Unique fingerprint of marine ectotherm body size change during hyperthermal crises

The term Lilliput Effect describes a substantial decrease in the average body size of fossil assemblages during major environmental perturbations in Earths history, which is reported in many paleontological studies. The limited regional, temporal and taxonomic focus of most studies, however, has sparked discussions concerning its generality. Additionally, even though a negative relationship between warming and body size has been established in recent marine ectotherms, the environmental and mechanistic drivers of the Lilliput effect are still debated. We compiled close to 9000 body size changes from fossil, historical and modern body size studies, to show that a decrease in body size is indeed a general response of marine ectotherms to environmental crises. The magnitude and temporal variability of size changes at the species-level are significantly higher during hyperthermal than non-hyperthermal events, suggesting differing mechanisms of body size decrease depending on the environmental stressor. Our results further show that ancient environmental perturbations with a higher magnitude of warming were associated with a greater dwarfing. This implies that warming was a major driver of body size decreases during hyperthermal events throughout the Phanerozoic, and future warming will impact current trajectories of body size reduction in modern marine ectotherms. Significance StatementDwarfing is a common response of marine ectotherms to both anthropogenic warming and similar environmental crises throughout Earths history. However, the underlying mechanisms and parallels between fossil and modern observations are still debated. We show, using newly compiled data from fossil, historical and modern studies, that warming events elicit especially strong species body size changes, suggesting similar mechanisms linking modern and fossil body size reductions to warming stress. A correlation between the magnitude of warming during ancient crises and the observed dwarfing indicates that trajectories of body size reduction can be projected into the future depending on climate-change scenarios.