The late Ediacaran to early Cambrian interval witnessed extraordinary radiations of metazoan life. The role of the physical environment in this biological revolution, such as changes to oxygen levels and nutrient availability, has been the focus of longstanding debate. Seemingly contradictory data from geochemical redox proxies help to fuel this controversy. As an essential nutrient, nitrogen can help to resolve this impasse by establishing linkages between nutrient supply, ocean redox, and biological changes. Here we present a comprehensive N-isotope dataset from the Yangtze Basin that reveals remarkable coupling between δ15N, δ13C, and evolutionary events from circa 551 to 515 Ma. The results indicate that increased fixed nitrogen supply may have facilitated episodic animal radiations by reinforcing ocean oxygenation, and restricting anoxia to near, or even at the sediment–water interface. Conversely, sporadic ocean anoxic events interrupted ocean oxygenation, and may have led to extinctions of the Ediacaran biota and small shelly animals.

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, exposes one of the world's oldest ecosystems. It includes kerogen-rich laminae and thin chert bands interbedded with coarse-grained and gravelly sandstones. The strata record a medium-energy, tidal coastal environment. Analyses of the microscopic structure and chemical composition of the chert bands through petrographic microscopy, Raman microspectroscopy, laser-induced breakdown spectroscopy (LIBS) analyses, C isotopes, and scanning electron microscope (SEM) photography of macerated material, supported by textural observations of hand samples, suggest that these laminae represent variably compressed and early-silicified microbial mats. Internal wavy laminations, amorphous carbon composition, and negative d 13 C values strongly imply a biogenic origin. Complete HF maceration of chert bands revealed polygonal cell structures in a formerly extracellular polymeric substance matrix. The tuft-and dome-micromorphology of the laminations resembles that of recent photosynthetic filament-dominated microbial mats. Facies interpretations indicate that microbial mats extensively colonized subtidal to intertidal Archean siliciclastic coastlines.

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