When the subject of extinctions in the geological past comes up, nearly everyone’s thoughts turn to dinosaurs. It may well be true that these long-extinct beasts mean more to most children than the vast majority of living creatures. One could even go so far as to paraphrase Voltaire and maintain that if dinosaurs had never existed it would have been necessary to invent them, if only as a metaphor for obsolescence. To refer to a particular machine as a dinosaur would certainly do nothing for its market value. The irony is that the metaphor is now itself obsolete. The modern scientific view of dinosaurs differs immensely from the old one of lumbering, inefficient creatures tottering to their final decline. Their success as dominant land vertebrates through 165 million years of the Earth’s history is, indeed, now mainly regarded with wonder and even admiration. If, as is generally thought, the dinosaurs were killed off by an asteroid at the end of the Cretaceous, that is something for which no organism could possibly have been prepared by normal Darwinian natural selection. The final demise of the dinosaurs would then have been the result, not of bad genes, but of bad luck, to use the laconic words of Dave Raup. In contemplating the history of the dinosaurs it is necessary to rectify one widespread misconception. Outside scientific circles the view is widely held that the dinosaurs lived for a huge slice of geological time little disturbed by their environment until the final apocalypse. This is a serious misconception. The dinosaurs suffered quite a high evolutionary turnover rate, and this implies a high rate of extinction throughout their history. Jurassic dinosaurs, dominated by giant sauropods, stegosaurs, and the top carnivore Allosaurus, are quite different from those of the Cretaceous period, which are characterized by diverse hadrosaurs, ceratopsians, and Tyrannosaurus. Michael Crichton’s science-fiction novel Jurassic Park, made famous by the Steven Spielberg movies, features dinosaurs that are mainly from the Cretaceous, probably because velociraptors and Tyrannosaurus could provide more drama.
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