Siphuncular structure in Silurian discosorid and ascocerid nautiloids (Cephalopoda) from Gotland, Sweden: implications for interpretation of mode of life and phylogeny

The connecting ring structure is studied in the discosorids Phragmoceras and Trimeroceras, as well as in the ascocerid Choanoceras. These taxa have the Nautilus type of connecting rings, composed of an outer spherulitic–prismatic layer and an inner organic fibrous layer, the latter destroyed by diagenesis. As indicated by its dense granular structure and high resistance against diagenesis, the spherulitic–prismatic layer was probably less porous and had higher mechanical strength than the corresponding layer in Nautilus. Discosorids and ascocerids were closely related. In both Phragmoceras and Choanoceras, the spherulitic–prismatic layer of the connecting ring is thickened on the inner surface of the septal neck forming a bullette, and multiple muscle scars surround the base of the body chamber. Mature shells of discosorids, ascocerids and oncocerids occur in large numbers in shallow-water carbonate facies at a few localities on Gotland. It is probable that the corresponding palaeoenvironments have been the spawning grounds for these nautiloids. Modified body chambers may have been used for the internal brooding of eggs.