Ammonite and inoceramid radiations after the Santonian‐Campanian bioevent in Sakhalin, Far East Russia

Abstract

Study of several marine Santonian‐Campanian successions from Sakhalin Island, Far East Russia, has revealed that evolution of the ammonites and inoceramid bivalves proceeded at different rates after the major faunal turnover at the (locally dee ned) Santonian‐Campanian boundary. Sometimes changes in the inoceramid assemblages were more frequent and rapid than changes in the ammonite assemblages, and sometimes vice versa. Signie cant levels of inoceramid turnover and radiation events have been identie ed at the (locally dee ned) Santonian‐Campanian and lower-upper Campanian boundaries. Changes in ammonite and inoceramid diversity, and in the proportions of endemic and cosmopolitan species, were investigated in the context of the local relative sea-level curve and inferred environmental changes. In Far East Russia, the main ammonite and inoceramid radiation after the local Santonian‐Campanian faunal turnover occurred in the early Late Campanian Pachydiscus (P.) aff. egertoni ammonite Zone and the coeval Schmidticeramus schmidti inoceramid Zone. This condensed interval of ammonite and inoceramid maximum diversity provides a perfect stratigraphic marker that is recognizable in Sakhalin, North-East Russia and Japan. The succession of Santonian‐Campanian assemblages identie ed in Sakhalin enabled the establishment of seven ammonite and six inoceramid zones, which correlate relatively well with those of North-East Russia and Japan. The problems of placing the Santonian‐Campanian boundary in Sakhalin and in the adjacent Japanese island of Hokkaido are reviewed. & Ammonites, Campanian, diversity, endemism, inoceramids, radiation, Sakhalin, Santo