Barremian – Aptian ammonite biostratigraphy of the Sanchu Cretaceous, Japan

Abstract

Seventeen species belonging to 14 genera are described, which include six new species (Pseudosaynella pacifica, Acantholytoceras costaevariavilis, Shasticrioceras nakazawai, Kutatissites sudai, Procheloniceras yajimai and Acanthohoplites albedocostatus). To date, 41 species belonging to 28 genera have been described from the Sanchu Cretaceous. This ammonite assemblage can be subdivided into three ammonite biostratigraphic zones (Shasticrioceras nakazawai Zone, Heteroceras sp. Zone and Macroscaphites yvani Zone): and two subzones (Heinzia ishidoensis Subzone and Procheloniceras yajimai Zone), and five ammonite biostratigraphic horizons (Kutatissites sudai Biostratigraphic horizon, Deshayesites sp. Biostratigraphic horizon, Dufrenoyia sp. Biostratigraphic horizon, Acanthohoplites albedocostatus Biostratigraphic horizon, and Anagaudryceras cf. sacya Biostratigraphic horizon). These zones and horizons are identified in the Ishido, the Upper Member of the Sebayashi, and the Sanyama formations. These biozones and biostratigraphic horizons can be correlated with the upper Barremian and Aptian of the ammonite standard zonation of the Barremian-Aptian stages for the western Mediterranean Province of the Tethyan Realm and that of Georgia, Northern Caucasus, and the Middle Volga valley in Russia. Ammonites of the Sanchu Cretaceous diversified morphologically during the Barremian transgression and were dominated by smooth or ornate planispiral forms in the latter part of the transgression. The ammonite fauna of the Sanchu Cretaceous contains Tethyan, Boreal, and northern Pacific paleobiogeographic components. Based on these ammonites, the paleobiogeography of the Sanchu Cretaceous is discussed.