Near-equatorial peak diversities are a prominent first-order feature of today’s latitudinal diversity gradient (LDG), but were not a persistent pattern throughout geological time. In an analysis of Ordovician (485–444 Ma) fossil occurrences, an equatorward shift of the latitudinal diversity peak can be detected. A modern-type LDG and out-of-the-tropics range shift pattern were synchronously established during emerging icehouse conditions at the climax of the Great Ordovician Biodiversity Event. The changes in the LDG pattern and range shift trends can be best explained as a consequence of global cooling during the Middle Ordovician and of diversification in the tropical realm following a greenhouse period with temperatures too hot to support diverse tropical marine life. These results substantiate a fundamental role of temperature changes in establishing global first-order diversity patterns.