Ammonoids and inoceramid bivalves from the Upper Cretaceous shallow marine deposits of Taneichi Formation in Hirono Town, Iwate Prefecture, northeastern Japan: Implication for biostratigraphy

Abstract

We examined 47 specimens of ammonoid and inoceramid bivalves from the Cretaceous Taneichi Formation in Iwate Prefecture, northeastern Japan and assigned them to seven species of ammonoids and two species of inoceramids. Of these, five species of ammonoids, Protexanites (Anatexanites) fukazawai (Yabe and Shimizu, 1925); Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara, and Karasawa, 2017; Hyphantoceras transitorium Matsumoto, 1977; Polyptychoceras yubarense (Shimizu, 1935); and Po. obatai (Matsumoto, 1977); and one species of inoceramid, Platyceramus japonicus (Nagao and Matumoto, 1940) are reported for the first time from this Formation. The inoceramid Pl. japonicus and a few biostratigraphically informative ammonoids indicate that the middle Member of the Taneichi Formation was deposited in Santonian–early Campanian (Late Cretaceous). Ammonoid species found in the Taneichi Formation were generally similar to those of the Santonian–lower Campanian in other regions of the northwestern Pacific. Heteromorph taxa were the most abundant ammonoids, accounting for approximately 70 % of the total. Additionally, it is notable that “Leiostraca” taxa, such as Tetragonitoidea and Desmoceratoidea, were not included in the examined specimens. These patterns of occurrence might suggest the palaeoecology of ammonoids, but determining whether they reflect the true distribution requires verification of taphonomic processes, such as post-mortem drifting.