocerida. Unique structural preservation of the connecting ring and the actinosiphonate deposit is here described in the oncocerid Octamerella sp. from the Silurian of Gotland, Sweden. The connecting ring of Octamerella is of the Nautilus type. It consists of an outer porous spherulitic–prismatic layer and an inner thin fibrous organic layer, the latter destroyed by diagenesis. The actinosiphonate deposit consists of two, structurally different, spherulitic–prismatic sublayers. The thin outer sublayer has a fragile porous structure and low mechanical strength, similar to the outer spherulitic–prismatic layer of the connecting ring. The inner spherulitic–prismatic sublayer that forms the actinosiphonate lamellae has a more rigid, less porous structure and higher mechanical strength than the outer sublayer. Each lamella is longitudinally traversed by a calcareous central plate that has a solid structure and consists of thin acicular crystallites that are orientated parallel to the long axis of the plate. The secretion of the actinosiphonate deposit probably began immediately after a new siphuncular segment was completed. The connecting ring together with the actinosiphonate deposit functioned as a reservoir and transport route for the cameral liquid improving the buoyancy regulation.