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| Makrokylindrus itoi Luque & Gerken, 2024 |
Abstract
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
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| Fossil comma shrimp (Cumacea: Diastylidae) Makrokylindrus itoi sp. nov., holotype YPM 549224, from the Plio-Pleistocene Hijikata Formation, Kakegawa Group, Shizuoka prefecture, Japan. (A) Photograph of whole holotype. (B) Close-up of the carapace, the Pereonites 1–4, first and second pereopods, and third maxilliped. (C) Close-up of the cuticular ultrastructure of the branchiostegite. (D) Close-up of the uropod, telson, and Pereonites 1 and 2. (E) Close-up of the cuticular ultrastructure of the second pereonite. (F) Close-up of the fourth pereopod. (G) Close-up of the first to third pleonites and some pleopods. (H) Schematic line drawing of holotype, showing details of the body parts and cuticular ornamentation. White numbers 1–5, Pereonites 1–5; Mxp3, third maxilliped; P1, Pereopod 1; P2, Pereopod 2; P4, Pereopod 4; pl1=, pleopod 1; pl2=, pleopod 2; Pl1–Pl6, Pleonites or Pleonal somites 1–6; psr, pseudorostrum; t, telson; U, uropod. Images and line drawing by J. Luque and S. Gerken |
Systematic paleontology
Arthropoda von Siebold, 1848
Malacostraca Latreille, 1802
Cumacea Krøyer, 1846
Diastylidae Bate, 1856
Makrokylindrus Stebbing, 1912
Makrokylindrus itoi sp. nov.
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.
Javier Luque and Sarah Gerken. 2024. A beautifully preserved comma shrimp (Pancrustacea: Peracarida) from the Plio-Pleistocene of Japan and the fossil record of crown Cumacea. Invertebrate Biology. DOI: doi.org/10.1111/ivb.12440
