Article: A Palaeogene record of extant lower photic zone calcareous nannoplankton
In this paper, we document Palaeogene occurrences of the extant nannoplankton genera Gladiolithus, Algirosphaera and Solisphaera from claystones of the Kilwa Group microfossil lagerstätte, coastal Tanzania. In the modern ocean, these taxa are restricted to tropical and subtropical lower photic zone habitats (100–200 m). All three genera produce small and/or fragile coccoliths, which have not previously been found as fossils or are only found in late Quaternary sediments. Their presence in the Kilwa Group sediments demonstrates the exceptional quality of preservation and provides a minimum origination time for these ecologically specialized nannoplankton. Gladiolithus is present in the oldest Palaeogene Kilwa Group material studied (late Paleocene, c. 59 Ma), which pushes back the origination time for lower photic zone coccolithophores by over 40 million years. The co-occurrence of several species of Gladiolithus alongside Algirosphaera and Solisphaera suggests that a relatively diverse lower photic zone assemblage was established by at least the Eocene. The abundant and consistent occurrence of specimens that are indistinguishable from the modern species Gladiolithus flabellatus has been observed in samples of late Paleocene (Nannofossil Zone NP6) to early Oligocene (Nannofossil Zone NP21) age and support an open ocean palaeoenvironmental interpretation for the Kilwa Group sediments. We describe three new species of Gladiolithus, G. brevis, G. ornatus and G. contus and one new combination, Algirosphaera fabaceus.