ONE-BILLION-YEAR-OLD EUKARYOTIC EPIBIONTS IN NORTH CHINA

Symbiotic associations, such as mutualism, commensalism, and parasitism, play a pivotal role in accelerating eukaryotic evolution and diversification. The Proterozoic fossil record of symbiosis, however, is poorly documented despite the abundance of Proterozoic eukaryotes and their increasing diversity in the Neoproterozoic. The dearth of direct fossil evidence for symbiosis impedes our ability to analyze the ecological impact of symbiosis on the evolution and diversification of early eukaryotes. Here we present epibiont fossils, preserved as abundant dark discoidal, semicircular, or ovate structures and associated with the early Neoproterozoic eukaryotic fossils Tawuia and Sinosabellidites from North China. Our integrated data of optical and electron microscopy and Raman spectroscopy show that these epibionts were likely eukaryotic symbionts that lived on the surface of and may have benefited from an association with their Tawuia and Sinosabellidites hosts, representing a case of ectosymbiotic commensalism. The new material represents some of the earliest known fossil evidence of eukaryotic symbiosis which, together with other contemporaneous evolutionary innovations, may have contributed to the ecological expansion of eukaryotes in the Neoproterozoic.