Microstructures des zones calcitiques et aragonitiques des rostres de Goniocamax (Cephalopoda, Belemnitida), du Turonien de Sibérie du Nord

The posterior part of the rostra of Goniocamax (Belemnitida, Late Cretaceous, Siberia) is calcitic, the anterior part being aragonitic. Microstructures are best preserved in the aragonitic part of the rostra, but examination of other Molluscan shells from the same site shows that the aragonite is slightly altered by diagenetic changes. Goniocamax shows a complex structure with sectors in the aragonitic part, similar to those observed in Triassic Aulacoceras and Tertiary Belopterina. Thus the calcitic rostrum of Belemnitida and the aragonitic telum of Sepiida seem to be homologous.