New insights into Late Cretaceous araphid diatoms from the Central Arctic Ocean

Despite being one of the most consequential events in the evolutionary history of diatoms, the emergence of pennates remains poorly understood both in terms of its timing and their ancestry. Here, we re-examined several Late Cretaceous (Campanian) diatom-rich samples from the Alpha Ridge in the Central Arctic Ocean using scanning electron microscopy with a focus on pennate taxa. Greater than anticipated species richness was recovered, increasing known Cretaceous araphid diatom flora by 25%. No raphid pennates were encountered. The majority of taxa found have heteropolar valves, with either field pores, ocelli and/or ocellulimbi, simple pores making up striae, and one rimoportula. Only one species found was previously known to science. One new genus (Cesaria) and three species (Cesaria palaiospennata, Kannoa archaea and K. simswilliamsorum) are formally proposed. For six others, the considered taxonomic affinity ranges from species to family and remains tentative.The plasticity of the structural characters of Cesaria frustules may serve as an example of a valve design that could have been advantageous among the earliest araphid pennates. Extinct Kannoa species show overall structural similarity to extant Asteroplanus and Asterionellopsis. Our unnamed species show some affinity to the genera Sceptroneis, Lancineis, Neodelphineis, and possibly other broadly defined Rhaphoneidaceae. All our diatoms are relatively small and delicate and can easily go unnoticed among the abundance of accompanying large and heavily silicified polar and non-polar centrics. If our assumptions regarding their taxonomic affinity based on morphological similarities are correct, the molecular phylogenies place them all among the earliest divergences of the pennates.