Paleontologists have described a new species of fossil comma shrimp based on a well-preserved specimen found in the Japanese prefecture of Shizuoka.
Makrokylindrus itoi. Image credit: Javier Luque & Sarah Gerken, doi: 10.1111/ivb.12440.
“Comma shrimp, or cumaceans, are a group of small peracarid crustaceans with over 1,900 living species currently described worldwide,” said Dr. Javier Luque from the University of Cambridge and Dr. Sarah Gerken from the University of Alaska Anchorage.
“Despite their global distribution, their anatomical diversity, species richness, and abundance, the fossil record of cumaceans is one of the poorest among marine arthropods.”
“This is puzzling because they typically live in fine-grained sediments with usually high fossilization potential.”
“Such a sparse fossil record is likely due to their small size (1-35 mm); their delicate and lightly biomineralized integument, which may reduce preservation in the fossil record; and considerable taxonomic biases due to their unfamiliar anatomy.”
“These factors make fossil cumaceans easy to misidentify, be confused with crustacean larvae of unknown affinities, or be interpreted as fragments of other crustaceans.”
“In fact, living cumaceans were initially interpreted as possible larvae, and there was controversy as to what they were until mature females bearing broods were discovered.”
The newly-described comma shrimp, Makrokylindrus itoi, lived around 2.5 million years ago (Plio-Pleistocene).
The beautifully preserved specimen was found in the siltstones of the Hijikata Formation in Japan.
“The singular fossil is exceptionally preserved, including details of the carapace, cuticle, and some appendages,” the paleontologists said.
“The holotype was a subadult male, measuring 8.5 mm in length from the tip of the pseudorostral lobe to the posterior end of the pleon.”
“The carapace, pereon, and pleon were covered with a mixture of large and small pits in a scale-like pattern.”
Makrokylindrus itoi represents the first known fossil of the family Diastylidae and only the fourth fossil crown group comma shrimp known to date.
The discovery indicates that the fossil record of comma shrimp is more widespread and diverse than previously known.
“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),” the researchers said.
Their paper appears in the journal Invertebrate Biology.
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Javier Luque & Sarah Gerken. A beautifully preserved comma shrimp (Pancrustacea: Peracarida) from the Plio-Pleistocene of Japan and the fossil record of crown Cumacea. Invertebrate Biology, published online September 10, 2024; doi: 10.1111/ivb.12440
Source : Breaking Science News