[Bivalve fauna succession in the north Pacific during the paleogene-neogene transition]

Cluster analysis was used to compare species composition of marine and brackish water bivalves in the Cenozoic deposits of the Russian Far East, Hokkaido, and California. Bivalvia species composition allows us to divide these deposits into two groups of clusters. The first group includes both extant and extinct species, while the second one is largely composed of extinct species. The drastic change in the bivalve species composition suggests that the base of the Uinin Horizon in the northern Sakhalin, Kuluven Horizon in the western Kamchatka, Tsubetsu Formation in eastern Hokkaido, and, possibly, the central part of the provincial Vaqueros Formation in California correspond to the Paleogene-Neogene boundary. The change in species composition was accompanied by the taxonomical diversity changes. At a higher level, the equitability of species richness among families significantly decreased.