When the dinosaurs were wiped off the face of the planet, how did they leave? Was it a slow, plodding decline or a short sharp bang? Back in the 1960s and 1970s, debate about this question was mainly taking place on the ground, at fossil sites in places like Montana. Paleontologist Robert Sloan and his colleagues documented evidence for the long-term decline of dinosaurs over a 10m to 20m-year period. Dinosaurs had been losing out, ever so slowly, to the rising mammals, mainly as a result of cooling climates.
Indeed, climates at this time were cooling. And because dinosaurs relied on the external environment to maintain their body temperatures, this would have hurt them.
But two revelations dramatically switched the consensus against gradual decline. First, the geological field evidence suggested no gradual decline in dinosaur fossils in the rocks. Plus the overlap of declining dinosaurs and rising mammals noted by Sloan turned out to be based on faulty fieldwork and fossil dating. Fossils can be moved from one type of rock to another by being “reworked” or eroded, moved along and then deposited for a second time in younger rocks, providing misleading information about their true age.
The other revelation was the 1980 discovery by Luis Alvarez, which showed that the Earth had been struck by a huge meteorite 66m years ago. It was a collision that threw up vast tonnages of black dust into the atmosphere, which blacked out the sun, leading to freezing and darkness for some months. This was accepted reluctantly at first by geologists, but then enthusiastically as the evidence accumulated.
Impact and sudden death of the dinosaurs made complete sense. The last dinosaurs, such as Triceratops and Tyrannosaurus rex, were imagined as dumbfounded by the asteroid streaking through the sky, and killed wholesale by a consequent fireball and then freezing darkness.