The type species, Mimetaster hexagonalis is known from the Lower Devonian (Pragian-Emsian) Hunsrück Slate, and amongst the most common arthropods from the locality, with over 120 specimens including three juveniles.

The trunk consisted of up to 32 short segments, each of which except the last bore a pair of biramous appendages, which gradually decreased in size posteriorly.

[1][2] M. hexagonalis probably lived in small groups on the seafloor,[1] walking in a tilted, upright posture propped up on its two uniramous legs.

[2] Many specimens have been found associated with tentaculitoids and sponge remains, which suggests that these taxa grew on the surface of Mimetaster as epibionts, which likely acted as camouflage.

It was found in the Lower Ordovician (Tremadocian) Floresta Formation of Argentina, the first occurrence of this group in South America.