Taxonomy and phylogeny of <i>Forticellula wolfei</i> gen. et sp. nov., (Fragilariophyceae), an Eocene diatom with a unique cingulum from the Horsefly fossil locality, British Columbia, Canada

A new araphid diatom genus in the class Fragilariophyceae is described from laminated, Eocene, lake sediments at the Horsefly fossil locality, British Columbia, Canada. Forticellula wolfei gen. et sp. nov. was observed in 16 samples from diatomaceous shale and mudstone rock specimens archived at the University of Alberta, Edmonton, Alberta, Canada. Forticellula frustules possess small, oval to subcircular valves each with a narrow sternum, large apical pore fields, a rimoportula, and striae that are continuous onto a deep mantle. The taxon did not produce spines. The cingulum is deep and consists of numerous open copulae (girdle bands), each with multiple rows of pores on both sides of a thickened hyaline strip, and a wide ligula with short striae. Up to 22 girdle bands were recorded, and practically all cells were still fully intact, indicating a strong and stable cingulum. The suite of characters was unique and different from all known genera in the class Fragilariophyceae, and specifically in the families Staurosiraceae, Fragilariaceae and Tabellariaceae, justifying the description of a new genus. The new genus is best placed as a stem taxon within the Tabellariaceae lineage. The Horsefly fossil locality is also the type locality of the extinct diatom genus, Eoseira, which was reported from 13 of the samples harbouring Forticellula. Samples with abundant numbers of Forticellula typically had numerous remains of heliozoans and paraphysomonad chrysophytes, Mallomonas spp., other araphid diatoms, and low numbers of Eoseira. Conversely, samples with abundant concentrations of Eoseira also harboured Aulacoseira spp., but minimal Forticellula frustules. It is concluded that Forticellula lived as single cells, probably attached to substrates, and favoured shallow, littoral environments.