Chemosymbiotic trophic strategy in an Ediacaran tubular animal.

Abstract

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 (δ⁹⁸Mo) values in pyritized fossils of the terminal Ediacaran <i>Conotubus</i>, a nonbiomineralized cloudinomorph representing some of the earliest metazoans. Such low δ⁹⁸Mo values are otherwise only known in modern cold-seep chemosymbiotic tubeworms hosting sulfur-oxidizing bacterial symbionts. Homogeneously low δ³⁴S signatures of fossils indicate that rapid pyritization in early diagenesis under euxinic taphonomic settings preserved primary <i>Conotubus</i> chitinous δ⁹⁸Mo. Substantial δ³⁴S fractionation relative to seawater sulfate and high Δ³³S values indicate that active microbial H₂S oxidation occurred within their habitat. Our findings provide evidence that <i>Conotubus</i> may have engaged in chemosymbiosis with sulfur-oxidizing bacteria-the earliest geochemically supported case in the fossil record. This trophic strategy likely enabled <i>Conotubus</i> to thrive in the redox-stratified Ediacaran ocean where sulfide and oxidants coexisted, facilitating its ecological success.