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

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. • A taxonomic study of 47 specimens of ammonoid and inoceramid bivalves from Cretaceous shallow marine deposits (middle to lower shoreface) of the Taneichi Formation, in northeastern Japan was conducted. • 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 newly recorded from the Taneichi Formation. • The depositional age of the Taneichi Formation was re-estimated to range from the Santonian to the lower Campanian, based on the occurrence of a stage diagnostic inoceramid and a few biostratigraphically informative ammonoid species described. • Focusing on the faunal composition of ammonoid, heteromorph taxa occurred abundantly, while “Leiostraca” taxa were not included in the examined specimens. These patterns might reflect the palaeoecology of ammonoids.