Bryozoans and serpuloideans in skeletobiont communities from the Pleistocene of Sicily: spatial utilisation and competitive interactions

Abstract

Sessile encrusters with mineralised skeletons in the fossil record generally retain their original spatial relationships to the substrate and each other. Being short living and not significantly time averaged, communities on shelly substrates represent excellent systems to study such relationships. Bryozoan and serpuloidean skeletobionts on molluscs and rhodolites from Lower Pleistocene localities in Sicily have been studied. Species composition and specimen sizes testify to a short exposure of the shells on the sea floor. Skeletobiont community structure is characterised by the dominance of a few species (5 bryozoans out of 87 and 3 serpuloideans out of 17). Substrate coverage is usually low (<5%), rarely reaching 50-60% or more. On bivalves, skeletobiont distribution does not exhibit a clear trend for inner/outer sides or left/right valves. Oriented growths, differential patterns in microenvironment utilisation of the substrate and spatial competition have been analysed. Several of the recorded overgrowths resulted from superimposition of specimens growing on skeletons of previous, already dead encrusters. True competitive interactions mainly involved bryozoans and only a few serpuloideans. Within bryozoans interspecific encounters usually led to overgrowth or abutment whereas intraspecific encounters commonly resulted in standoffs and growth side by side in cheilostomes, and to fusion of colonies in some cyclostome species.