Symbiosis in late Silurian (Ludfordian) bryozoans from Saaremaa Island, Estonia

Six new symbiotic associations between erect branching bryozoan colonies and tentaculitoid tubeworm-like organisms have been described from argillaceous carbonate rocks of the Ludfordian of Saaremaa Island, Estonia. Several symbiotic associations occur in the Kuressaare Formation: Conchicolites sp.– Fistulipora sp. A and Fistulipora sp. B, Palaeoconchus sp. – Fistulipora sp. A, Conchicolites sp. – ?Eridotrypella sp., Conchicolites sp.– ?Anisotrypa proavus, Conchicolites sp. –?Leptotrypella versimilis and Monotrypa sp. – unknown endozoobiont association. The colonization of bryozoans by cornulitids and microconchids likely occurred because the former provided a suitable hard substrate on an otherwise soft clay sea floor. Most cornulitids had endobiotic life modes and were completely intergrown with their host bryozoans. There is no evidence of how cornulitids might have provided some advantage to the bryozoan host. However, given the likelihood of feeding competition, these associations are more appropriately qualified as mildly parasitic.