What killed the dinosaurs?

A crater 180 to 200 kilometres across lies buried at the northern edge of the Yucatán peninsula, its centre just off the coast. Its age matches the boundary layer to within the resolution of the dating.

The boundary clay itself carries the signature worldwide: an excess of iridium, the anomaly that first pointed to an asteroid, shocked quartz, and glass droplets that were molten rock thrown into the air.

A review by forty-one researchers in 2010 concluded that the impact triggered the extinction, and that the pattern of who died agrees with what its modelled effects, darkness, cooling and the collapse of photosynthesis in the ocean, would do.

The Deccan Traps in India poured out between half a million and one and a half million cubic kilometres of lava across the same interval, depending on the estimate, releasing carbon dioxide and sulphur on a scale that affects climate.

Dating published in 2015 found that the eruptions changed character close to the boundary, shifting to fewer and larger flows within about fifty thousand years of the impact, and proposed that seismic energy from the impact may have provoked the change.

So the two need not be independent candidates competing for the blame. They may be one sequence.

A 2020 analysis of climate records across the boundary found warming consistent with Deccan carbon release before the impact, and concluded that the extinction itself tracks the impact rather than the volcanism.

Others read the volcanism as the setup. A 2015 review of the dinosaur record allowed that Deccan climate change may have left communities less able to absorb the impact, and a 2025 recalculation of the lava volumes put about seventy per cent of the lava before the boundary and argued that their gases set the stage for the extinction. The dating precision needed to settle this is at the edge of what the methods deliver.

Birds are dinosaurs, and only one branch of them, the lineage of modern birds, came through. Many lizards, snakes and crocodile relatives died too. Freshwater amphibians and turtles lost less, and every group of mammals was hit hard.

Among mammals, the larger-bodied and the specialised feeders fared worst, and life in fresh water appears to have helped. One explanation offered for the freshwater advantage is food chains running on detritus rather than on sunlight.

The pattern is suggestive rather than explained. A 2015 review listed why certain birds survived, while many bird-like feathered dinosaurs died, among the questions for the decade ahead.