A new Middle Ordovician torticonic nautiloid

A new, dextrally coiled torticonic nautiloid, Pakrioceras holtni, 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 describcd. On the basis of the postcrior (‘ventral’) position of the siphonal tube and the occurrence of the thick spherulitic-prismatic layer of the connecting rings, Pakriorerus is assigned to thc family Estonioceratidae of the order Tarphycerida. Studies with scanning electron microscopy and energy-dispersive analy- sis of X-rays (EDAX) show that aragonite in the spheruliticprismatic layer was substituted by calcium phosphate during early diagenesis. This layer is therefore ultrastructurally well prcserved 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.