Approximation of skeletal density in tabulate corals

he density of coral skeletons is an important feature, often overlooked in coral paleontology. In the study of present-day scleractinians it is a metric useful in assessing the influence of such environmental stressors as bioerosion and ocean acidification. It can be measured e.g. on the basis of the weight and volume (Archimedes’ Principle), or using x-ray densitometry. In case of fossil corals however, it is far more difficult to quantify skeletal density. Usually fossil skeletons are filled by cements which are comprised of calcium carbonate, and therefore have the same density as skeletal elements, making measurements difficult. In addition, the coralla commonly occur embedded within a solid rock, and in an incomplete state of preservation. The presented study proposes an easily replicable method of quantification of skeletal density in tabulate corals despite these factors. The approximated skeletal density is here understood as a proportion of skeletal elements to the volume of the corallum, given as an area percentage value. It is measured in oriented sections of coralla. In longitudinal section it offers a rough estimate of skeletal density of the corallum. In transverse sections it allows the approximation of skeletal density at a given stage of astogeny. The oriented thin sections or acetate peels of the coral are photographed in darkfield. The digital photographs are then converted to black and white bitmaps, and basic graphic processing software is used to remove any artifacts and to change the background to a neutral colour. Subsequently the image is analysed using a pixel-counter tool. The result is the number of white (=skeleton) and black (=empty/cement) pixels. The percentage of white pixels is the approximated skeletal density. In the presented study, a juvenile corallum of Favosites goldfussi was used as an example. It comes from the Eifelian of Ouihlane in the Anti-Atlas (Morocco), where a monospecific community of juvenile, short-lived F. goldfussi occurred in deep-water facies of the Mader basin. Ten serial transverse acetate peels were made from the lower parts of the studied corallum, presenting the earliest stages of astogeny. While the representatives of the genus Favosites are generally characterised by relatively thin corallite walls, the approximation of skeletal density yielded results in the range of 85-98%. This anomalously dense skeleton was most likely an adaptation to soft bottom conditions. The corals from the studied strata lived short, probably dying buried by gravitational mass movements. Thick, heavy skeleton could provide them with an anchor in the soft seafloor during early life.