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Gastropods have lost the quintessential snail feature, the coiled shell, numerous times in evolution. In many cases these animals have developed a limpet morphology with a cap–shaped shell and a large foot. Limpets thrive in marginal habitats such as hydrothermal vents, the high–energy rocky intertidal areas and fresh water, but they are considered to be evolutionary dead–ends, unable to re–evolve a coiled shell and therefore unable to give rise to the diversity seen among coiled snails. The re–evolution of a coiled shell, or any complex character, is considered unlikely or impossible (Dollo's law) because the loss of the character is followed by the loss of the genetic architecture and developmental mechanisms that underlie that character. Here, we quantify the level of coiling in calyptraeids, a family of mostly uncoiled limpets, and show that coiled shells have re–evolved at least once within this family. These …