Question: How did the dinosaurs die out?

Susan Cartwright answered on 23 Jun 2015:

The key point about the dinosaur extinction is that it isn’t just the dinosaurs. A wide range of species died out at the same time: estimates suggest over 3/4 of ALL species became extinct at the end of the Cretaceous, including ALL large land animals (I have seen various estimates for the maximum size that survived, from 25 kg down as low as 1 kg; either way, humans would have had it). Therefore, any theory based purely on dinosaurs is bound to fail, because the dinosaurs were just the largest and most spectacular victims.

There is evidence of widespread environmental catastrophe – the so-called “fern spike”, which is a sudden sharp increase in the number of fossil fern spores just after the extinction. In modern times, sudden increases in fern populations occur after fires, when a lot of the competing vegetation has been killed – fern spores are very tough and survive the blaze. So one interpretation of the fern spike is that it points to widespread serious wildfires. There is also a large increase in fossilised fungal spores – again, as fungi feed on dead plant material, this points to a sudden catastrophe, not a gradual decline.

Right at the extinction, there is a thin layer of rock which has a very high concentration of a metal called iridium. Iridium is very rare in the Earth’s crust – it is a heavy metal, and sinks to the Earth’s core along with the nickel-iron early in the Earth’s history – but much more common in asteroids, because asteroids are too small for the heavy elements to have sunk into the middle. So, when this iridium spike was discovered by the geologist Walter Alvarez, his father, Nobel prize-winning physicist Luis Alvarez, suggested that it marked a large asteroid impact. The likely site for the impact was subsequently found: the buried Chicxulub crater in the Yucatan (Mexico).

An asteroid impact would undoubtably cause havoc: there would be a blast wave of superheated air, which could explain the fires, followed by a depression of global temperatures caused by all the dust kicked up into the atmosphere (the 1815 eruption of Mt Tambora, which was peanuts by comparison, caused “the year without a summer”, with widespread crop failures). Reading some of the accounts of what would happen, the only real mystery is why some land species survived! (For example, we lost all the small carnivorous dinosaurs, but we kept the very closely related birds. Was flight crucial to their survival? Why?)

Not everyone believes that the asteroid impact was the main cause of the extinction, though it can’t have helped. At about the same time, there was a massive volcanic episode in India, producing a gigantic lava flow now called the Deccan Traps (“traps” from a Scandinavian word for “stairs”, because of how it looks – not because you can get caught in it!). Large volcanic eruptions also have the potential for causing widespread devastation, so some people think this was the main cause and the asteroid strike a contributory factor. There were also major changes in sea level at about the same time. Of course, it could easily be that all these causes worked together: the fall in sea level stressed the ecosystem, so that it was less able to survive the effects of the impact and the volcanism.