How and why did dinosaurs become extinct?

The simple answer to this question is: it really is not known. The time when the last of the dinosaurs lived - the end of the Cretaceous Period 66 million years ago - also marked the extinction of a wide range of other organisms on Earth, it was actually a time of mass-extinction. Although many groups of organisms became extinct, a whole host of others were largely unaffected. This difference presents a real problem, because whatever happened at the end of the Cretaceous Period it affected some groups much more than others. At present there are two theories popular with palaeontologists who are interested in the end Cretaceous extinctions. Firstly there is the catastrophe theory, which is linked with the idea that a large meteorite hit the Earth at the end of the Cretaceous Period. The evidence for this history has been presented quite recently (1987) following the discovery of a layer of clay at just about the time of the extinction, in which are found unusually high levels of the element Iridium. This relatively rare element - usually found in very small percentages in the Earth's crust - may have been brought to the Earth by a large meteorite which vaporised as it collided with the Earth. The meteorite impact would have created a huge dust cloud which shrouded the Earth for a while, cutting out sunlight, and dramatically affecting the biological systems on Earth, perhaps causing the patterns of extinction which mark the close of the Cretaceous Period. The alternative theory is one which suggests a more gradual pattern of extinctions which was driven by progressive climate change. Careful analysis of the fossil vertebrates living just before and just after the end of the Cretaceous Period suggests that there was a fauna of early mammals living before the end of all the dinosaurs in areas of the world which were much cooler and more seasonal. It is suggested that these spread more and more widely, displacing the dinosaurs which preferred the warmer subtropical conditions that had prevailed until then. Global deterioration of climate is seen as a factor which may have caused extinctions in the sea as well as on land, and is in turn linked to the changing positions of the continents at this time in Earth history. There is no strict agreement between these two theories. It is certainly the case that the coincidence of extinctions among so many groups points to some sort of catastrophe, while there is little doubt that some groups were in decline anyway. Perhaps the steady decline of many groups caused by long-term climatic change was hastened by a timely meteorite strike.