Comma shrimp, or cumaceans, are diverse benthic crustaceans, yet they are one of the groups with the poorest fossil record, hindering our understanding of the evolution of the group in deep time. Here, we describe a new species of fossil comma shrimp, Makrokylindrus itoi sp. nov., from the Plio-Pleistocene of Japan. The singular fossil is exceptionally preserved, including details of the carapace, cuticle, and some appendages, and it represents the first known fossil of the family Diastylidae and only the fourth fossil crown group cumacean known to date. We suggest that the scarce and sparsely known fossil record of cumaceans likely reflects a lack of recognition due to their small size and unfamiliar anatomy, making fossil cumaceans easy to misidentify, be confused with crustacean larvae of unknown affinities, or be interpreted as fragments of other crustaceans (taxonomic bias), rather than a lack of fossilization potential (taphonomic bias).

Keywords: Cenozoic, Diastylidae, exceptional preservation, Neogene, quaternary 

Arthropoda von Siebold, 1848

Malacostraca Latreille, 1802

Cumacea Krøyer, 1846

Diastylidae Bate, 1856

Makrokylindrus Stebbing, 1912

 Diagnosis: The carapace, pereon, and pleon were covered with small and large pits arranged in a scale-like pattern. The carapace bore a line of large pits that paralleled the ventral margin. The pseudorostral lobes were 0.5× the total carapace length. The telson was cylindrical, and it was at least 3× longer than pleonite 6 length, with the post-anal portion much shorter than the pre-anal.

 Etymology: The specific epithet honors the late Mr. Takashi Ito, an avid fossil collector and artist from Japan who discovered and collected the holotype, made it available for study, and donated it to the Yale Peabody Museum.