Morphological Variability of Alveolophora antiqua from a Freshwater Early Miocene Paleolake in The Barguzin Valley (Baikal Rift Zone)

Using scanning electron microscopy, the morphology of Alveolophora antiqua (Moisseeva) Moisseeva from the freshwater Early Miocene paleolake in the Barguzin Valley was studied. We extended the species diagnosis relative to features such as spines, structure of areolae, costae, alveolae, rimoportulae and the external opening of rimoportulae and show a wide morphological variability with regard to valve shape (round, oval, ovoid and triangular). The percentage of these shapes varied with the core depth. Oval valves dominated in lower layers, while triangular ones occur in upper layers. We propose that the shape variability of valves might be due a sharp short-term cooling event or polyploidization. The finding of A. antiqua in the Early Miocene deposits of the Barguzin Valley expands both the georaphic and stratigraphic ranges of this species.