Early Cretaceous Small-Sized Gastropods from the Shallow Marine Deposits of the Kimigahama Formation, Choshi Group, Japan

Abstract. Twelve species of small gastropods are extracted using the sodium tetraphenylborate method from the strongly lithified shallow marine deposits of the Barremian (Lower Cretaceous) Kimigahama Formation of the Choshi Group, central Japan. They belong to the following families and subfamilies; Eudaroniidae, Pseudomelaniidae, Ampezzopleurinae, Metacerithiidae, Procerithiidae, Nystiellidae, Metaxiinae, Stuoraxidae and Ebalinae, including six new species and one new genus. Pseudomelania yamadai sp. nov., Ampezzopleura barremica sp. nov., Choshipleura striata gen. et sp. nov., Metacerithium boshuae sp. nov., Antiphora aurora sp. nov. and Stuoraxis kasei sp. nov. are described as new species. Pseudomelania yamadai, Metacerithium boshuae and Cirsocerithium subspinosum are closely related to or identical with the species from the Lower Cretaceous in western Europe. Ampezzopleura barremica and Choshipleura striata are the first Cretaceous records of the subfamily Ampezzopleurinae that has hitherto been limited to the Triassic deposits. Antiphora aurora is the first record of the family Triphoridae from Mesozoic deposits, suggesting that the sinistral triphorids originated from the dextral ancestor. Stuoraxis kasei is the youngest occurrence of the heterostrophan family Stuoraxidae that has hitherto been reported from the Permian to Middle Jurassic deposits. The sodium tetraphenylborate method for finding the small molluscs from the strongly lithified deposits is an effective tool with which to reveal the true diversity of the fossil molluscan fauna.