Simple methods for detection of microborings produced by coral-associated microendoliths

Well-preserved skeletons of Paleocene and Eocene scleractinians and octocorals (Polytremacis sp.) from Poland and Ukraine were studied to reveal microborings produced in vivo by coral-associated microendoliths. Microborings (mostly < 5 μm in diameter) are hardly visible, if at all, under a petrographic microscope. Their resin casts are obtained, however, through the epoxy vacuum cast-embedding technique and observed under a scanning electron microscope (SEM). Three-dimensional resin-filled (cast) microborings are also clearly visible under SEM in acid-etched petrographic thin-sections. Backscattered scanning electron microscopy imaging (BSE) is useful for visualization of the microborings during SEM study of both etched and non-etched thin-sections. A simple but very effective method to reveal the dense network of resin casts of microborings is observations of etched thin-sections under the petrographic microscope. Fluorescence microscopy (FL), especially with application of blue and green filters (Nikon’s B-1A and G-2A filter cubes), is recommended if etching thin-sections or polished samples is not possible. However, color contrast between the resin casts and the calcium carbonate of the coral skeleton was strong enough only in some examined thin-sections. The cathodoluminescence microscopy, the other method, does not require the etching of the thin-sections and is potentially useful for detection of microborings filled with calcite cement, although this technique was not applicable for the samples studied. Symbiotic coral-microendolith association (in broad meaning of the term symbiosis) is a common phenomenon in modern corals, but its fossil record is very sparse. This study shows that empty microborings can be common in fossil corals, allowing preparation of the resin casts. Some of the tested methods permit rapid detection of resin-filled microborings in thin-sections even by non-specialists, and selection of samples for SEM studies. Corals from claystones and mudstones, usually less affected by diagenesis, have higher taphonomic potential for preservation of empty microborings than corals from reef facies. The methods discussed here can be also applied for rapid detection of post-mortem microborings occurring in other substrates.