Spathian (Late Olenekian, Early Triassic) Ammonoids from the Osawa Formation, South Kitakami Belt, Northeast Japan

Thirty-one species of early to middle Spathian (late Olenekian, Early Triassic) ammonoids are reported from the Osawa Formation in the Yamaya, Akaushi–Osawa, Utatsu, Kamiwarizaki and Ozashi areas in the southern part of the South Kitakami Belt, Miyagi Prefecture, Northeast Japan. Four distinct Spathian ammonoid zones (taxon-range zones) are recognized in the upper lower to middle upper parts of the Osawa Formation, in ascending order: the upper lower Spathian Deweveria kovalenkoi Zone, the middle Spathian Subcolumbites perrinismithi Zone, Subfengshanites multiformis Zone and Isculitoides sp. Zone. The Subfengshanites multiformis Zone is characterized by its occurrence in various lithologies in different areas, i.e., alternating beds of mudstone and fine-grained sandstone in the south, and more offshore mudstone in the north, suggesting a northward-deepening setting. Because a primitive ichthyopterygian Utatsusaurus hataii occurs in the Subcolumbites perrinismithi to Isculitoides sp. zones, its age is likely constrained to the middle Spathian. The ammonoid faunas in the Osawa Formation exhibit a great affinity with those of South Primorye as well as the Tethys, and such faunal similarity suggests that the South Kitakami Belt may have been located near South Primorye.