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.

[1] Wisshak, Knaust and Bertling (2019) state that Anoigmaichnus odinsholmensis resembles Trypanites weisei, which occasionally shows elevated apertures in its type material, and therefore they regard Anoigmaichnus as a junior synonym of Trypanites.