Climatic controls on the ecological ascendancy of dinosaurs

The ascendancy of dinosaurs to become dominant components of terrestrial ecosystems was a pivotal event in the history of life, yet the drivers of their early evolution and biodiversity are poorly understood. During their early diversification in the Late Triassic, dinosaurs were initially rare and geographically restricted, only attaining wider distributions and greater abundance following the end-Triassic mass extinction event. This pattern is consistent with an opportunistic expansion model, initiated by the extinction of co-occurring groups such as aetosaurs, rauisuchians, and therapsids. However, this pattern could instead be a response to changes in global climatic distributions through the Triassic–Jurassic transition, especially given the increasing evidence that climate played a key role in constraining Triassic dinosaur distributions. Here, we test this hypothesis and elucidate how climate influenced early dinosaur distribution by quantitatively examining changes in dinosaur and tetrapod ‘climatic niche space’ across the Triassic-Jurassic boundary. Statistical analyses show that Late Triassic sauropodomorph dinosaurs occupied a more restricted ‘climatic niche space’ than other tetrapods and dinosaurs, being excluded from the hottest, low-latitude climate zones. A subsequent, earliest Jurassic, expansion of sauropodomorph geographic distribution is linked to the expansion of their preferred climatic conditions. Evolutionary model fitting analyses also provide evidence for an important evolutionary shift from cooler to warmer climatic niches later on, during the origin of sauropods. These results are consistent with the hypothesis that global abundance of sauropodomorph dinosaurs was facilitated by climatic change and provide support for the key role of climate in the ascendancy of dinosaurs.