Description: Flood basalt volcanism has been implicated in several episodes of mass extinctions and environmental degradation in the geological past, including at the Triassic–Jurassic (Tr–J) transition, through global warming caused by massive outgassing of carbon dioxide. However, the patterns of biodiversity loss observed are complicated and sometimes difficult to reconcile with the effects of global warming alone. Recently, attention has turned to additional volcanic products as potential aggravating factors, in particular sulphur dioxide (SO 2 ). SO 2 acts both directly as a noxious environmental pollutant and indirectly through forming aerosols in the atmosphere, which may cause transient global dimming and cooling. Here, we present a range of morphological changes to fossil plant leaf cuticle surfaces of hundreds of Ginkgoales and Bennettitales specimens across the Tr–J boundary of East Greenland. Our results indicate that morphological structures of distorted cuticles near the Tr–J boundary are consistent with modern cuticle SO 2 -caused damage and supported by recent leaf-shape SO 2 proxy results, thus identifying cuticle surface morphology as a potentially powerful proxy for SO 2 . Recording the timing and duration of SO 2 emissions in the past may help distinguish between the driving agents responsible for mass extinction events and thus improve our understanding of the Earth System.