[M-IS24] Biogeosciences of cold seeps, mud volcanoes, and hydrothermal vents

[MIS24-05] Taxonomy of fossil gastropods from an Early Cretaceous hydrocarbon seep: Focus on Provannidae (Gastropoda: Abyssochrysoidea)

*Haruka Fukaki¹, Robert Jenkins¹, Kenji Kashiwagi² (1.Division of Geosciences and Civil Engineering, Graduate School of Natural Science and Technology, Kanazawa University, 2.Department of Environmental Biology and Chemistry, Faculty of Science, University of Toyama)

Keywords:Gastropods, Chemosynthesis-based community, Early Cretaceous

Chemosynthetic communities that depend on the primary production of chemosynthetic bacteria have been found in hydrocarbon seeps, which are located along subduction zones and continental margins and where methane-rich fluids gush out along faults. The communities are also found in hydrothermal vents, whale-falls, and sunken woods. Those communities include endemic taxa, such as Provannidae (Gastropoda: Abyssochrysoidea). Although they are representative of gastropods belonging to the Recent chemosynthetic communities, paleontological studies on their origins and evolution have been insufficient. The fossil record of Provannidae dates back to the Middle Cenomanian, Late Cretaceous (Kaim et al., 2008). In this study, we conducted taxonomic research for the gastropod fossils from Early Cretaceous hydrocarbon seep carbonates distributed along Utagoesawa Creek (Yubari-City, Hokkaido, Japan) to investigate the origin and early evolution of provannid gastropods. We collected several seep carbonates as floats collected during fieldwork along the Utagoesawa Creek. The floats are considered to be originated from upper Albian outcrop because of the distribution of floats along the creek. Preliminary results of radiolarians from floats haven’t conflict with the age designation. The fossils were picked up through acid treatments because gastropod fossils preserved in the hydrocarbon seep carbonates from Utagoesawa are replaced by silica. As a result, at least 13 species were found including Provannidae and Hokkaidochinchidae. The most abundant species is the gastropod with morphological features intermediate between Provannidae and Hokkaidoconchidae (ancestral taxon of Provannidae; Kaim et al., 2013). The finding of the Provannidae from the Early Cretaceous is the oldest fossil record of the family so far. The presence of gastropods with intermediate morphology between Provannidae and Hokkaidoconchidae indicates that Provannidae was derived from Hokkaidoconchidae, provably in Albian, Early Cretaceous. Morphological change from Hokkaidoconchidae to Provannidae occurred in reticulated sculpture in protoconch, cylindrical to elongated shell shape, and the clarification of spiral ribs in teleoconch.