The Early Jurassic Toarcian oceanic anoxic event (T-OAE, ca. 183 Ma) was accompanied by a major biotic turnover in the oceans and substantial vegetation change on land. The marine biotic crisis has been attributed to several triggers, e.g., anoxia, warming, ocean acidification, yet the processes underlying the collapse of the terrestrial ecosystem are poorly understood. New high-resolution geochemical and palynological data across the T-OAE from a lacustrine succession in North China reveal elevated occurrences of spore dwarfism, asymmetrical Classopollis tetrads, and aberrant spores coeval with increases in heavy metal (Hg, Cu, Cr, Cd, Pb, As) abundances. The occurrence of teratological spores and pollen in multiple plant groups suggests overall vegetation-scale ecological pressure. Our data indicate that the combination of a widespread floral crisis with higher terrestrial organic matter oxidation and decomposition, enhanced hydrological cycle, and coeval large-scale volcanism resulted in higher concentrations of toxic heavy metals in terrestrial ecosystems. These heavy metals could poison plants, causing mutations and disrupting their reproductive cycle, and making them more vulnerable to secondary stresses such as climatic extremes and/or habitat shifts, eventually leading to widespread collapse across all terrestrial trophic levels.

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