Aptian (Lower Cretaceous) ammonite fauna of the Todai Formation, Nagano Prefecture, Japan

Twenty Aptian ammonite species including one new genus (Inaptychoceras) three new species (Hulenites kitamurai, Inaptychoceras koikei and Parahoplites inaensis) from the Todai Formation, central Japan, are described. The ammonite fauna of the Todai Formation can be divided into one horizon and two ammonite biostratigraphic zones (Aconeceras (A.) sp. Horizon, Epicheloniceras cf. claudii Zone and Diadochoceras nodosocostatiforme Zone, in ascending order), which are defined. This horizon and the two zones are correlated with the upper Aptian of the ammonite standard zonation in the west Mediterranean Province and the Tethyan Realm, and with the upper Aptian succession of England, Georgia and North Caucasus. The Diadochoceras nodosocostatiforme Zone of the Todai Formation corresponds to both Hypacanthoplites subcornuerianus and Diadochoceras nodosocostatiforme zones of the Miyako Group in Japan. The succession from the Aconeceras (A.) sp. Horizon to the Epicheloniceras cf. claudii Zone correlates with the succession beneath of the Hypacanthoplites subcornuerianus Zone of the Miyako Group. The ammonite fauna of the Todai Formation includes ornate planispiral forms, smooth or weakly ornate planispiral forms and heteromorph forms. There are almost an even ratio of morphotypes, and are similar to that of the Aketo Formation of the Miyako Group. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study describes twenty Aptian ammonite species, including one new genus and three new species, from the Todai Formation in central Japan. The ammonite fauna of the Todai Formation is divided into one horizon and two biostratigraphic zones, which are correlated with Upper Aptian standard zonation in various regions. The fauna includes ornate planispiral forms, smooth or weakly ornate planispiral forms, and heteromorph forms, with a nearly even ratio of morphotypes similar to the Aketo Formation in the Miyako Group.