| Summary: | The most well preserved and diverse upper Eocene to lower Oligocene assemblage of calcareous nannofossils (coccolithophores) known to date is described from the Pande Clays, Kilwa Group, Tanzania. The assemblage is exceptionally diverse, with a total of 115 species described herein, which significantly exceeds the current globally compiled nannofossil species diversity of 67 for the latest Eocene (NP19/20). The enhanced diversity observed in these sections is concentrated in the numerous Rhabdosphaeraceae (20 species), Syracosphaeraceae and holococcolith taxa (19 species) that are unknown from any other contemporaneous location. Scanning electron microscope (SEM) studies reveal exquisite preservation down to the sub-micron scale in many of these taxa, including the architecture of <3 mu m holococcoliths and the details of grills and processes in other very small fragile taxa - a size class which is rarely preserved even in Recent sediments. A distinct assemblage of at least four specialist lower-photic zone taxa - three Gladiolithus species and Algirosphaera fabaceus - is present in these sediments. The occurrence of these highly specialised coccoliths in the Palaeogene sediments of Tanzania extends their previously known late Quaternary fossil record by tens of millions of years. The controls on the exquisite preservation of such a diverse nannofossil assemblage are difficult to determine but we speculate that a diverse open-ocean, oligotrophic coccolithophore assemblage was being rapidly buried and sealed within a clay-rich facies that is more characteristic of shelf-environments, a combination that, to date, makes the Palaeogene sediments of Tanzania unique. One new genus, Pocillithus, is described, consisting of very small spine-bearing muroliths that may be related to extant narrow-rimmed muroliths of uncertain affinity. Ten new species are described: Pocillithus spinulifer, Reticulofenestra macmillanii, Calcidiscus parvus, Syracosphaera monechiae, Syracosphaera raffiae, Blackites cutter, Blackites shafikii, Acanthoica backmanii, Orthozygus occultus and Orthozygus arcus.
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