Evolutionary history of three mollusk classes (Bivalvia, Gastropoda, and Cephalopoda), regarded as components of the Phanerozoic marine biota, is discussed based on the comparison of dynamics of quantitative parameters obtained from the analysis of the global paleontological databases. The main trends in the evolution of the role of mollusks in Phanerozoic marine ecosystems and relationships between the diversification of this group and biodiversity of paleocommunities are considered. Certain parameters show similarity between the diversity dynamics of mollusks and the whole marine biota, including the paleolatitudinal distribution of diversity. At the same time, mollusk classes differ considerably in certain aspects. The evolutionary history of Bivalvia, Gastropoda, and Cephalopoda was different and determined presumably by deep ecological divergence which occurred as early as the Early Paleozoic adaptive radiation. Bivalves and gastropods followed the trend of a gradual and constant increase in their role in marine communities; they are characterized by high and constantly growing duration of genera, high (and also growing) frequency in paleontological collections. Cephalopods show more chaotic macroevolutionary dynamics, relatively low mean duration of genera and low relative frequency.