ULTRASTRUCTURE OF SYNORISPORITES-DOWNTONENSIS AND RETUSOTRILETES CF CORONADUS IN SPORE MASSES FROM THE PRIDOLI OF THE WELSH BORDERLAND

Scanning and transmission electron microscopy has been employed to des cribe the ultrastructure of constituents of two kinds of spore masses recovered on bulk maceration of basal Downtonian (Pridoli, Silurian) s edimentary rocks from the Welsh Borderland. Spores of Synorisporites d owntonensis, occurring as tetrads in narrow, elongate masses, possess a thick homogeneous exospore. Adjacent tetrads are linked by sporopoll enin bridges. Variation in distal sculpture is discussed in relation t o spore ontogeny. Discoidal masses contain Retusotriletes cf. coronadu s also isolated from coeval Pertonella dactylethra, the sporangia of w hich are of similar shape to those of Cooksonia pertoni and also termi nate smooth isotomously branching axes. However, differences in ultras tructure of exospore, that in R. cf. coronadus shows faint striations, with a darker layer bordering the lumen, compared with the bilayered exine of Cooksonia spores, suggest that the two megafossil species are not closely related.