Phylogenetic Affinities and Paleoenvironmental Implications of Fossil Mantis Shrimps from the Miocene Duho Formation, Pohang, Southeastern Korea

Stomatopods (mantis shrimps) are common members of tropical and subtropical marine benthos, but their fossil record is sparse. Squillidae is the largest stomatopod family, having first appeared in the Late Cretaceous, and morphology-based cladistic analysis recognizes six major generic groups, comprising the Meiosquilla-, Anchisquilloides-, Clorida-, Harpiosquilla-, Squilla-, and Oratosquilla-groups. Because many modern squillids are soft-bottom dwellers and most common in shallow-marine environments, squillid fossils from the Miocene Duho Formation of Pohang have traditionally been interpreted as evidence of shallow-water conditions. However, recent reports of deep-sea faunas from the same formation call this interpretation into question. This study reassesses the phylogenetic affinities and paleoenvironmental significance of fossil squillids from the Duho Formation. Previous work identified two genera (Leesquilla, Pohsquilla) and four species from the formation. Our reexamination of 129 specimens recognizes a putative new third genus and six new species. Cladistic analysis based on 77 morphological characteristics places the three Pohang genera as successive sister taxa to the Harpiosquilla–Squilla–Oratosquilla assemblage, with the Meiosquilla-, Anchisquilloides-, and Clorida-groups as successive outgroups, implying that the Pohang genera represent a mid-Squilloidea grade, with their divergence from the Harpiosquilla–Squilla–Oratosquilla assemblage predating the Miocene. Many specimens are preserved as 2D to 3D carbonaceous compressions to impressions, and several occur as complete to near-complete body fossils rather than isolated fragments. Thin sections of fossil-bearing rocks show homogeneous to finely laminated fabric, suggesting in-situ preservation rather than redeposition by turbidity currents. Together, these results indicate that Miocene stomatopod diversity in the East Sea (Sea of Japan) was greater than previously recognized and suggest that the Pohang taxa may have inhabited bathyal settings, an interpretation consistent with the broad depth range of living squillids from shallow waters into bathyal depths.