Trompe l’oeil fossils from Silurian quartzites of Sierra Morena (South Spain): a taphonomic approach on the study of a coral-like structure

Abstract

Some circular fossils showing radial structures concentrated in a small area have been recorded from the Criadero Quartzite, 1 km north from El Centenillo village (Iberian Central Zone, SE Spain). The fossils resembling cnidarians have been studied in detail, both at the field and in the laboratory. The specimens have been preserved in thick-grained quartz sandstones. They were analysed in thin sections and rock samples using optical microscopy, micro-reflectance spectroscopy, Fourier transform infrared–attenuated total reflectance spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy and epifluorescence microscopy. The radial structure is composed of ridges and grooves, which correspond to clear and dark ochre bands filled with middle-sized, angular-elongated quartz grains without tractive structures. Mineralogical analyses reveal that composition is based on quartz and illite in both areas. The morphological and compositional features of the fossils have been compared with a set of ichnofossils of multiple taxonomic origins and ages without conclusive results. The preservation is probably produced by the existence of microbial mats on the sand floor and the impression of moulds by authigenic cementation by an unidentified organism. To elucidate the nature of the producer, potentially soft-bodied cnidarian, including fossil medusae and sea anemone, further investigations are needed.