The early–middle Cambrian (Fortunian to Age 4) is characterized by a significant increase in metazoan diversification. Furthermore, this interval is marked by a prominent environmental and ecological expansion of arthropod- and echinoderm-rich biotas. Recent redox work has suggested that this shift occurred during stable or decreasing marine oxygen levels, suggesting that these paleobiological and paleoecological transformations were decoupled from a redox control. We tested this idea by conducting new paleoredox analyses on Age 2–Age 4 Cambrian outer shelf (Jiuqunao-Wangjiaping), slope (Wuhe-Geyi), and basinal (Zhalagou) sections of the South China Craton. Multiple sections indicate that mid-depth waters transitioned from anoxic conditions during Cambrian Age 2 to stable oxic conditions during Cambrian Age 4. These findings suggest a stepwise expansion of oxic waters from shallow to deep settings during the early–middle Cambrian, consistent with a redox control of metazoan diversification and ecological expansion. More broadly, despite the surge in redox work over the past decade, this study highlights the need for continued coupled redox and paleontological studies to directly test models about the links between the evolution of animals, ecosystems, and marine redox conditions.

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