Continuing investigations of early land plant spore wall ultrastructure are beginning to reveal a considerable degree of diversity, despite a limited number of taxa defined at the level of the light microscope. Previous analyses of in situ cryptopsore monads showed several ultrastructural types within a single taxon, Laevolancis divellomedium. A similar situation seems to exist with some dispersed trilete spores (e.g., Ambitisporites). Variable characteristics of these trilete spores include the relative thickness of the proximal and distal walls, the nature of the presumed proximal suture, and the number of sublayers comprising the wall. Thus far, no lamellae have been identified in any dispersed trilete spore. These trilete spores are from geographically diverse locations (eastern US, eastern Canada, central US, eastern Europe and Saudi Arabia), range in age from Middle Ordovician to Upper Silurian, and have different degrees of thermal maturity. Therefore, much additional work will need to be done to determine the extent to which these factors are contributing to the apparent structural diversity. Although broad categories of ultrastructural types will no doubt ultimately become apparent, as yet, many dispersed spore taxa seem to represent heterogeneous assemblages.

Key words: cryptospores, Ordovician, palynology, Silurian, spores, trilete