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This paper presents a biostratigraphic revision and a quantitative study of the calcareous nannofossils from DSDP Leg 80 – Hole 550B. A total of 42 samples were analyzed, revealing 78 species whose preservation varied from slightly to moderately dissolved with slight to severe overgrowth. Biostratigraphy indicates deposition during the late Campanian to late Maastrichtian, incorporating zones UC15d to UC20b. The quantitative nannofossil data recovered shows that Watznaueria barnesiae and Micula staurophora dominate the assemblages, which can be attributed to a low preservation potential for the depositional environment. Cold-water taxa ( Kamptnerius magnificus , Ahmuellerella octoradiata, and Arkhangelskiella cymbiformis ) are present in very low percentages, with their maximum values occurring inside the UC17 biozone.

The Albian–Cenomanian was an interval of extreme warmth due to greenhouse climatic conditions, as well as significant changes in the paleogeography and paleoceanography of the oceans, which affected the evolution of marine ecosystems on a global scale. This study analyzed the calcareous nannofossil assemblages from 72 samples that were recovered from Site 364 (Kwanza Basin, Angola), which was drilled by the Deep Sea Drilling Project (DSDP) Leg 40. A total of 103 calcareous nannofossil species were recovered, enabling the identification of biozones (CC 8 to CC 10a; or BC 23 to UC 1b/UC 4a) allowing to assign an Albian–Cenomanian interval for the deposition of the studied section. The recovered assemblages have a subtropical-tropical affinity and are indicative of a surface water connection between the Central Atlantic and the South Atlantic oceans, that extends at least as far as offshore Angola. The area sedimentation was predominantly calcareous and pelagic during this interval, with the exception of the basal section, where Albian black shales interbedded with dolomitic and marly limestones were deposited. Throughout the section, high concentrations of TOC and sulfur and the presence of dissolution-susceptible species likely indicate low euxinic conditions for the Kwanza Basin. Dissolution is a significant process for Site 364 in the majority of the studied samples, although not to the point where species richness was severely impacted, as small amounts of dissolution-susceptible taxa were observed. Dissolution showed a marked decrease in intervals in which paleontological and geochemical data indicated an increase in surface water fertility/productivity.