The microspores of Pleuromeia rossica Neuburg (Lycopsida; Triassic): comparative ultrastructure and phylogenetic implications

The ultrastructural features of the microspores of the Triassic lycopsid Pl euromeia rossica are similar to those of the microspores of the living Isoe tes, except that the latter have an additional outermost wall. These featur es differ greatly from those of the spores of all the other extant Pteridop hyta. Likewise, the megaspores of P. rossica, of which the comparative anal ysis will be presented later, are also ultrastructurally analogous to the m egaspores of Isoetes. These ultrastructural similarities very likely imply close relationships between Pleuromeia and Isoetes. Moreover they suggest t hat these genera belong to a distinct group among the heterosporous lycopsi ds, in which the ultrastructural features of the microspores remained outst andingly steady. ((C) 1999 Academie des sciences / Editions scientifiques e t medicates Elsevier SAS.).