Encrusting foraminiferan assemblages as palaeoecological tools in reef environments. A case from the Late Eocene of northern Italy

Coral-encrusting foraminiferan associations and their variations along a palaeobathymetric gradient represented by the different facies that characterize the shallowing upward parasequences of the Nago Limestone (Upper Eocene, Trentino, northern Italy) have been investigated. From a relatively deep reef slope up to the shallow shelf-edge, corals have been recognized to be encrusted by different types of foraminiferan assemblages that change in composition and distribution and differ on the basis of relative abundance of species, growth form and type of encrusted coral surface. Three main assemblages have been recognized and named according to the dominant and statistically significant taxa: 1) Miniacina aff. multiformis assemblage (Facies 1: reef slope); 2) Acervulina-Fabiania-Haddonia assemblage (Facies 2: mid-depth reef slope); and 3) Solenomeris-Carpenteria assemblage (Facies 3: shallow reef front). The succession of encrusting foraminiferan assemblages is interpreted as controlled mainly by light, competition with coralline algae, hydrodynamic energy, and coral growth fabric, suggesting that these organisms can be used as ecological tools in palaeoenvironmental reconstructions of ancient reefs. Foraminiferans encrusting corals directly (not associated with algal crusts) mostly occur in cryptic habitats and especially on coral lower surfaces, whereas on exposed surfaces they generally contribute to form crusts together with coralline algae. These foralgal crusts preferentially develop along the mid-depth reef slope (Facies 2), on both upper and lower surfaces of the platy corals. Test morphology, and especially the flat vs. globose morphotypes ratio, positively correlates with increasing water energy across the shallowing upward sequence. However, within this general trend, their preferential encrusted surface is mainly controlled by light and consequent competition with coralline algae. Flat specimens mostly encrust coral lower surfaces where low light levels generally reduce competition for space with coralline algae. In contrast, globose morphotypes are successful on coral upper surfaces, where lateral spatial competition with algae is higher. The dense coral growth fabric that characterizes the shallower portion of the Nago reef front (Facies 3), provided cryptic habitats for the development of an encrusting foraminiferan assemblage partly similar to the one recognized within the relatively deep and poorly illuminated reef slope (Facies 1).