CLIMATE AND COMPETITION AS CONTROLS ON THE ECOLOGICAL ASCENDANCY OF DINOSAURS (Invited Presentation)

Dinosaurs are one of the most extensively studied groups of fossil vertebrates, but the impacts of large-scale environmental changes on their evolutionary history remain contentious. Climate has been hypothesised to have been a fundamental control on dinosaur, particularly sauropodomorph, distribution and evolution during their rise to ecological dominance in the Late Triassic and Early Jurassic (237–174 million years ago). However, hypotheses of climatic influences on early dinosaurs are based on indirect, localised or qualitative observations, and have not yet been explicitly tested. We test early dinosaur climatic niche evolution using, for the first time, a general circulation climatic model (HadCM3L), combined with comprehensive phylogenetic and fossil occurrence data. We find that, in the Late Triassic, early sauropodomorph dinosaurs occupied generally cooler regions with more seasonably variable temperatures. Unlike other tetrapods, including non-sauropodomorph dinosaurs, Late Triassic sauropodomorphs were excluded from the hottest, low-latitude climate zones. The Triassic/Jurassic boundary saw a reorganisation of these climate distributions in which sauropodomorphs expanded their climatic niche to match that of other tetrapods. This climate niche expansion co-occurs with geographical expansion and phenotypic radiation, and occurred independently in multiple lineages of sauropodomorph, suggesting an extrinsic driver. We propose that these patterns result from ecological release following extinctions of Triassic herbivorous groups such as aetosaurs, which had their diversity peaks in hot, low latitude regions. These patterns are consistent with biotic competitive exclusion of sauropodomorphs from some regions in the Triassic, supporting the key role of the end-Triassic mass extinction in the ecological ascendancy of dinosaurs.