All known groups of fossilized siliceous and calcareous phyto- and zooplankton experienced major, sudden extinctions of unique severity at the very end of the Cretaceous. Evolutionary diversification of the earliest Tertiary nannofossil assemblages was rapid and highly variable in space and through time. Biogeographic and stratigraphic studies of calcareous nannofossils suggest stable ecologic conditions in oceanic environments during the last several million years of Cretaceous time. Well-preserved earliest Danian calcareous nannoplankton assemblages from several apparently continuous deep sea sequences dispel the notion of a major global dissolution event related to the mass extinctions. Available paleontological evidence indicates that the extinctions were highly selective and affected the following paleoenvironments with decreasing severity: open ocean surface waters, shallow coastal waters, terrestrial environments, fresh water habitats, deep ocean waters. A total blackout of a few weeks’ to months’ duration appears consistent with the fossil evidence and with light requirements of living phytoplankton. However, stratigraphic resolution in boundary sections is insufficient, and present ignorance of the geochemistry of noble elements too extensive, to exclude terrestrial metal enrichment processes unrelated to an extraterrestrial impact.