Simple methods for detection of microboringsproduced by coral-associated microendoliths

Abstract

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