Limpets break Dollo's Law.

A new molecular phylogeny of the limpet molluscs (Calyptraeidae) reveals that coiled shells have independently re-evolved at least once in this family, which is a violation of Dollo's Law that complex ancestral states, once lost, are never reacquired. Reacquisition of the coiled ancestral state is remarkable in that uncoiled shells have been the most recent ancestral state for 20 million-100 million years. Adult coiling might have re-evolved by the mechanism of prolonging the period during which genes for coiling are expressed in larvae. This and other developmental mechanisms could provide general routes for maintaining the potential to produce traits lost in distant ancestors.