Taphonomy of soft-tissue preservation in ferricrete at the McGraths Flat Lagerstätte

Locating new fossil sites with exceptional soft-tissue preservation is vital for addressing taphonomic bias and accurately reconstructing the fossil record. McGraths Flat, a Miocene rainforest lake Konservat-Lagerstätte, uniquely preserves nanometre-scale soft-tissue structures in laminated Fe-oxyhydroxide (goethite) or ‘ferricrete’. However, constraints on the source of Fe, depositional conditions, and mechanisms that drove the exceptional soft-tissue preservation were lacking. Here, we demonstrate that warm, seasonally wet rainforest conditions generated acidic soils and transported dissolved Fe(II) from weathering basaltic bedrock into an oxbow lake. Fe(III)-oxyhydroxide precipitation then drove fossil burial and mineralization. Our findings suggest that ‘McGraths Flat-type’ fossil preservation may be common and could represent an untapped record of life on land.