This study provides an exceptional, high-resolution, specimen-level record of marine bivalve body-size dynamics across the Permian–Triassic mass extinction, one of the most extreme climate crises in Earth’s history. By distinguishing species- from genus-level responses, the authors show that post-extinction size reductions are driven mainly by faunal turnover and the emergence of smaller species, rather than by universal dwarfism within surviving taxa, thereby refining interpretations of the “Lilliput effect.” The identification of two distinct recovery phases highlights the complex interplay of evolutionary and environmental controls during prolonged climate stress and offers valuable context for understanding biological responses to rapid warming in the past and future.