Climate state controls the strength of the latitudinal diversity gradient

The latitudinal diversity gradient (LDG)-a decline in taxonomic richness from the Equator to the poles-is a fundamental biogeographic pattern on Earth today, yet its primary regulators remain debated. Whereas many of the proposed climatic and environmental drivers covary with latitude in the present day, these variables have fluctuated substantially over millions of years. The fossil record therefore offers a unique perspective on the response of diversity and, by extension, the LDG, to global climatic change. We test the hypothesis that Earth's long-term climate state controls LDG strength, determining the conditions under which steep, 'modern-type' gradients with low-latitude-centred diversity peaks emerge. By analysing sampling-standardized marine invertebrate diversity across the past 500 million years, we reveal a systematic link: 'modern-type' LDGs typically prevail during coldhouse-coolhouse intervals, whereas flattened LDGs or temperate diversity peaks occur predominantly within transitional, warmhouse and hothouse climates. This robust phenomenon demonstrates that long-term climate exerts a primary control on global biodiversity patterns, whereas palaeogeography, including the distribution of shallow-marine habitat and continental configuration, modulates the regional expression of the LDG, including hemispheric variation. As anthropogenic warming pushes Earth towards hothouse conditions, our results offer a critical geohistorical perspective on the impending restructuring of global biodiversity.