ARCTIC ICE AND THE TRIASSIC ORIGIN AND ECOLOGICAL ASCENT OF THE DINOSAURS

The Mesozoic is usually portrayed as having a CO₂-driven warm and equable climate, in which dinosaurs and pterosaurs evolved during ice-free times. But, four sets of observations suggest that cold, in fact, freezing, was key in not only to their origin, but also their survival through the end-Triassic extinction (ETE) and consequent ecological ascent. First, the most parsimonious phylogenetic hypothesis has pterosaur and dinosaur insulating filaments as feather homologs. Insulation was thus present in basal dinosaurs conferring selective advantages for cold-climate resource use. Second, Late Triassic continental biotas were profoundly provincial with herbivorous dinosaurs restricted to higher-latitudes where they are among the most abundant tetrapods. High-latitude regions were also extensively vegetated. In contrast, non-insulated pseudosuchians and small non-dinosaurian dinosauromorphs dominated low-latitudes. Third, only small protosuchian and sphenosuchian pseudosuchians survived the ETE, while dinosaurs and pterosaurs were virtually unaffected. The continental ETE is consistent with volcanic-winters of the Central Atlantic Magmatic Province decimating large non-insulated pseudosuchians, allowing dinosaurian ecological ascent. Low-latitude plants were similarly decimated, while highlatitude forms were not. Finally, we have found abundant lake-ice-rafted debris in the Early Mesozoic, coal-bearing, ~70°N Junggar Basin (China) associated with dinosaur footprints. Thus, that region had freezing winters, and dinosaurs were there and could take advantage of the vegetation. Thus, both the high-latitude restriction of herbivorous dinosaurs during the Triassic and the survival of dinosaurs though ETE volcanic winters, suggest that dinosaurian and pterosaurian success was fundamentally connected with adaptations for cold, specifically insulation and high metabolism.