Episkeletobionts of large rugose corals from the Middle Devonian mesophotic palaeoenvironment recorded in the Pokrzywianka Beds (Holy Cross Mountains, Poland)

Organisms encrusting corals from a coral horizon encountered in a trench in the Middle Devonian (Givetian) Pokrzywianka Beds of the classic Grzegorzowice-Skały section in the Holy Cross Mountains, Poland, are described and analyzed in the context of their palaeoecological and palaeoenvironmental background. These episkeletobionts form rather a low-diversity community, dominated by microconchid tubeworms, crinoids, and tabulate corals. The last group, however, is especially diverse at the family level, represented by auloporids, alveolitids, coenitids and favositids. These episkeletobionts are considered to have developed in a low-light, lower mesophotic palaeoeonvironment, as evidenced by the presence of platy, alveolitid tabulate coral in the deposits studied. This microconchid-crinoid-tabulate-coral community differs from other Givetian communities from the Holy Cross Mountains (Laskowa and Miłoszów), which also are considered to have developed in low-light habitats. The differences in taxonomic composition of episkeletobionts between these three localities most probably resulted from specific local conditions, related to bathymetry (light levels, nutrient levels), the specific nature of the hosts/substrates occupied, and also differences in larval dispersal patterns. This, in turn, shows that various encrusting communities may have inhabited seemingly similar, marine habitats within a given time interval and neighbouring areas, which may have serious implications for large-scale comparisons of biodiversity within a given palaeoenvironment.