Wall ultrastructure in some dispersed megaspores and seed-megaspores from the Middle Devonian of northern Poland

A detailed investigation using light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) has been undertaken on three dispersed megaspore taxa recovered from Givetian deposits of northern Poland. Morphology of Contagisporites optivus is the same as that of megaspores reported from sporangia of Archaeopteris, but the producers of the seed-megaspores Granditetraspora zharkovae and of Biharisporites? capillatus are unknown. The affinity of the former is of special interest as monomegaspory is one of the botanical innovations that characterizes the seed plants (though it also occurs in other heterosporous plants). Comparisons at ultrastructural level with megaspores and large spores of plant groups believed to have attained, in the Middle Devonian, some level of heterospory are discussed. It is suggested that the alveolate outer wall of the seed-megaspores studied, consisting of granular units, is similar to that of archaeopteridalean megaspores. Specimens of G. zharkovae bear also some morphological resemblance to anisodiametric tetrads from the Devonian and Carboniferous ovules of presumed or certain pteridosperms in being enclosed in resistant meshy membranes (mesh-sack). Wall thickness of aborted versus functional megaspores of G. zharkovae, and the lack of indication that the mature functional megaspore wall was stretched, point to a non-lycopsid origin of G. zharkovae. It is suggested that the anisodiametric tetrads discussed may have been produced by a plant related to archaeopteridaleans.