The Mesoproterozoic-Neoproterozoic transition represents a crucial interval of evolutionary innovation and phylogenetic diversification. A spotty fossil record indicates that several clades of multicellular eukaryotes, including red and green algae, as well as fungi, may have diverged and diversified at this transition. However, the Meso-Neoproterozoic fossil record remains poorly documented, particularly at the assemblage level, greatly limiting our understanding of the marine ecosystem in this critical time interval. Here we report a well-preserved assemblage of organic-walled microfossils from the Meso-Neoproterozoic Diaoyutai Formation (ca. 1050–950 Ma) in Southern Liaoning Province of the North China Block. Raman geothermometric analysis of these organic-walled microfossils suggests that the Diaoyutai Formation experienced relatively low-grade metamorphism, with apparent peak metamorphic temperatures of ∼ 215 ℃. The Diaoyutai assemblage consists of 14 species belonging to 9 genera, as well as several unnamed or unidentified forms. In addition to simple forms such as aggregated coccoidal and unbranched filamentous fossils that are often interpreted as cyanobacteria, a moderate diversity of eukaryotes is also observed. Probable eukaryotic fossils in this assemblage include the branched multicellular alga Proterocladus antiquus that has been previously interpreted as a green alga, several morphologically complex spheroidal taxa (e.g., Germinosphaera bispinosaPterospermopsimorpha insolitaP. pileiformis and Squamosphaera colonialica), and relatively large leiospheres (50–700 μm in diameter). We note that the occurrence of Proterocladus antiquus in the Diaoyutai assemblage predates its previously reported occurrence from the overlying Nanfen Formation. Overall, the Diaoyutai assemblage presents a glimpse of the coastal marine ecosystem at the Meso-Neoproterozoic transition, which consisted of benthic microbial mats dominated by filamentous cyanobacteria, millimeter-sized benthic green algae with thallus elevating above the sediment–water interface, and micrometer-sized planktonic prokaryotic and eukaryotic organisms. From the perspective of primary producers, the rise of photosynthetic eukaryotes in the Meso-Neoproterozoic represents a transformative innovation to modernize the coastal marine ecosystem in terms of ecological functions.

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New record of the green algal fossil Proterocladus and coexisting microfossils from the Meso-Neoproterozoic Diaoyutai Formation in southern Liaoning, North China