Dino killer struck in spring
Ateam of European scientists have worked out the time of year when a huge asteroid struck Earth 66 million years ago, and say it could help explain why the impact destroyed so many living things. The asteroid, which hit Mexico’s Yucatán Peninsula, was the final blow in the extinction of the dinosaurs that had ruled Earth for 170 million years. It left birds as the only surviving dinosaur relatives and cleared the way for the expansion of mammals (warm-blooded, usually furry animals that make milk for their young).
Until now, experts have struggled to explain just why some species survived and others didn’t. The new research offers important clues. It suggests the asteroid struck in spring for the northern half of the world – a time when many animals were emerging from the snug winter burrows that might otherwise have protected them.
The evidence that places the asteroid impact in spring comes from a remarkable site called Tanis, in the US state of North Dakota. Here, scientists have found the fossils (preserved remains turned into rocky minerals) from fish that died as their gills were choked with debris falling from the impact itself. A team led by Marianne During of Uppsala University in Sweden realised that the fish grew their bones more at some times of year than others. By looking at the state of the fossilised bones, the team could work out the season when the fish had died.
The impact caused an enormous blast of heat and fires that probably killed most large animals immediately. Small animals living closer to the impact site, and those that were waking up in spring, would also have died almost instantly. Then, as dust from the explosion spread through the air and blocked out the Sun’s heat and light, the whole planet was plunged into a long period of cold and darkness.
This killed plants and led both plant-eating and meat-eating animals to die off – about 76% of all species. If smaller, southern-hemisphere animals were already in their winter shelters when the asteroid struck, those that could find food when they emerged months later might have spread across the recovering planet.