First record of Chaetosalpinx bioclaustrations in the rugose corals from the Late Ordovician of Baltica

Bioclaustrations are extremely rare in the rugose corals from the Ordovician of Estonia. A specimen of Kenophyllum subcylindricum from the Vormsi Regional Stage (Katian) contains multiple shafts of Chaetosalpinx siberiensis?. The latter bioclaustration also occurs in the Pirgu Regional Stage (Katian). The growth lines of the host coral show a reaction to the infesting organism, suggesting that the shafts in the calyx are bioclaustrations and not post-mortem borings (Trypanites). The worm-like Chaetosalpinx tracemaker initially caused damage to the coral’s soft tissues and used the coral’s resources to build its domicile, resulting in a negative impact. The symbiotic worms benefited from having a domicile and protection against predators from the host’s nematocysts. Chaetosalpinx siberiensis?-Kenophyllum subcylindricum association was likely parasitic. Keywords: symbiosis, rugosans, worms, parasites, Katian, Estonia.