Early Jurassic origin of avian endothermy and thermophysiological diversity in dinosaurs

A fundamental question in dinosaur evolution is how they adapted to long-term climatic shifts during the Mesozoic and when they developed environmentally independent, avian-style acclimatization, becoming endothermic.^{1 , 2} The ability of warm-blooded dinosaurs to flourish in harsher environments, including cold, high-latitude regions,^{3 , 4} raises intriguing questions about the origins of key innovations shared with modern birds,^{5 , 6} indicating that the development of homeothermy (keeping constant body temperature) and endothermy (generating body heat) played a crucial role in their ecological diversification.⁷ Despite substantial evidence across scientific disciplines (anatomy,⁸ reproduction,⁹ energetics,¹⁰ biomechanics,¹⁰ osteohistology,¹¹ palaeobiogeography,¹² geochemistry,^{13 , 14} and soft tissues^{15 , 16 , 17}), a consensus on dinosaur thermophysiology remains elusive.^{1 , 12 , 15 , 17 , 18 , 19} Differential thermophysiological strategies among terrestrial tetrapods allow endotherms (birds and mammals) to expand their latitudinal range (from the tropics to polar regions), owing to their reduced reliance on environmental temperature.²⁰ By contrast, most reptilian lineages (squamates, turtles, and crocodilians) and amphibians are predominantly constrained by temperature in regions closer to the tropics.²¹ Determining when this macroecological pattern emerged in the avian lineage relies heavily on identifying the origin of these key physiological traits. Combining fossils with macroevolutionary and palaeoclimatic models, we unveil distinct evolutionary pathways in the main dinosaur lineages: ornithischians and theropods diversified across broader climatic landscapes, trending toward cooler niches. An Early Jurassic shift to colder climates in Theropoda suggests an early adoption of endothermy. Conversely, sauropodomorphs exhibited prolonged climatic conservatism associated with higher thermal conditions, emphasizing temperature, rather than plant productivity, as the primary driver of this pattern, suggesting poikilothermy with a stronger dependence on higher temperatures in sauropods.