Unique fossil preservation in ferruginous Silurian deposits from the Carnic Alps, Italy

Conodont residues from Silurian calcareous levels exposed in the Rio Tamer area of the Carnic Alps, Italy, have produced three-dimensional fossil skeletal remains that are typically not preserved in organisms with calcareous shells, as such structures dissolve during standard acid laboratory processing. This unique preservation has been facilitated by early precipitation of iron oxides and hydroxides, which replicated the original skeletons. The fossil assemblage is dominated by benthic fauna and includes echinoderms, trilobites, bivalves, ostracods and gastropods, preserved in the form of ferruginous external coatings or internal moulds. These often retain fine details of the original shell structure. The material was characterised by using optical and scanning electron microscopy (SEM), environmental scanning electron microscopy coupled with microanalyses (SEM/ESEM-EDX), X-ray powder diffraction (XRPD) and confocal laser Raman microscopy. The combined results indicate that the skeletal replicas are primarily composed of goethite, chamosite and hematite, with subordinate amount of siderite, birnessite, quartz and amorphous material. Additionally, the presence of carbonaceous material within the coatings suggests a biologically mediated process in crust formation. Overall, the findings from this study highlight the role of microbial activity in triggering unusual and unique fossil preservational pathways.