The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt global warming event at ∼55 Ma. This event had profound effects on the biosphere. Quantitative investigations of calcareous nannofossil assemblages were performed across the PETM, at Deep Sea Drilling Project Site 213, in the tropical Indian Ocean, and Site 401, in the Bay of Biscay, North Atlantic Ocean, at a temperate latitude. Calcareous nannofossil assemblages from both sites show major changes before, during, and after the PETM. Fluctuations at Site 213 are affected by selective dissolution of the least robust taxa. By contrast, assemblages in the lower part of the PETM at Site 401 show less diagenetic alteration and display high abundances of genera such as Discoaster, Fasciculithus, and Ericsonia. These taxa reflect relatively warm and, probably, oligotrophic surface waters. Abundances of the genus Chiasmolithus, an indicator of relatively cool and eutrophic waters, decreased dramatically. The upper portion of the PETM is characterized by an increase in the abundance of Toweius spp. and Zygrhablithus bijugatus, interpreted as a return to cooler and more eutrophic conditions. The elimination of Fasciculithus was probably due to competition with the holococcolith species, Z. bijugatus, which occupied the same ecological niche. The occurrences of long-armed and asymmetrical Discoaster species such as Discoaster araneus, Discoaster anartios, and Discoaster okadai, and Rhomboaster species, such as Rhomboaster calcitrapa and Rhomboaster cuspis, are characteristic of the PETM interval.
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