VOLCANIC WINTER SCENARIO FOR THE SELECTIVE NATURE OF THE TRIASSIC-JURASSIC TETRAPOD TRANSITION ON LAND

A major challenge facing our understanding of mass extinctions is constraining the causes of the selectivity of mass extinctions. The end-Triassic mass extinction (ETE) was among the largest or these, and until recently one of the more poorly constrained. But, advances in magnetostratigraphy and geochronology have illuminated a pattern among continental tetrapods that suggests an origin for the selectivity. Prior to the ETE in the Late Triassic, there was strong zonal biotic provinciality, perhaps linked to high-CO₂ conditions. Herbivorous dinosaurs were limited to high latitudes, and pseudosuchians were strongly dominant in low latitudes. After the ETE the provinciality seems to have been largely eliminated, pseudosuchians and labyrinthodonts were nearly wiped out, and both herbivorous and carnivorous dinosaurs became abundant globally, while semi-aquatic tetrapods were decimated. The pattern suggests enhanced survivorship for higher latitude groups, particularly homeothermic endotherms with filamentous proto-feather insulation, as well as small, plausibly burrowing ectothermic forms. This includes dinosaurs, pterosaurs, small crocodylomorphs, lepidosaurs, turtles, mammaliformes, and lissamphibia. In contrast, large, primitively poikiolothermic, uninsulated ectotherms, such as the large psedosuchians dominant in the tropics were wiped out, and labyrinthodont amphibians were also nearly eliminated. The ETE has been convincingly related to the emplacement of the Central Atlantic Magmatic Province (CAMP), and while the greenhouse effects of CO₂ have been stressed, especially for marine extinctions, a plausible cause for the continental selectivity could have been multiple, short, extreme cold spells caused by sulfur aerosols, for which the large poikiolothermic ectotherms were ill adapted. These many “volcanic winters” punctuated at least 4 much longer CO₂ doublings, which, although they could have produced lethally high temperatures in the tropics, would not have been lethal at high latitudes. While both effects of the CAMP plausibly played a role, the biotic pattern is more consistent with survivors being pre-adapted to cold, rather than being resistant to extreme warmth.