CENOZOIC HISTORY OF TURRITELLOIDEA AND BUCCINOIDEA (MOLLUSCA, GASTROPODA) IN THE NORTH PACIFIC

Trends in the taxonomic diversity and geographic distribution of two g astropod superfamilies, Turritelloidea and Buccinoidea, in the North P acific during the Cenozoic are discussed. The progressive thermal dete rioration beginning in the Late Eocene is considered as the main cause of gradual taxonomic decrease and restriction of the area of warm-wat er Turritellinae and Siphonaliidae, and simultaneous origin and divers ification of boreal Tachyrhichiinae, Buccinidae and Beringiidae. The f ollowing three kinds of migrations of these groups are recognized: exp ansion of boreal genera to the Northeastern Pacific, migrations of som e warm-water taxa northward due to temperature fluctuations, and migra tion of some boreal groups to the Arctic through the Bering Strait dur ing the Pliocene and Pleistocene. A number of stages are distinguished in the Cenozoic history of each superfamily in different regions of t he North Pacific; among them several stages are common to both the gro ups in most regions. Some assumptions on possible time of isolation of main biogeographic regions and subregions within the North Pacific ar e made on the base of distribution of Turritelloidea and Buccinoidea d uring the Cenozoic.