Sixty-five million years ago, at the end of the Cretaceous period, one of the five mass extinctions of Earth history occurred. It was not the greatest of the mass extinctions (the Permian extinction resulted in the disappearance of more species), but has long been the subject of intense speculation and research because it was the most recent of the mass extinctions, and it involved the disappearance of the dinosaurs, the prehistoric animals that have attracted perhaps the most attention from scientists and the general public.

Part of the problem in explaining the Cretaceous extinction, as with the other mass extinctions, was that not all groups of organisms suffered the same rate of disappearance. The largest land animals were dinosaurs, all of which suffered extinction, and the large marine reptiles also became extinct. However, not all dinosaurs were large; even the small ones became extinct. Most groups of mammals, amphibians, crocodiles, turtles, insects, and land plants survived the Cretaceous extinction. Among the marine mollusks, the ammonoids became extinct but the nautiloids, which appeared to have very similar ecological roles, did not. Birds survived, but only a few lineages of them. Some plankton, the floating microscopic organisms of the oceans, experienced a dramatic set of extinctions (for example, the foraminiferans), while others, such as diatoms, did not. Extinction rates were much higher on land (about 90 percent of species) than in freshwater (about 10 percent of species).

There was also the question of whether the Cretaceous extinction was a single catastrophic event or the outcome of gradual declines toward extinction. Several lineages of dinosaurs were already in decline during the Cretaceous period; however, others were not. And although land plants survived the extinction, their populations were apparently set back by some catastrophe. At the end of the Cretaceous, ferns and fern spores were very numerous. Ferns often lived in areas from which the larger vegetation had been removed by some disturbance. The Cretaceous extinction, then, appeared to be an event in need of an explanation. . .

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