The lycophyte genus Selaginella has been the subject of several taxonomic treatments in the past one hundred and fifty years. While most have considered megaspore morphology to some extent, no one has successfully reconciled this feature with a comprehensive phylogeny of the group. Recent molecular phylogenies generated using rbcL, ITS and 26S rDNA along with electron microscopy of the megaspores now afford the opportunity to do so, at least in part. Granules of sporopollenin on the innermost exospore surface are found in all members of the monophyletic subgenus Tetragonostachys and its sister taxon S. lepidophylla. Tetragonostachys includes approx. fourty drought adapted species of worldwide distribution. So far, this feature has been found in thirteen species, and no where else within the genus. A highly ordered colloidal crystal-like exospore structure is found only within an as yet unnamed, well supported clade, which includes the monophyletic Articulatae. All examined members of the Articulatae have this feature. The species that possess this wall structure do not appear to form a monophyletic group, but the number of reversals and/or parallelisms involved is uncertain because of some weakly supported nodes. The species of Selaginella now known to possess this unusual structure throughout their exospore are: S. articulata, S. diffusa, S. exaltata, S. galeottii, S. kraussiana, S. kunzeana, S. lingulata, S. lyalii, S. marginata, S. myosurus, S. polymorpha, S. remotifolia, S. sericea, S. silvestris, S. suavis, S. sulcata. S. pygmaea and S. willdenovii show this pattern in places. A final emerging correlation involves the possession of a coarse complete reticulum with high muri by most members of this unnamed clade. The ability to recognize megaspores with particular ultrastructural features of phylogenetic significance without the use of the EM (i.e., by correlation with surface features) would allow the incorporation of dates from fossil material in the analysis.

Key words: megaspores, phylogeny, Selaginella