What really killed the dinosaurs?

ONE THING about the prehistoric past which almost everybody thinks they know is that the dinosaurs (those, at least, that did not belong to the group of animals now known as birds) were wiped out more or less instantaneously by a collision between Earth and a very large space rock. The crater from that collision was discovered decades ago in southern Mexico. The effects of the giant waves created by the impact can be seen in places like Hell’s Creek, near Bowman, North Dakota, where marine creatures were swept far inland. And modelling suggests the planet would have been a pretty uncomfortable place for quite some time afterwards, with ejecta suspended in the atmosphere blotting out the sun, and acid rain changing the chemical composition of the oceans.And yet... a small group of holdouts paint a different picture. Yes, they say, Earth was indeed hit by an asteroid or comet some 66m years ago at the end of the Cretaceous period. But that was either a coincidence or the straw that broke the planet’s ecological back. For the rocks also show that a series of huge volcanic eruptions was happening at the time in what is now India. Toxic and climate-changing gases from these eruptions, they suggest, were the underlying cause of the mass extinction that did for the dinosaurs—a point of view backed up by the fact that two earlier mass extinctions, those at the ends of the Permian and Triassic periods, coincided with similar eruptions while showing no sign of an asteroid strike. Conversely, several other large bolides are known to have arrived at various times in the past without accompanying extinctions.