By Jeff Hecht
Forget the giant meteor. When the dinosaurs expired it was simply because they ran out of breath. This latest suggestion for how the dinosaurs died emerged last week when four researchers presented the results of their researches at the annual meeting of the Geological Society of America in Boston.
Gary Landis, of the US Geological Survey, measured oxygen in microscopic air bubbles trapped in amber, a fossilised tree resin. He found that the amount of oxygen dropped from 35 per cent 2 million years before the end of the Cretaceous period, to 28 per cent just after the end of the Cretaceous - the time when the dinosaurs all but disappeared.
Today, the oxygen level is at 21 per cent, but unlike the dinosaurs today's animals are built to breathe in this atmosphere. Rich Hengst, a physiologist at Purdue University in Indiana, studied a skeleton of Apatosaurus, one of the largest of the dinosaurs. With nostrils the size of a horse's and lacking a diaphragm to help push air in and out of the lungs, he says, it had a limited capacity to breathe. This was fine in the oxygen-rich environment it evolved in, but not good enough when oxygen levels fell, he says.
Already faced with a cooling climate at the end of the Cretaceous, the drop in oxygen levels would have caused the dinosaurs the same degree of stress as people would suffer if they were transplanted to the Himalayas. Palaeontologists Keith Rigby of the University of Notre Dame in Indiana and Robert Sloan of the University of Minnesota in Minneapolis, say this finished off the dinosaurs slowly. They blame extreme respiratory stress for the reduction in the diversity of the dinosaurs in North America, which shrank from 35 genera 10 million years before the end of the Cretaceous to 12 genera at the end.
Many palaeontologists doubt the catastrophic impact theory, mainly because there was a gradual decline in the diversity of dinosaur species towards the end of the Cretaceous. The oxygen theory is the latest attempt to explain this decline, but it raises its own questions - in particular about how well amber preserves ancient air.
This has not stopped the four from proposing what they call the 'Pele hypothesis', named after the Polynesian goddess of volcanoes. They suggest that changes in the volcanic activity on Earth determine the various proportions of atmospheric carbon dioxide and oxygen levels, which in turn influence life on the planet. The theory 'provides a way to explain evolution by deep-earth processes,' says Landis