Identifying biotic and abiotic roles in marine clade extinction.

Clade extinction is a crucial process that contributes to the loss of evolutionary history. The extinction of abundant and diverse clades has been linked to a range of abiotic and biotic factors, but their relative contributions and importance remain debated. Here, by analysing the fossil record of 12 major extinct marine clades, we show a consistent extinction trajectory comprising two phases: a slow, age-dependent decline followed by a fast, environment-triggered wipe-out. In the first phase, the clade proportional diversity (the diversity of a clade relative to the combined diversity of its ecologically similar taxa) steadily declined, largely independent of environmental factors such as temperature, dissolved oxygen, sea level and carbon-cycle variations, and instead reflecting biotic drivers such as competition and niche contraction. Once proportional diversity fell below a critical threshold (mean of approximately 0.13), major abiotic perturbations acted as the 'last straw', causing the clade extinction. Simulation and analytical models corroborate this pattern, showing near-zero extinction risk at high proportional diversity, but a steep risk rise once thresholds are crossed owing to reduced ecological dominance and narrowed functional roles. These findings reveal additive effects of biotic-abiotic factors in driving clade extinction, providing a predictive hypothesis for assessing future extinction risk among modern clades.