Barnacle taphonomy and ultrastructure: comparing modern and fossil shells from the western Mediterranean

Fossil acorn barnacles are excellent palaeoenvironmental proxies in shallow-marine settings. However, the impact of post-depositional and diagenetic alterations on the barnacle shell remains poorly understood. Opercular plate preservation and observations on abrasion and fragmentation of the wall plates are the most valued taphonomic indicators. Although the shell microstructure of marine invertebrates usually provides useful insights for understanding these processes, conventional microscopic observations are often insufficient for assessing the degree of micro-and ultrastructural alteration in fossil barnacles. This study compares the ultrastructure of modern and fossil (Pliocene and Pleistocene) barnacle shells from the western Mediterranean to explore their ultrastructure and investigate the impact of taphonomic and diagenetic processes. Under Scanning Electron Microscopy (SEM), modern barnacles show a porous ‘dragon-scale pattern’. SEM and cathodoluminescence analyses reveal microstructural changes in fossil barnacles, including dissolution features and diagenetic imprints. Evidence of microbial activity is also observed, in both modern and fossil specimens, in the form of microbioerosion traces made by cyanobacteria. Despite their stable low-magnesium calcite composition, barnacle shells from coarse-grained deposits usually show significant diagenetic alteration. In contrast, specimens from fine-grained sediments typically exhibit minimal alteration, preserving ultrastructural features almost comparable to those of their modern counterparts. This study underscores the importance of high-resolution analytical approaches in palaeontological research to advance our understanding of barnacle ultrastructure, taphonomy and diagenesis.