Co-occurrence patterns of ice-rafted dropstones and brachiopods in the Middle Permian Wandrawandian Siltstone of the southern Sydney Basin (southeastern Australia) and palaeoecological implications

The ecological significance of ice-rafted dropstones in present-day glacial marine benthic environments has received considerable attention from ecologists, but similar studies based on the geological records of dropstones and associated fossils are rare. In this study, we report statistically significant co-occurrences of ice-rafted dropstones with brachiopod shells in multiple stratigraphic horizons at multiple sites from the Middle Permian Wandrawandian Siltstone of the southern Sydney Basin in southeastern Australia. We analysed the distribution patterns of both dropstones and brachiopod-dominated fossil assemblages by using a quadrat-based sampling method and spatial point pattern analysis. It was revealed that the co-occurrences of ice-rafted dropstones and brachiopod shells are not random; rather, they demonstrate statistically significant and stratigraphically recurring associations that are here interpreted to represent dropstone-associated, brachiopod-dominated palaeoecological communities. In these recurring palaeocommunities, the presence of ice-rafted dropstones is considered to have added habitat complexity and heterogeneity to the benthic environment, especially suited to the settlement of brachiopods. In addition, the sinking of dropstones, from floating ice masses through the water column to the silty seafloor, is interpreted to have enriched the nutrient and oxygen supply to the benthic environment, further aiding the flourishing and maintenance of a diverse and stable marine benthic fauna.