Palaeoberesellids are septate, tubular microfossils usually attributed to the green algae. They occur widely in Upper Palaeozoic carbonate sediments, where they are normally seen in thin sections as cross-sections or short lengths of thallus. Detailed study of late Dinantian (Asbian) limestones from two areas of the UK. South Wales and northwest England, show that palaeoberesellids. particularly Kamaenella. are the most important carbonate-producing organisms in shallow. low to moderate energy environments and supplied grains to higher energy environments as a result of storm breakage and transport. Where palaeoberesellids were the dominant organisms they formed low-growing ‘thickets’ on the sea-floor which trapped fine sediment. to create a bafflestone texture. The late Dinantian was a time of great instability with rapid sea-level changes. Palaeoberesellids were opportunistic organisms which thrived in such an environment. The volume of carbonate produced by these organisms in shallow water may have been a contributory factor in the progradation of shallow marine facies and the establishment of relatively flat-topped shelves from the ramps of the Early Dinantian.