Freshwater diatoms belonging to the genus Eunotia Ehrenberg 1837 are well preserved in Middle Eocene lake sediments from the Northwest Territories, Canada. Although two distinct valve morphologies are recognized, both have features that are entirely compatible with modern congeneric taxa, including dorsiventral valve morphology, striae spanning the valve face and extending onto the mantle, raphe structures on the mantle that extend distally onto the valve face, two helictoglossae per valve, and one apical rimoportula per valve. These findings establish that the genus Eunotia, and likely freshwater raphid diatoms as a whole, are more ancient than previously believed, dating back to at least 40 Ma. One of the observed morphotypes has a raphe spanning more than 80% of the valve length as well as a narrow hyaline area extending between proximal raphe endings and interrupting mantle striae. These features imply that the raphe system among certain ancient representatives of Eunotia was more developed relative to modern counterparts. The notion of progressive degeneration of the raphe within Eunotia is supported by several extant species. Collectively, these observations confirm molecular phylogenetic arguments that position Eunotia as a basal raphid diatom.
National Geochemical Reconnaissance: reanalysis of 1775 lake sediments from regional surveys on central Baffin Island, Northwest Territories (parts of NTS 27B, 27C, 37A and 37D)
