南部北上志津川地域における下部ジュラ系韮ノ浜層の汽水域および浅海域底生動物化石群集

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Molluscan (bivalve and gastropod) fossil associations are recognized in the earliest Jurassic (Hettangian) Niranohama Formation in the South Kitakami Belt, Northeast Japan. The Niranohama Formation is composed of tidal flat, tidal channel fill, tidal channel margin, lower shoreface, and inner shelf deposits. Molluscan fossil associations occur from the tidal flat, tidal channel margin, and lower shoreface deposits. In the tidal flat deposits, five molluscan fossil associations are recognized. These associations consist of a non-siphonate, infaunal bivalve Geratrigonia, short-siphonate infaunal bivalves (Eomiodon and Integricardium), semi-infaunal bivalves (Modiolus and Bakevellid), and epifaunal bivalve Ostrea, with scattered occurrences of gastropods. An association dominated by non-siphonate ivalves, such as Vaugonia and Coelastarte, occurs from tidal channel margin and lower shoreface deposits. In the earliest Jurassic of South Kitakami, non-siphonate, suspension-feeding bivalve was a dominant life habit group in shallow marine benthic environments. Short-siphonate bivalves without pallial sinus, such as Integricardium and Eomiodon, occur only in tidal flat deposits. No long-siphonate bivalves with pallial sinus occur in the earliest Jurassic (Hettangian) of South Kitakami.
2004-12-31