In Cenomanian time, the deeply weathered Precambrian bedrock of southern Sweden was transgressed by a shallow sea, resulting in the formation of an archipelago with low islands and peninsulas during early Campanian time. On one of these islands, a high-diversity fauna characteristic of a rocky coast ecosystem is found comprising nearly all levels of the food pyramid from phytoplankton and zooplankton to reptiles. The fauna is dominated by bivalves, especially oysters, and includes the northernmost Upper Cretaceous reef corals and rudists. Three distinct epifaunal zones exist: the lowest zone is characterized by large numbers of serpulids and occupies the underside of the boulders; the next higher zone is found on the vertical sides of the boulders and is characterized by oysters and a large, vertically orientated craniacean brachiopod; and the highest zone occupies the upper part of the vertical sides and the rounded upper surfaces of the boulders and is characterized by spondylid bivalves. All species normally had a clumped distribution. The zonation is interpreted as a result of the combined effect of negative phototropism (serpulids), protection against sedimentation (craniaceans), and tolerance of strong turbulence, occasional exposure, and limited competition by other species for space (spondylids).