Post-crisis recovery of Campanian desmoceratacean ammonites from Sakhalin, far east Russia

Abstract

Abstract The Santonian-Campanian boundary in Sakhalin marks a major Cretaceous mass extinction event. In the late Santonian there was a decrease in the taxonomic diversity of ammonoids as well as significant changes in shell morphology which were probably related to a global regression event. However, the fauna of the Pacific region is endemic and precise correlation between the Pacific and European regions is impossible. In this paper the stratigraphic position of the boundary between the Santonian and Campanian in Sakhalin is revised and the post-crisis recovery of the ammonoid diversity is discussed. Detailed sampling of the Santonian-Campanian sequence of Sakhalin (Naiba section, Orlovka section and others) allows identification of the phyllogenetic links between the different ammonite morphotypes in the pre-crisis communities. Comparison with adjacent regions in northeast Russia and Japan is also attempted. The ammonite extinctions at the Santonian-Campanian boundary are a good marker for stratigraphic correlation. During the crisis event ammonites of the superfamilies Acanthocerataceae and Desmocerataceae were affected most strongly whereas phylloceratids and lytoceratids were not significantly changed at that time. Heteromorphic ammonites are not discussed here. The pre-crisis community is characterized by a predominance of Acanthocerataceae, while the Desmocerataceae had a subordinate role. Representatives of these two groups form a community with large-size shells and strong ornamentation and this is not seen in the ammonites of the post-crisis community. The maximum occurrence of the Superfamily Desmocerataceae is recorded in the Middle Campanian, when the first diversification occurred (Family Pachydiscidae). Zonal correlation of the Santonian to Campanian succession of northeast Russia and Japan is discussed.