A piston core drilled at a water depth of 2809 m off Rio de Janeiro, southeastern Brazil, recovered a 1.83-m thick section of unconsolidated muddy sediments, representing a complete stratigraphic succession across the Cretaceous–Paleogene (K/Pg) boundary. The sediments yielded abundant, strikingly pristine tests of planktic and benthic foraminifera, unaffected by diagenetic alteration or recrystallization.

The sedimentary record ranges from the uppermost Maastrichtian (P. hantkeninoides Zone) to the lower Danian (zones P0–P1c), spanning a time interval of about 3–4 Myr through the K/Pg transition. A detailed taxonomic appraisal of the recovered planktic foraminiferal assemblages is presented, together with an evaluation of lineage relationships, phylogenetic trends, and the sequence of evolutionary events that took place among the Globigerinina at the beginning of the Cenozoic.

The numerous recent taxonomic studies of Danian planktic foraminifera notwithstanding, the significance of phenotypic varieties has never been assessed in a paleoecological context. Whereas the cancellate-walled, low trochospiral, large-sized Praemurica (Truncorotaloididae) and Parasubbotina and Subbotina (Globigerinidae) lineages evidently underwent rapid speciation over a relatively narrow phenotypic range, the small-sized Praemurica, the distinctly elevated trochoid Eoglobigerina (Globigerinidae), the Parvulorugoglobigerina, and the coiled to elongate morphotypes of the Guembelitria and Woodringina (all members of the family Guembelitriidae) and Chiloguembelina (Chiloguembelinidae) lineages show phenotypic plasticity over a much broader range. This dichotomy correlates with inferred differences in life-history parameters among these planktic taxa, including pelagic-dwelling preferences, related to reproductive and feeding strategies and to water-mass trophic levels.

Two new early Danian planktic foraminiferal species are established herein: Praemurica nikolasi n. sp., the earliest recorded Truncorotaloididae and cancellate species, likely evolved directly from Hedbergella monmouthensis near the base of Zone P0 and probably giving rise to all subsequent cancellate lineages of the early Danian; and Globoconusa victori n. sp., evolved from Globoconusa daubjergensis within Zone P1c.

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