An emended description of the freshwater araphid genus Ambistria: a rare diatom from North American Eocene localities

Abstract

In 1968, Lohman and Andrews described the freshwater araphid diatom genus Ambistria from the Wagon Bed Formation, a Late Eocene deposit in Wyoming, U.S.A. The genus was distinguished from other genera on the basis of the distinctive shape of the valve coupled with short striae restricted to the valve margins, the latter resulting in a wide hyaline axial region. These authors further described two species of Ambistria based on differences in size and the presence/absence of randomly spaced isolated pores in the hyaline axial region. Further details could not be discerned from the specimens illustrated, and the genus has not been reported since the original description. Numerous specimens of Ambistria were uncovered from the Giraffe Pipe locality, an Early Eocene deposit located in the Northwest Territories, Canada, near the Arctic Circle. Examination of these specimens with light and scanning electron microscopy revealed an additional suite of characters, which have now been used to emend and expand the original generic description. These features include the presence of marginal ridges, a unique mechanism used to link frustules together into chains, areolae with reniform-shaped volae, and laterally opening girdle bands. Ambistria belongs in the class Fragilariophyceae Round, and is best placed within the order Fragilariales (Silva) emended Round. Additional characters that could be used to delineate between species include shape of the central region, length to width ratio, and possibly the number of girdle bands. The recent discovery of Ambistria in the Giraffe Pipe locality extends the range of this extinct diatom to the Early Eocene.