Calcareous nannofossil assemblages in four middle to low latitude Cretaceous/Tertiary boundary sections in different paleogeographic settings have been analysed. Correlation between the sections is based on the first occurrence of Neobiscutum romeinii, N. parvulum. Cruciplacolithus primus and Futyania petalosa and planktonic foraminiferal zones. Latest Maastrichtian assemblages are abundant and diverse, coccolithophores did not seem to be in decline in the latest Maastrichtian, though periods of stress are indicated by peaks in relative abundance of Micula decussata and increase in abundance of the genera Cribrosphaerella and Prediscosphaera. At the K/T boundary there was a drastic decrease in almost all Cretaceous species, blooms of Thoracosphaera, followed by acmes of few opportunistic survivors including Braarudosphaera bigelowii, Cyclagelosphaera reinhardtii, Markalius inversus, Octolithus multiplus and new Tertiary, dwarf species (Neobiscutum romeinii, N. Parvulum). The blooms are different in the studied sections probably as a result of their different paleogeography, paleodepth and consequent distribution of nutrients at the different sites. A significant change in the stratification of surface waters may have occurred in the earliest Tertiary : an influx of continental, runoff led to eutrophycation of the uppermost photic zone, triggering Braarudosphaera blooms at El Kef, Caravaca and Petriccio. Dwarf, early Tertiary coccoliths may indicate the ecosystem adjustment to unfavourable environmental conditions, increased primary productivity, poor stratification of surface water and anomalous nutrient availability. During the earliest Tertiary, nannoplankton are not the major producer of carbonate as they were in the late Cretaceous. Conditions not favourable to calcification of coccoliths are also invoked to explain the long-lasting calcareous nannofossil low-production and diversity in the earliest Danian. In this work, the rebuilding of the calcareous nannofossil community has been informally fixed by the entry of the first, "complex" Tertiary taxon Cruciplacolithus primus (<5mu ), indicating the long return to more stable conditions. The complete return of the nannoplankton community to equilibrium conditions occurred later than other planktonic organisms, above the studied interval. Cretaceous species occurring after the K/T boundary are mainly considered as reworked though survival of some taxa in the earliest Danian cannot be completely ruled out.

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