Middle Devonian brachiopod-hosted sclerobiont assemblage from the southern shelf of Laurussia, Holy Cross Mountains, Poland

Brachiopod shells coming from the Middle Devonian (latest Eifelian ensensis conodont Zone) offshore marly shales deposited on the south-eastern shelf of the Laurussia and cropping out at Skały locality (Holy Cross Mountains, Poland), were investigated with respect to encrusting and bioeroding organisms. These sclerobionts colonized the shells during a time of global warming and eustatic sea-level rise related to the Kačák event. The sclerobiont assemblage is diverse and dominated by colonial ascodictyids and bryozoans, as well as foraminifers, followed by Clionolithes traces and other microborings. Sphenothallus cnidarians, Orbiculoidea lingulid brachiopods, and Rothpletzella algae, which are rare or entirely absent in other Devonian sclerobiont communities, are present in Skały. Many brachiopods may have been colonized during life, as suggested by the growth patterns of their encrusters close to the shell commissure. However, unequivocal evidence in the form of shell malformations induced by sclerobiont growth is mostly lacking. Conversely, preference of sclerobionts for the convex ventral valve of the productide Poloniproductus suggests that colonization of this host took place postmortem. The taxonomic composition of the assemblage from Skały is most similar to the rugose coral-hosted sclerobiont assemblage from the Givetian of the nearby locality of Miłoszów and the uppermost Eifelian/lowermost Givetian brachiopod-hosted assemblage from northern Gondwana (Mader Basin, Morocco). It is clearly distinct, however, from Middle Devonian assemblages from the western part of Laurussia (USA) and South China craton. The compositions of the studied sclerobiont assemblages primary reflect specific local habitat conditions and thus their palaeobiogeographic comparisons are still difficult. However, the reliability of such large-scale analyses can be improved by focusing on sclerobiont assemblages hosted by similar groups of organisms and integrating them with well resolved palaeoceanographic reconstructions, as in the case of the Skały and Moroccan assemblages