A new Middle Ordovician torticonic nautiloid

Abstract

Abstract A new, dextrally coiled torticonic nautiloid, Pakrioceras holmi, n.gen., n.sp., is described from the lowermost Middle Ordovician (Viruan) Aseri Stage of northern Estonia. This is the next to oldest torticonic cephalopod hitherto described. On the basis of the posterior (‘ventral’) position of the siphonal tube and the occurrence of the thick spherulitic-prismatic layer of the connecting rings, Pakrioceras is assigned to the family Estonioceratidae of the order Tarphycerida. Studies with scanning electron microscopy and energy-dispersive analysis of X-rays (EDAX) show that aragonite in the spherulitic-prismatic layer was substituted by calcium phosphate during early diagenesis. This layer is therefore ultrastructurally well preserved and consists of bundles of acicular crystalline elements, separated by interspaces. Its ultrastructure is identical to that in the corresponding layer of the Recent Nautilus.