The concept of a sudden extinction of the dinosaurs, consequent upon the impact 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. The wave of extinctions, so often attributed to the impact, is equally disputable. 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 extinct. The dating of the extinction of the larger saurischians and of the ornithischians, based as it is upon evidence from only one small corner of the globe, is equally disputable. Whenever it happened, that extinction appears 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 questionable; if so, it appears to have been not worldwide, but confined to a limited region of the Americas.