Anoigmaichnus

Anoigmaichnus is an ichnogenus of bioclaustrations (a type of trace fossil). Anoigmaichnus includes shafts perpendicular to host's growth surface or tilted (up to 45°); conical to cylindrical; circular to oval cross-sections; lacking separate wall. Their apertures are elevated above host's growth surface forming a short chimney-like structures. Anoigmaichnus is the worlds earliest known macroscopic endobiotic symbiont and it may have been a parasite. It occurs in the Middle Ordovician bryozoans of Osmussaar Island, Estonia.