Prior to the early Silurian, trilete spores are rare in the microfossil record. If these spores can be considered a proxy for the non-existent macrofossil record (as their gradually increasing number and diversity might suggest) of bryophytic grade ancestors of both modern bryophytes and vascular plants, any identifiable trends would be of great significance in reconstructing the evolution of early land plants. The very limited information available to date on their wall ultrastructure suggests a considerable degree of variability. This variability could be considered evidence of a diversifying land flora including plants that produced spores with a simple proximal thinning for a suture as well as spores with a more complex proximal suture. These two categories could reflect the early divergence of bryophytes and vascular plants, but this evolutionary scenario is based on too few data points. Firstly, there is a major gap in our understanding of dispersed fossil spore wall ultrastructure throughout the Silurian. New information on this part of the record will serve as a test of the timing/existence of this divergence. Secondly, there is a dearth of information on modern hornwort spores (information on suture morphology in this group is virtually nonexistent). Only one genus of modern hornwort spores has a homogeneous wall anything like that present in most fossil spores. That genus, Phaeoceros, also appears, based on molecular phylogenetic analyses, to be derived.

Key words: cryptospores, hornwort spores, Silurian, spore, ultrastructure