Cryptic encrusting communities in a Middle Devonian mesophotic paleoenvironment of the Holy Cross Mountains, Poland

Abstract

The Middle Devonian (Givetian) chaetetid sponges and alveolitid tabulate corals that inhabited mesophotic paleoenvironments of the southern shelf of Laurussia (Holy Cross Mountains, Poland) have been studied with respect to encrustation patterns. These epibiotic communities are characterized by modest diversity and high dominance, and show distinct polarization with respect to the exposed and cryptic sides of the host's skeletons. The most abundant davidsoniid brachiopods and microconchids show strong preferences toward the lower (cryptic) sides, while auloporid tabulate corals dominate on the exposed sides of the hosts. The remaining and much rarer epibionts, cornulitids and rugose corals, show no clear preferences. In some shallow-water Silurian paleoenvironments, auloporids are known to dominate the cryptic sides of the hosts. Thus, considering the auloporid tabulates as photosymbiotic corals, their absence or rarity in the Middle Devonian cryptic habitats might have resulted from insufficient irradiance levels in otherwise depleted light conditions. As a result, the available space and lack or lower competition for the area with faster-growing colonial auloporids, enabled solitary encrusters to colonize the cryptic niches. The scarce bioerosion traces and rather low diversity of epibionts in our case may indicate that these mesophotic reefs developed in a nutrient-poor, oligotrophic habitats. The davidsoniid-dominated, cryptic encrusting communities were widespread in some paleoenvironments during the Middle Devonian, being known from both Laurussia and South China craton. However, such mesophotic cryptic communities, characterized by distinct polarization in comparison to open-surface environments, contrast with other Givetian deep-water, aphotic cryptic communities reported so far, where such polarization is hardly visible.