Encrustation patterns on post-extinction early Famennian (Late Devonian) brachiopods from Russia

Abstract

A study of patterns of episkeletobiont encrustation on the shells of two early Famennian brachiopods, Cyrtospirifer zadonicus and Ripidiorhynchus huotinus from Russia, revealed assemblages comprising seven skeletonised encruster taxa. The assemblages are dominated by spirorbiform microconchids, followed by cornulitids. Features such as shell malformations, frequent encruster growth towards the commissure of the shells, and encruster distribution in the areas adjacent to zones of possible water intake by the brachiopods indicate that these may have been colonised syn vivo. The lack of differences in encruster abundance between the two valves in C. zadonicus indicates that this species may have lived with both valves inclined at a high angle to the substrate. The significantly higher abundance of episkeletobionts on the brachial valves of R. huotinus may indicate that shells of this species were oriented with the brachial valve at a lower angle to the substrate, providing sheltered sites for coelobiotic larvae. Compared with pre-Famennian brachiopod-hosted encruster communities, this early Famennian community is impoverished in species richness and is dominated by microconchids. It is proposed that the epibionts witnessed a regression-related Frasnian–Famennian extinction event. During the recovery interval, opportunistic microconchids may have been among the first encrusters to colonise the pioneering brachiopods.