Spore ultrastructure of Selaginellites leonardii and diversity of Selaginellalean spores

The morphology and ultrastructure of spores of Selaginellites leonardii Kustatscher et al. 2010 from the Anisian (Middle Triassic) of the Dolomites is studied. The microspores are assignable to Uvaesporites Döring, 1965. Distally and equato-rially they are covered with verrucae fused into rugae; proximally they are smooth or finely granulate. The sporoderm includes two layers, which appear homogeneous; the outer layer greatly varies in thickness at the expense of the sculptural elements, is much thicker and slightly less electron dense than the inner layer. The microspores were pro-bably originally acavate, with an homogeneous sporoderm. Although a multi-layered sporoderm forming a cavum is the most common type occurring in selaginellalean microspores, acavate sporoderms are also known with a very high ratio between sporopollenin units and the spaces between them. The megaspores are rounded to rounded-triangular, with a very dense two-layered sporoderm, with the outer layer many times as thick as the inner layer. We suppose that originally the megaspore sporoderm was granular, formed by fused spheroid units, and could belong to the irregularly granular type or to the laterally fused type of selaginellalean sporoderms. The occurrence of various spore types in the Selaginellales is known from the Carboniferous until the present day. Available data on the sporoderm ultrastructure in the Selaginellales and Isoetales, similarities and dissimilarities between the two groups are discussed in light of the newly obtained information.