Field and laboratory data indicate that all four species of Thais (Gastropoda, Muricacea) from rocky shores of the northeastern Pacific selectively attack barnacles at the margins of parietal (lateral) and opercular plates. Attacks are also more likely to be successful at plate margins. Such preferential attack and differential attack success may account for the evolutionary reduction in the number of parietal plates exhibited within seven of the eight families of balanomorph barnacles. Additional evidence suggests that predation by drilling gastropods may have also favored the evolution of strong external ribbing on these plates within some balanomorph lineages.
Radiation and plate reduction in the Balanomorpha occurred in concert with extensive radiation in muricacean gastropods, starting in the Late Cretaceous. Of the three skeletal conformations exhibited by open-surface dwelling barnacles (8, 6 and 4 parietal plates), only genera of the four-plated form have become proportionally more common in the Recent. Further, in a rather striking evolutionary experiment, a lineage of barnacles evolving free from gastropod predation retained primitive skeletal features. This supports the interpretation that predation by drilling gastropods has been an important selective force behind the widespread, parallel evolutionary tendency towards plate reduction in the Balanomorpha; a trend recognized by Darwin (1854), but for which he offered no explanation.