Ultrastructure of selected Silurian trilete spores and the putative Ordovician trilete spore Virgatasporites

Seven trilete spores from lower Wenlock to lower Ludlow sediments, all assignable to the form genus Ambitisporites, display a range of ultrastructural features. Various combinations of distal vs. proximal wall thickness, prominence of crassitude and structure/presence of trilete mark and suture distinguish the various types. Comparisons with coeval in situ spores support affinities for some of the specimens, but not others. A review of modern plant spores suggests possible production by lycophytes and hornworts, but these associations are speculative. The controversial Ordovician spore Virgatasporites possesses a robust wall and equatorial crassitude, but its relationship to the embryophyte lineage remains equivocal. (C) 2003 Elsevier B.V. All rights reserved.