Iterative evolutionary changes are of special interest because they imply that the recurring morphological changes had a cause that also repeated itself and might therefore be possible to uncover. We describe a set of iterative morphological changes in melanopsid gastropods from the ancient, long-lived Lake Pannon. First in the Pannonian Age, and again in the Pontian Age approximately three million years later, a smooth-shelled ancestor gave rise to a shouldered descendant. In both cases, the morphological change was probably coincident with a shift from habitats just outside the lake (e.g., rivers and streams) to habitats within the lake itself. Many other convergent examples exist in which a smooth-shelled river dweller is closely related to a shouldered and/or ribbed lacustrine snail. The frequency of this type of morphological change suggests that it has an adaptive basis; response to differing predators or hydrodynamic conditions seem the most plausible explanations, but the functional nature of these morphological changes remains unknown.

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