Heliolitid corals and their competitors: a case study from the Wellin patch reefs, Middle Devonian, Belgium

Wellin patch reefs are small Upper Eifelian build-ups within the fine-grained argilla-ceous limestone of the Hanonet Formation. Whereas the reefs themselves are not well exposed, their fossil assemblage is accessible in the hills near the town of Wellin, approximately 40 km SE of Dinant in Belgium. It is especially rich in massive stromato-poroids, heliolitids and other tabulate corals. They exhibit predominantly domical and bulbous morphologies. This paper focuses primarily on the palaeoautoecology of the heliolitid corals and their relationships with other organisms. Cases of mutual over-growth between heliolitids, other corals and stromatoporids suggest a high degree of competition for space on the reefs, possibly related to the scarcity of hard substrates. Coral and stromatoporoid growth forms, as well as the prevalence of micritic matrix, point to a relatively low energy environment. However, abundant growth interruption surfaces, sediment intercalations and rejuvenations of corals suggest episodically increased hydrodynamic regime and sediment supply. It is inferred that the patch reefs developed in a relatively shallow environment, where the reefal assemblage was regularly affected by storms. Heliolitids exhibited high sediment tolerance and relied on passive sediment removal for survival. They also could regenerate effectively and commonly overgrew their epibionts, after the colony's growth was hampered by the sediment. This is recorded in extremely abundant growth interruption surfaces, which allow the analysis of the impact of sediment influxes on the heliolitid corals.