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Mass extinctions and adaptive radiation.
Why did the dinosaurs become extinct at the end of the Cretaceous, after having been so successful for so long? This is a question that has taxed geologist for many years. Various explanations have been proposed, including climate change, volcanic eruptions, and mutations caused by cosmic rays. Probably several factors were at play, and the dinosaurs appear to have been in decline for at least a few hundred thousand years before the knockout blow was delivered by the impact of the large meteorite. The probable impact site is the 180-km-diameter Chicxulub crater off the Yucatan coast of Mexico, which was discovered by geophysical techniques in the 1980s.
Such an impact would have injected a vast amount of dust into the atmosphere. Supplemented by smoke from widespread burning, this would have reduced the sunlight reaching the surface to such an extent that photosynthesis became impossible. Plants would have died, and the whole food web depending on them would have fallen apart. Recovery of the vegetation on land and marine planktonic plants would have begun within a few years (from seeds and spores), but many groups of animals and some plants became extinct. The effects were much wider than merely the demise of the dinosaurs. Many important marine groups vanished. Notably the ammonoids, reef-building mollusks called rudists, and the large marine reptiles. Most species of marine planktonic animals of the kind known as foraminiferans disappeared too, although the group as a whole recovered.
Because it was global in extent, and has effects that can be seen among organisms living in virtually all environments, this end-Cretaceous episode is described as a «mass extinction». The fossil record shows several mass extinctions during the Cambrian Period, and a more dramatic one at the close of the Ordovician Period. Because there was no life on land yet, these mass extinctions are purely marine. However, a mass extinction at the end of the Permian Period (and thereby marking the end of the Palaeozoic Era) wiped out whole groups of land and marine animals, and seems to have been an even more severe event than the end-Cretaceous mass extinction.
The causes of these earlier mass extinction are uncertain. Statistically it is unlikely that the end-Cretaceous meteorite impact was the only one of that size to have happened in the Phanerozoic, so it is likely that some of the earlier mass extinctions were triggered in the same way.
What is certain is that after each mass extinction, those varieties or organisms that had survived were able to take advantage of the demise of their competitors to adapt, through evolutionary change, to new lifestyles. This is described as adaptive radiation.
The sudden blossoming of the mammals at the start of the Cenozoic Era is a good example. The disappearance of the dinosaurs meant that on land there were no longer any large herbivores or predators. The sudden lack of competition made it possible for the tiny mammals to diversify, and within a few thousand generations some groups that had moved into new environments were well on the way to evolving into different species better suite to take advantage of the new opportunities. A classic case of the meek inheriting the Earth, if ever there was one!