Comparison of methods: Micro-CT visualization method and epoxy cast-embedding reveal hidden details of bioerosion in the tube walls of Cretaceous polychaete worms

Bioerosion in three serpulid tubes of the (sub-)genera Cementula, Pyrgopolon (Septenaria), and Placostegus from the Bohemian Cretaceous Basin was studied by a combination of micro-computed tomography and vacuum cast-embedding technique producing polymer resin casts. Results gained from both methods were evaluated and compared in terms of material usability, destructive force, quality of the resulting image, and hardware/software requirements. The advantage of the micro-CT methodology is its non-destructiveness and the ability to make three-dimensional images, animations, and serial sections through the object; this method is suitable for most examined materials, but it is limited by low quality of the resulting image and demanding hardware/ software requirements. To ensure good visibility of the borings in the resulting X-ray reconstructions, a material contrast between hard substrate and borings is necessary. The advantage of the vacuum cast-embedding technique followed by SEM observation is high quality of the resulting image, but it is limited by destructive force and material composition. Vacuum casting is unusable for undissolvable silicified or pyritized substrates, as happened in our Placostegus zbyslavus specimen, which suffered from incomplete dissolution in acid. However, epoxy casting provided detailed morphology of borings that were beyond the detection limit of the given spatial resolution of the micro-CT scanner, and helped with ichnotaxonomic identification. Tiny branching canals of Iramena in Cementula sp. and the finer apertural canals connecting the Entobia chambers to the substrate surface in Pyrgopolon (Septenaria) cf. tricostata and Cementula sp. were not detected using micro-CT, but were visible in SEM images of epoxy resin casts.