Different numbers of species are found in different regions of the globe and in different environments. The tropics house incredible numbers of species, whereas polar environments house far fewer. This pattern of decreasing number of species from the equator to the poles is referred to as the latitudinal biodiversity gradient. The spatial distribution of life on Earth is well characterised today, but we know relatively little about how spatial patterns of biodiversity have varied over millions of years, during times in which Earth's climate and continents were dramatically different to today. This knowledge gap prevents us from understanding the causes of variation in richness among regions and environments, leaving a fundamental and unanswered question at the heart of biodiversity studies. We will characterise how latitudinal biodiversity gradients in the oceans have varied during the past 545 million years, using the rich fossil record of skeletonising marine invertebrates. This will allow us to ask what environmental factors control the distribution of biodiversity among regions and environments. These deep time patterns will provide important historical context for understanding the distribution of life on Earth, yielding unprecedented insight into the generation and maintenance of marine biodiversity. It will also help us to understand the long-term effects of major shifts in climate state, such as those occurring today, on biodiversity.