Single grain analysis of the Late Silurian spore Cymbosporites echinatus from the Welsh Borderland

The ultrastructure of the Late Silurian patinate spore Cymbosporites echinatus has been studied using single grain analysis, which allows individual specimens to be examined with light, scanning and transmission electron microscopy. In combination with serial sectioning this allows an extraordinarily thorough assessment of the ultrastructural variability to be made. C. echinatus has an unusual two-part spore wall; the thin inner exospore surrounds the entire spore, and the considerably thicker outer exospore, or patina, only covers the distal and equatorial walls of each trilete spore and tetrad. The patinate ultrastructure of C. echinatus is unlike any extinct or extant trilete spore yet examined, though it is most similar to early Paleozoic dyads and monads such as Dyadospora murusdensa and Laevolancis divellomedia. The occasional presence of lamellae in the inner exospore and the inner part of the distal patina suggest some variability in the timing of tetrad separation. This is not surprising given the dispersal of identical spores both singly and as tetrads.