Lucky rugose corals on crinoid stems: unusual examples of subepidermal epizoans from the Devonian of Morocco

In the fossil record, evidence for true epizoans, i.e. living animals inhabiting other living host-animals, is rather rare. A host reaction is usually needed to proof the syn vivo-settling of the epizoan. Herein, we provide a first report of such an epizoan biocoenosis from various strata of the Early Devonian of Hamar Laghdad, the world-renowned Moroccan mud-mound locality. In this case, solitary rugose corals settled as larvae on crinoid stems, perhaps at a spot where the epidermis was missing for some reason (injury, disease). Both the crinoid and the coral began to grow around each other. By doing so, the affected crinoid columnals formed a swelling, where ultimately only an opening slightly larger than the coral orifice remained. We discuss both macroecological and small-scale synecological aspects of this biocoenosis. The coral profited from its elevated home because it reached into more rapid currents providing the polyp with more food than at the densely populated seafloor, which was probably covered by a coral-meadow around the mounds and hydrothermal vents.