Repeated evolution: the case of columellar folds in gastropods.

Most adaptive traits evolved multiple times independently, but the conditions of their evolution remain obscure. Here I examine the origins of columellar folds, spiral ridges on the axial wall of the interior of many gastropod shells, in both extant and fossil gastropods. Analyses indicate that these features arose at least 46 times, primarily in caenogastropods and heterobranchs. Columellar folds do not form in shells in which the coiling axis rises steeply above the apertural plane and in which the rim of the rounded aperture is a logarithmic spiral. More derived clades have shells with a low angle of elevation of the coiling axis, which permits the evolution of a non-circular aperture, a longer columella, and the formation of columellar folds. This architectural predisposition to form folds long preceded the realization of this potential. Functions of columellar folds include reducing apertural access to predators and preventing anteroposterior displacement of soft parts in the shell as parts of the body slide into and out of the shell. These benefits became important late in gastropod history as metabolic rates and activity levels of gastropods and the threat of predation by way of the aperture increased in the later Mesozoic and Cenozoic eras. [ABSTRACT FROM AUTHOR]