Article: Are conchiolin sheets in corbulid bivalves primarily defensive?

Conchiolin sheets in corbulid bivalves have long been considered to offer some defence against predation by boring gastropods. Vermeij argued that most defences are exaptions rather than adaptations, but it has never been demonstrated whether conchiolin sheets in corbulids evolved in response to predation pressure or as a fortuitous benefit of an otherwise selected character (e.g. to retard shell dissolution, to prevent crack propagation, or to assist with hermetic sealing). A three pronged investigation was undertaken to resolve this question: a survey of failure rate of gastropod boreholes in corbulids over geological time, a survey of the distribution of conchiolin sheets in Recent corbulids in relation to various selection pressures, and an investigation into the temporal appearance of the sheets by direct (microstructural) and indirect (inferred from morphology and taphonomy) means. It is concluded that well developed conchiolin sheets first appeared at a time coincidental with the first reported gastropod boreholes and that they evolved primarily in response to that threat. Possession of conchiolin sheets may then have further influenced corbulid evolution in allowing the assumption of gross valve discordance in post-Jurassic taxa.