spores of the triassic lycopsid Pleuromeia sternbergii (Munster) corda: morphology, ultrastructure, phylogenetic implications, and chronostratigraphic inferences

In situ microspores and megaspores from two reference cones of Pleuromeia sternbergii have been investigated. Their morphological and ultrastructural features are compared with those of the spores of another Triassic Pleuromeia species, Pleuromeia rossica, and with those of extant Isoetaceae. These comparisons clearly show that the Triassic and recent spores share a number of features that distinguish the isoetalean spores. Because the walls of both the microspores and megaspores of P. sternbergii were variably damaged owing to their poor preservation and/or low resistance to chemical extraction treatments, their study also proved to be particularly instructive in several other respects. They clearly showed that the "laminated zones"-i.e., the proximal small structural differentiations of the exospore, which initially were known only in living lycopsid microspores-(i) are present in both microspores and megaspores of the Triassic genus Pleuromeia and (ii) correspond to the papillae (cushions, dark spots, etc.) described in microspores and megaspores from numerous extinct and sometimes very ancient lycopsids, most of which belong to the isoetaleans. These papillae/laminated zones thus appear to represent a taxonomically and phylogenetically significant feature that might be very useful for recognizing and tracing the isoetalean lineage within lycopsid history. Moreover, comparison of the taphonomic and processing damage present in these spores allows identification of close homologies between the wall components in the microspores and megaspores of Pleuromeia. As these data are the first reliable ones about these relationships, they open up new prospects about the connections linking many seemingly different sporoderm types. This study also demonstrated that both the microspores and megaspores of P. sternbergii that are assigned to Densoisporites nejburgii and Trileites polonicus, respectively, may show degradational forms that are attributed to other dispersed spore taxa, Endosporites papillatus and Talchirella daciae, respectively, with E. papillatus also representing a degradational form of Densoisporites playfordii that belongs to another lycopsid. These new palynological data undoubtedly have important implications, such as those regarding the distribution of Pleuromeia within the Triassic of Germany.