Benefits and limits of x-ray micro-computed tomography for visualization of colonization and bioerosion of shelled organisms

Micro-computed tomography (micro-CT) allows non-invasive imaging of internal structures of various objects. Micro-CT devices (x-ray microscope, CT scanner) utilize x-rays to see inside the object and enable its three-dimensional (3D) reconstruction. Here, a large set of shells (>60 specimens) of varying composition and origin were visualized using micro-CT to test its efficiency for investigation of (micro-)borings and other colonization structures. The set covered various materials and structures of marine shells, from the Ordovician to recent, some of them being influenced by diagenetic changes; it comprised internal moulds of invertebrate body fossils preserved in the siliceous nodules, bryozoan colony from marls to clayey limestones, epibiontic interactions of bryozoan colonies and conulariid specimens with apatitic periderm from clay limestones sediments, calcareous shells of platycerid gastropods from organodetritic limestones, microborings in peloidal grainstone, calcitic or calcitic/aragonitic serpulid tube worms, organic-walled lacustrine egg-like bodies, and recent foraminiferas from the Mediterranean. We focused on various structures from minute microborings to macroborings, burrows, and epibionts. Optimal settings of micro-CT devices for different types of shells combined with different types of borings and their fillings, burrows, and epibionts are suggested. Three-dimensional visualization of the surfaces of fossil shelled organisms using surface modelling is proposed. The main benefits of micro-CT include its non-destructive nature (measurements can be repeated, valuable specimens can be preserved for further studies, etc.) and reasonable 3D visualization of inner structures. On the other hand, resolutions of less than 1 µm cannot be effectively achieved, and this may limit studies on microborings by bacteria and certain fungi.