Radiolitid rudist colonisation strategies and biostrome development in moderate-energy inner-platform environments (Campanian, Brač Island, Croatia)

During the Late Cretaceous radiolitid rudist bivalves were abundant calcifiers on Tethyan shallow-water carbonate platforms. Previous studies have demonstrated various low to high-energetic subtidal environments in which vertically growing rudist bivalves lived. However, the precise mode of colonisation in moderate to higher energetic environments remains unclear. The purpose of this study is to elucidate the attachment of rudist juveniles and colonisation strategies on loose grain sediment using three-dimensional reconstructions and other quantitative approaches. The present data demonstrate that small and thinly walled rudists colonised energetic environments over several generations. The main contributors to the biostrome, Distefanella, Bournonia, and Pseudopolyconites, followed different strategies within the same ecosystem. For instance, cylindrical elevator morphotype Distefanella built up a generally loose but anchored framework to resist sedimentation and wave energy. In contrast, Bournonia and Pseudopolyconites followed a solitary strategy of settlement on any hard substrate. Preferred growth direction affecting all genera was possibly linked to a predominant direction of water currents.