Heavy metal toxicity and its role as a major driver of past biodiversity crises

Abstract Whether today’s heavy metal pollution constitutes an unprecedented threat to biodiversity remains unresolved. Although evidence of metal enrichment exists for several deep-time biotic crises, direct causal links to extinction are lacking. This review synthesizes the current understanding of the relationship between elevated environmental heavy metal concentrations and biological responses, including malformations and extinctions, across marine and terrestrial ecosystems. Drawing on analogies from modern environments, we assess the role of heavy metal toxicity in past mass extinction events. While some studies implicate heavy metals as primary extinction drivers, shown for instance by sporomorphs malformations, others interpret these signals as post-extinction artifacts linked to organic matter decomposition. Our analysis highlights the influence of normalization methods, particularly sulfur-based approaches, and emphasizes the role of metal bioavailability as a key factor influencing biological effects. Further research is needed to clarify the role of heavy metals in Earth’s biotic history and ongoing environmental risks.