Biostratigraphic utility of species of the genus Orbitoides

Material from type-localities and other related areas have allowed us to re-evaluate species of the genus Orbitoides. The number of auxiliary chamberlets, reflecting the number of apertures in the wall of the embryo, and the number of nepionic chambers with only one aperture are considered to be the main features for distinguishing the species. These characters increase or decrease respectively with geological time and, therefore, are interpreted as genetically controlled. The size of the embryo is also diagnostic, but it may be influenced by environmental factors. Based on the stratigraphic distribution of species of the genus Orbitoides, seven lineage zones are proposed. A single lineage zone (Zone of O. hottingeri) characterizes the Santonian. Four lineage zones are identified within the Campanian (Zones of O. douvillei, O. tissoti, O. media and O. megaloformis), and two lineage zones are recognized in the Maastrichtian (Zones of O. gruenbachensis and O. apiculata).