Fossil Metal and Zinc Isotopic Compositions Reveal Potential Application in Palaeo-Metallomics: An Example of Jurassic Clam Shrimps

Bio-essential metals are fundamental mediators at the environment-life interface, with their distributions reflecting both biological regulation and environmental influences. However, the metal signatures of ancient organisms remain poorly constrained and their role in the evolution of life has been largely underestimated, mainly due to uncertainties in the preservation of these signatures during fossilization and diagenesis. In this study, we developed a standardized methodological protocol to precisely analyze elemental concentrations and zinc (Zn) isotopic compositions in exceptionally preserved clam shrimps (Spinicaudatans) fossils and their host sedimentary matrix from the Daohugou biota of Northeast China, paired with modern clam shrimp analog samples. Our objectives were to systematically investigate the taphonomic processes controlling metal preservation in phosphatized fossils, trace the sources of enriched bio-essential metals, and evaluate the potential of these fossils for reconstructing the primary biogenic palaeo-metallome. We demonstrate that organism-driven authigenic phosphate formation leads to selective enrichment of apatite-compatible elements (e.g., Mn, REEs) in the fossils, while bio-essential metals (Fe, Co, Ni, Cu, Zn) are largely retained from the original biological tissues, despite minor modification by late hydrothermal fluids. Zn isotope data confirm that Zn in the fossils is derived from two distinct end-members: an in-situ biogenic component with a heavy isotopic composition and a hydrothermal component with a light isotopic composition. By reconstructing the primary biogenic Zn isotope signature (δ66Zn ≥ 0.56‰), we show that this heavy signal reflects a benthic ecological niche with a trait has remained in remarkable evolutionary stasis relative to extant taxa. Our study confirms the ability of phosphatized invertebrate fossils to record the primary metal composition of ancient organisms, and provides an evidence-based methodological framework for future deep-time palaeo-metallomic research.