Sponges generally live firmly attached on firm- and hardgrounds but a few species are known to colonize mobile sediments in shallow water. These psammobiontic sponges are anchored to the bottom by incorporating sediment particles at their base. Herein we relate so-far enigmatic bowl-shaped structures from seagrass deposits of the lower Miocene (Burdigalian) Calcari a Briozoi e Litotamni Formation on the Latium-Abbruzzi carbonate platform (Central Apennines, Italy) to the in-ground parts of psammobiontic sponges. The sponge fossils yield a peculiar foraminiferal assemblage with an increased abundance of Bulimina and Bolivina compared to the surrounding sediments as well as with planktic foraminifers and spirillinids (Spirillina, Patellina), which are not recorded outside the sponges. Drifting planktic foraminifers became trapped on the sponge surface exposed to the water column before agglutination. In contrast, high quantities of Bulimina, Bolivina, and spirillinids indicate commensalistic relationships with the sponges. Generally, ecological interactions between foraminifers and living sponges are poorly documented. In the present case the infaunal Bulimina and Bolivina have been attracted by a low-oxygen and nutrient-rich environment, which developed in the progressively decaying buried part of the sponges along with early lithification, while the epifaunal spirillinids populated the internal network of water channels, which provided protection and food supply. The high abundance of generally rare Spirillina in Late Jurassic sponge reefs indicates a persistent ecological preference of spirillinids to sponge microhabitats.

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