Anoigmaichnus

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