Data from: Shell ornamentation as a likely exaptation: evidence from predatory drilling on Cenozoic bivalves

Predation is an important process in modern oceans and in the evolutionary history of marine ecosystems. Consequently, it has been hypothesized that shelled prey modified their ornamentation in response to predation. However, bivalve ornamentation has also been argued to be important in maintaining a stable life position in the sediment and in burrowing. To test whether concentric ribs were effective against drilling by carnivorous gastropods, we examined drill hole position and completeness for four Cenozoic bivalve species that differ in rib strength (Astarte radiata, A. goldfussi, Lirophora glyptocyma, and L. latilirata). The percentage of drill holes located between the ribs increases with increasing rib strength, whereas the percentage of drill holes on top of ribs decreases. This result suggests that gastropods select the drill hole site more effectively as rib strength increases, thereby saving time and energy, and that natural selection favors gastropods that select drill hole sites between ribs. Because of this greater stereotypy, the percentage of drill holes that are incomplete is generally lower in strongly ribbed species. The proportion of drill holes located on top of ribs is greater for incomplete than complete holes, implying that ribs can be effective against predators, but only when selected as the drilling location. We show that ribs are most effective against drilling predation for bivalves with moderately sized ribs, between which gastropods have difficulty siting drill holes. Concentric ribs are unlikely to have evolved as an adaptation against drilling predation because concentric ribs evolved in the Paleozoic and were already common in the Mesozoic, whereas drilling frequency increased later, in the Late Cretaceous–Paleogene. Moreover, rib strength of North American Astarte did not change through this time interval. Thus, the ribs considered here are a likely exaptation to drilling given their effectiveness at deterring drilling predation on bivalves with moderate ribs.捕食作用（predation）是现代海洋以及海洋生态系统演化历史中的关键过程。据此，学界提出假说：带壳猎物会通过演化改变其壳表纹饰以应对捕食压力。不过，双壳类（bivalve）的壳表纹饰也被认为在维持其在沉积物中的稳定生活姿态以及掘穴行为中发挥重要作用。为验证同心肋是否能有效抵御肉食性腹足类（gastropod）的钻孔捕食，我们针对四种肋纹强度存在差异的新生代（Cenozoic）双壳类物种（*Astarte radiata*、*A. goldfussi*、*Lirophora glyptocyma*及*L. latilirata*）的钻孔位置与钻孔完整度展开了分析。研究发现，位于肋纹间隙的钻孔占比随肋纹强度提升而升高，而位于肋纹顶部的钻孔占比则随之降低。该结果表明，随着肋纹强度增加，腹足类会更精准地选择钻孔位点，以此节省时间与能量；同时自然选择更倾向于选择在肋纹间隙钻孔的腹足类个体。由于这种更强的位点选择刻板性，强肋纹物种的不完全钻孔占比普遍更低。相较于完整钻孔，不完全钻孔中位于肋纹顶部的比例更高，这意味着壳表肋纹确实能够抵御捕食者，但仅当捕食者试图在肋纹顶部钻孔时才可发挥作用。我们的研究表明，对于肋纹尺寸适中的双壳类而言，同心肋的防钻孔捕食效果最佳——这类物种的肋纹间隙足以让腹足类难以精准定位钻孔位点。同心肋不太可能是为抵御钻孔捕食而演化出的适应性性状：因为同心肋早在古生代（Paleozoic）便已出现，且在中生代（Mesozoic）已颇为常见，而钻孔捕食的发生频率直至晚白垩世-古近纪（Late Cretaceous–Paleogene）才出现上升。此外，北美星蛤属（Astarte）的肋纹强度在该时间段内并未发生变化。综上，结合本次研究中肋纹在抵御肋纹尺寸适中的双壳类钻孔捕食时的有效性来看，本次研究涉及的壳表肋纹很可能是一种预适应（exaptation）性状，在后续演化中被赋予了抵御钻孔捕食的新功能。