Aenigmacaris cornigerum
Mississippian
Bear Gulch Limestone
Fergus County MT USA.

83mm shrimp on 170mm slab. Nice detail.

Crustacean.  Arthropod. Carboniferous.

Aenigmacaris is an extinct genus of crustacean.  Its closest modern relatives are 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.

This one seems to have been caught while still in a swimming motion.