Background and aims – The majority of freshwater diatom lineages appear to have evolved before or during the Eocene, with rapid radiation occurring in the Miocene. However, only a few accurately dated fossil localities are available to test this assertion, resulting in an overall poor understanding of the biostratigraphy and early evolution of these organisms. Exquisitely preserved diatoms have been uncovered from Eocene lake sediments deposited post-eruptively in the Giraffe kimberlite pipe (Northwest Territories, Canada). We describe three new pennate diatoms from this deposit and discuss their evolutionary implications. Methods – Both oxidized preparations and whole-rock mudstone samples from the Giraffe Pipe core were examined with light and scanning electron microscopy for diatom remains. Key results – Species belonging to the genera Actinella F.W.Lewis, Oxyneis Round and Nupela Vyverman & Compère are described from the Giraffe Pipe sediments. Oxyneis apporrecta is a unique species with valves that are centrally constricted, inflations midway between the center and apices, and protracted and rostrate apices. Actinella giraffensis is a small taxon that lacks a highly complex head pole, and is most closely related to a species known today only from the highlands of New Zealand. Nupela mutabilis has variably shaped valves possessing a raphe and unique areolae characteristic of this genus. These findings represent the first known fossils for both Oxyneis and Nupela, and the oldest record for Actinella. Conclusions – None of the species are known from extant floras and are therefore considered extinct. However, the morphological features they present, including the structure of the areolae, raphe, rimoportula and girdle bands, share pronounced affinities with modern congeneric taxa. The well-developed raphe found on Nupela mutabilis confirms that the evolution of this structure dates to at least the Middle Eocene, and supports the hypothesis that it may be older than the Paleocene.

ACTINELLA; EOCENE; FOSSIL DIATOMS; FRESHWATER; NUPELA; OXYNEIS; RAPHE