Fideliacyclus wombatiensis gen. et sp. nov. – a Paleocene non-marine centric diatom from northern Canada with complex frustule architecture

Abstract

Marine diatoms began colonizing freshwater habitats in the early Cenozoic, or possibly earlier, becoming well established by the Eocene. However, because of a sparse fossil record, little is known about the earliest diatom representatives that signalled this important ecological and evolutionary event. We describe a new centric diatom genus, Fideliacyclus, from lake sediments deposited during the Paleocene in the Canadian Arctic. This organism, one of the oldest known freshwater diatoms reported to date, has a unique, highly complex wall structure composed of areolae with large, bulbous, spherical shaped chambers that are open to the external environment but rest on a solid siliceous surface referred to as the solum, and are surrounded by an open space, or hypocaust. The solum is only perforated by marginal labiate processes, which largely isolates the protoplast from ambient conditions hence limiting exchange; this may have represented an adaptive response to living in freshwater. Given the complement of microfossils found in the fossil locality, the lake was an unambiguously softwater, slightly acidic, limnic system of moderate nutrient content. Although the exact taxonomic position of Fideliacyclus remains uncertain, it appears to be most closely allied to Actinocyclus (Class: Coscinodiscophyceae) or possibly Spumorbis given the range of synapomorphies.

Keywords:

• non-marine fossil diatoms
• Paleocene
• solum
• hypocaust
• Fideliacyclus
• Actinocyclus
• Spumorbis

Acknowledgements

We extend thanks to Marie Cantino and Xuanhao Sun from the Bioscience Electron Microscopy Laboratory (BEML), a facility funded jointly by the University of Connecticut and the National Science Foundation. The authors also thank Michaela Siver for help with Fig. 19, Shusheng Hu and the late Leo Hickey, Yale University and Peabody Museum of Natural History, for palynological analyses; Mike Wynne, University of Michigan, for discussions on the ICBN code; and BHP Billiton Inc. for access to exploration drill cores from the Wombat and other kimberlite localities.