The first record of symbiosis between cornulitids and stromatoporoids from the Hirnantian (Late Ordovician) of Estonia

Only stromatoporoids from the genus Ecclimadictyon contain symbiotic endobionts in the Ordovician of Estonia. Cornulitids formed symbiotic associations with Ecclimadictyon microvesiculosum and E. laminaeungulatum. The endobiotic lifestyle provided an advantage over the encrusting and free-living forms of Cornulites and Conchicolites, as it offered protection from boring and durophagous predators. The host selectivity of cornulitids suggests that the antifouling capacities of different stromatoporoid taxa were different. Both stromatoporoids and cornulitids were suspension feeders and presumably competed for the food, suggesting that the cornulitid stromatoporoid associations may have been mildly parasitic.