Abstract
This research uses the techniques of light and scanning electron microscopy to observe two species of Achnanthidium Kützing belonging to the A. minutissimum ‘sensu lato’ complex, collected from the Pleistocene sediments of the Colônia basin, Southeast Brazil. We compared Achnanthidium ectorianum Marquardt & C.E.Wetzel, sp. nov. and A. craterianum Marquardt & C.E.Wetzel, sp. nov. to their morphologically closest species, revealing different valve outlines, valve apices shapes, central areas, and raphe structures. Our results help to enlarge the taxonomic knowledge on the genus in South America, which is fundamental for a better understanding of the diatom distribution and diversity patterns in poorly studied areas. Our research also improves the current knowledge on the evolution of the paleolake in the Colônia basin (São Paulo) since the early Pleistocene.