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Abstract

Triceratium barbadense Greville, 1861a, T. brachiatum Brightwell, 1856, T. inconspicuum Greville, 1861b and

T. kanayae Fenner, 1984a, are among the most common diatoms reported worldwide from lower to middle Eocene

biosiliceous sediments. Due to complicated nomenclatural histories, however, they are often confused. A morphometric

analysis performed herein indicates that T. brachiatum is conspecific with T. inconspicuum, and that both

were previously often misidentified as T. barbadense. Triceratium barbadense sensu stricto is a distinct species

similar to Triceratium castellatum West, 1860. Triceratium brachiatum and T. kanayae are transferred herein

to a new genus, Fenneria, for which a close phylogenetic relationship with Medlinia Sims, 1998 is proposed.

A review of the geographic and stratigraphic distribution of Fenneria shows that the best constrained records of

its occurrences are found at DSDP Site 338, and ODP Sites 1051 and 1260. The ages of the base (B) and top (T)

of each species’ stratigraphic range are calibrated here to the Geomagnetic Polarity Timescale either directly or

inferred via correlation with dinocyst biostratigraphy. Latitudinal diachroneity of ~7 million years is documented

for F. brachiata, which disappears earlier in tropical and mid-latitude sites than in the northern high latitudes. These

observations, coupled with a preliminary compilation of the Chron C20n taxonomic composition of pelagic diatom

assemblages for Sites 338, 1051 and 1260, indicate that diatoms diversified palaeobiogeographically considerably

earlier than the Eocene−Oligocene Transition, as commonly believed. This study also emphasizes the importance

of the detailed examination of specimens from both museum collections and deep-sea cores as a step toward enhancing

the utility of Palaeogene diatoms in palaeoceanographic and palaeoenvironmental reconstructions.

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Authors and Affiliations

Jakub Witkowski

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