Campanian (Late Cretaceous) nautiloids from Sakhalin, Far East Russia

Three nautiloid taxa, Cymatoceras pseudoatlas (Yabe and Shimizu, 1924b), C. cf. bifidum Shimansky, 1975, and C. cf. honmai Matsumoto and Miyauchi, 1983, are recorded from the Campanian of Sakhalin, Far East Russia. These are the first biostratigraphically well dated nautiloids from Sakhalin, which show close affinities to nautiloid faunas from Japan (Hokkaido), the two areas having formed part of a southerly palaeobiogeographical subprovince of the North Pacific Province. Possible relationships between shell form/ornament and preferred habitats of Late Cretaceous nautiloids are discussed. Coarsely ribbed ("cymatoceratid"), depressed nautiloids seem to predominate in nearshore environments. This may be regarded as an adaptive response to increasing predation pressure by durophages, especially in shallow water settings, which may have triggered the development of defensive morphologies (i.e., ornamented, predation-resistant shells) in Cretaceous shallow-water nautiloids.