Mode of occurrence and taphonomy of the heteromorph ammonite Pravitoceras sigmoidale Yabe from the Upper Cretaceous Izumi Group,Japan

The heteromorph ammonite Pravitoceras sigmoidale Yabe, of the family Nostoceratidae, is the zonal marker of the upper Campanian P. sigmoidale Zone in southwest Japan, and is the main component of ammonite assemblages in this zone. We explain the taphonomic processes underlying the occurrence of P. sigmoidale in the Izumi Group, specifically in the Minato (Awaji Island), Anaga (Awaji Island), Koike–Omoizaki (Shikoku), and Hidonodani sections (Shikoku). The first two sections consist mainly of the non-turbiditic Northern Marginal Facies (NMF), while the latter two sections comprise the Main Facies (MF), a turbiditic facies deposited in waters deeper than those of the NMF. We recognise three modes of occurrence of P. sigmoidale, as follows: (1) In nodules crowded with P. sigmoidale (NCP); this mode occurs only in the NMF, and includes juvenile and adult specimens, together with other ammonites such as Solenoceras (Oxybeloceras) aff. humei (Douvillé), (2) In mudstone with isolated P. sigmoidale (MIP), which is found in all of the sections studied, (3) In sandstone with isolated P. sigmoidale (SIP), which occurs only in the MF. Adult individuals of P. sigmoidale are dominant in most of the sections, while juveniles were observed only in the Minato section, which originally was closer to land areas. The number of specimens of P. sigmoidale and Solenoceras spp. tends to increase in sections representing depositional environments proximal to terrestrial areas. Ammonite assemblages could have been transported from shallow- to deep-water settings by turbidity currents. The sorting of components by transport processes likely contributed to the formation of these fossil assemblages in different areas. This is an important clue to understanding the habitat of heteromorph ammonite life assemblages consisting mainly of P. sigmoidale.