Geographically separate biotas often show differences in species composition, diversity, and adaptations. Such differences, which often reflect historical differences in regimes of productivity and selection, have received little attention from ecologists. Here we concentrate on diversity and maximum body size in 18 guilds of shallow-water marine shell-bearing mollusks and barnacles from the European and North American sides of the temperate North Atlantic. These guilds represent suspension-feeders (epifaunal, shallow infaunal, and deep infaunal), chemosymbiotic bivalves, predators, and grazers. Geographic patterns among Recent guilds were compared to those during Pliocene and early Miocene time in order to determine how diversity and maximum size changed over the last 17 million years in the face of high levels of extinction and species invasion. Recent European guilds are generally more diverse than their American counterparts, a finding consistent with previous biota-wide analyses. Diversity within Pliocene guilds was often higher on the American side, but this stems in part from the large subtropical component in preserved temperate Pliocene guilds in Virginia but not in Europe. The largest species in hard-bottom guilds in Europe reach greater sizes than those in comparable American guilds, but for sand-bottom guilds, American species almost always attain greater sizes than their European counterparts. These size differences have changed little since early Miocene time despite high levels of Pliocene and Pleistocene extinction in invasion, particularly on the American side. Large-bodied species are overrepresented among both extinct and invading lineages, meaning that lineage replacement has been the rule for the largest species in guilds on both sides of the Atlantic. Together with previous studies, these results may imply that sand-bottom environments in North America have been more productive and have experienced more far-reaching escalation between shell-bearing species and their predators than their European counterparts since early Miocene time. The Pliocene to Recent record of hard-bottom guilds implies that productivity and/or escalation in these systems may have been higher on the European side than in eastern North America. Shallow-water guilds on opposite sides of the Atlantic have retained differences despite great upheavals caused by extinctions and invasions during the last 3 million years.