From Shell to Iron: Microbially Mediated Mineral Replication of Calcareous Skeletons

Conodont residues extracted from Silurian limestones of the Carnic Alps (Italy and Austria) yielded exceptionally preserved three dimensional ferruginous coatings and moulds of originally calcareous skeletal elements. Such preservation is highly unusual, as calcareous shells typically dissolve during standard acetic acid processing. The replicas mostly include bryozoans, echinoderms, trilobites, bivalves, ostracods, gastropods and brachiopods, and often retain fine morphological details of the original skeletal structures. Their formation was facilitated by early precipitation of iron oxides and hydroxides, which replicated the skeletons prior to dissolution. A multi analytical approach, including optical microscopy, SEM/ESEM EDX, XRPD, and confocal Raman spectroscopy, reveals that the ferruginous coatings consist predominantly of chamosite, magnetite, goethite and hematite, with subordinate siderite, birnessite, quartz, and amorphous phases. The consistent association of carbonaceous material with these ferruginous laminations indicates a strong microbial influence in crust formation. The mineral suite and textural relationships point to early diagenetic processes operating under fluctuating redox conditions within an iron-rich shallow marine environment. Alternations between oxidizing and reducing microenvironments likely governed episodic accretion of Fe oxide layers, generating the characteristic laminated patterns observed. Overall, the Carnic fossils demonstrate that microbially mediated ferruginous mineralization can enable exceptional preservation of calcareous organisms, providing valuable insights into the interplay between microbial activity, early diagenesis, and fossilization in Paleozoic marine ecosystems.