The Cretaceous-Tertiary boundary and the last of the dinosaurs

Abstract

Disaster theories of the K-T extinctions, more specifically dinosaur extinctions, are presently engendering much controversy. They require ( inter alia ) that those extinctions were sudden and simultaneous worldwide and that they coincided with an allegedly causal disaster at the K-T boundary. This paper reviews the evidence for and against those temporal requirements. The other major requirement is of a biological nature, namely an indication of the manner in which the specified disaster might have extinguished the organisms concerned; yet this causal mechanism, whatever it might have been, apparently had no effect whatever upon other, very similar organisms. In any particular geographical region, the precise stratigraphic level at which dinosaurs became extinct can be determined only if there is a virtually unbroken succession of potentially dinosaur-bearing continental beds that pass up from the level of the highest dinosaur known to a level well above the K-T boundary. Unfortunately there are surprisingly few regions where such conditions prevail. The problem is further complicated by the difficulties of worldwide stratigraphic correlation and by the fact that specialists in different fields define the position of the K-T boundary on different criteria. Although some alleged discoveries of Palaeocene dinosaurs have long been discredited (the beds were not Palaeocene, or the bones were not dinosaurian), there does seem to be some evidence that dinosaurs died out at different times in different places, sometimes surviving whatever it was that produced the iridium anomaly and sometimes co-existing with Palaeocene palynomorphs and Tertiary-type mammals, or both. In such cases it does not seem unreasonable to postulate a Danian age for the animals in question.