Aenigmacaris cornigerum
Mississippian
Bear Gulch Limestone
Fergus County MT USA.

83mm shrimp on 170mm slab. Nice detail.

Carboniferous. Crustacean. Arthropod.

Aenigmacaris is an extinct genus of crustacean.  Its closest modern relatives are said to be mantis shrimp.  It was an enigma to researchers when it was being described as they were unsure how to classify it within the Class Malacostraca.

An ancient shrimp preserved as a ghost-like impression with legs and tail visible.