Identification of the best sequence of planktonic foraminiferal events that is reproducible across the Coniacian–Santonian boundary interval is determined by comparison of data from three reference sections: the Cantera de Margas section at Olazagutia in northern Spain (Global Boundary Stratotype Section and Point [GSSP] for the base of the Santonian Stage), the Ten Mile Creek section in southern Texas (candidate GSSP stratotype for the base of the Santonian Stage), and Tanzania Drilling Project (TDP) Site 39 drilled in Tanzania. In the stratotype section, the GSSP is marked by the lowest occurrence of the inoceramid Cladoceramus undulatoplicatus (= Platyceramus undulatoplicatus), which occurs within the planktonic foraminiferal Dicarinella asymetrica Zone, and by secondary microfossil events and a negative 0.3‰ excursion in δ¹³C. The same bio- and chemostratigraphic records have been identified in the Ten Mile Creek section in Texas. In Tanzania, Globotruncana linneiana, a GSSP secondary planktonic foraminiferal marker event, has been used in the absence of Cl. undulatoplicatus and of correlative chemostratigraphic tie points.

The planktonic foraminiferal composition in the three stratigraphic sections is similar, although discrepancies are observed in the reproducibility of some bioevents. Similarities between sections include the same order of appearances of the index species Sigalia carpatica, Costellagerina pilula, and G. linneiana in the Cantera de Margas section and TDP Site 39, and the absence of the single-keeled globotruncanids (Globotruncanita stuartiformis, Globotruncanita elevata) in the Cantera de Margas and Ten Mile Creek sections. This apparent diachronism mostly pertains to the paleobiogeographic distributions and ecological preferences of species that developed and diversified under specific paleoenvironmental conditions. Overall, the study of the three sections allows derivation of a more accurate and reproducible sequence of planktonic foraminiferal events across the Coniacian–Santonian boundary interval. It demonstrates the reliability of the surface dweller C. pilula as the best proxy for recognition of the base of the Santonian Stage from epicontinental to open-ocean depositional settings and in a wide range of paleolatitudes.