Article: Ultrastructure of Synorisporites downtonensis and Retusotriletes cf. coronadus in spore masses from the Prídolí of the Welsh Borderland

Scanning and transmission electron microscopy has been employed to describe the ultrastructure of constituents of two kinds of spore masses recovered on bulk maceration of basal Downtonian (Pridoli Silurian) sedimentary rocks from the Welsh Borderland. Spores of Synorisporites downtonensis, occurring tetrads in narrow, elongate masses, possess a thick homogeneous exospore. Adjacent tetrads are linked by sporopollenin bridges. Variation in distal sculpture is discussed in relation to spore ontogeny. Discoidal mass contain Retusotriletes cf. coronadus also isolated from coeval Pertonella dactylethra, the sporangia of which a of similar shape to those of Cooksonia pertoni and also terminate smooth isotomously branching axes. However, differences in ultrastructure of exospore, that in R. cf. coronadus shows faint striations, with a dark layer bordering the lumen, compared with the bilayered exine of Cooksonia spores, suggest that the two megafossil species are not closely related.