Unlocking a hidden fossil archive: synchrotron micro-computed tomography illuminates ‘ghost’ radiolarians linked to Late Ordovician oceanic anoxic events

Abstract Recent advancements in synchrotron-based micro-computed tomography (microCT) enable non-destructive visualization of fossils that were previously inaccessible using mechanical preparation or acid digestion. Here, we present, to our knowledge, the first high-resolution synchrotron microCT imaging of Upper Ordovician radiolarians preserved in petra as bitumen- and dolomite-filled hollow moulds (‘ghost’ fossils) within black shales of the Wufeng Formation, Sichuan Basin, China. From a single 2.5 mm diameter mini-core, we digitally reconstructed more than 20 specimens, representing eight species, including one new taxon, revealing substantially greater radiolarian diversity than previously recognized from this interval. The method overcomes long-standing recovery biases caused by dissolution of siliceous skeletons during diagenesis, demonstrating that black shales globally may host rich yet overlooked radiolarian archives. This discovery has significant implications for understanding planktonic responses to Late Ordovician environmental change and the dynamics of oceanic anoxic events. Synchrotron microCT provides a scalable workflow for systematic reassessment of microfossil diversity and taphonomy in deeply buried, lithologically challenging successions. This approach opens a previously inaccessible fossil archive essential for refining models of ecosystem resilience during past climate warming.