Chemosymbiotic trophic strategy in an Ediacaran tubular animal

Chemosymbiosis may have been a key driver of early animal evolution and diversification. Yet evidence for animal–microbe nutritional partnerships in the fossil record of early metazoans remains scarce, largely owing to the limited availability of robust geochemical proxies and suitable fossilization pathways. Here, we report extremely low molybdenum isotope (δ 98 Mo) values in pyritized fossils of the terminal Ediacaran Conotubus , a nonbiomineralized cloudinomorph representing some of the earliest metazoans. Such low δ 98 Mo values are otherwise only known in modern cold-seep chemosymbiotic tubeworms hosting sulfur-oxidizing bacterial symbionts. Homogeneously low δ 34 S signatures of fossils indicate that rapid pyritization in early diagenesis under euxinic taphonomic settings preserved primary Conotubus chitinous δ 98 Mo. Substantial δ 34 S fractionation relative to seawater sulfate and high Δ 33 S values indicate that active microbial H 2 S oxidation occurred within their habitat. Our findings provide evidence that Conotubus may have engaged in chemosymbiosis with sulfur-oxidizing bacteria—the earliest geochemically supported case in the fossil record. This trophic strategy likely enabled Conotubus to thrive in the redox-stratified Ediacaran ocean where sulfide and oxidants coexisted, facilitating its ecological success.