The "Great Extinction" that never happened: the demise of the dinosaurs considered

The concept of a sudden extinction of the dinosaurs, consequent upon the im pact of some extraterrestrial object, is so dramatic that it has taken hold upon the imaginations of many scientists, as well as of the general public . The evidence for an impact, at approximately the level of the Cretaceous- Tertiary boundary, is impressive. Whether it was the cause for the iridium concentrations, so widely distributed at that level, remains disputable. Th e wave of extinctions, so often attributed to the impact, is equally disput able. It is now evident that no clear line can be drawn between the smaller theropod dinosaurs and the birds. In that sense, the dinosaurs are not ext inct. The dating of the extinction of the larger saurischians and of the or nithischians, based as it is upon evidence from only one small corner of th e globe, is equally disputable. Whenever it happened, that extinction appea rs to have been the product of natural causes - a slow decline, occasioned by environmental changes, and not an extraterrestrially induced catastrophe . Whether the impact had any effect at all upon the dinosaurs is questionab le; if so, it appears to have been not worldwide, but confined to a limited region of the Americas.