Many phenomena, which have recently been regarded as mass extinctions, are now recognised to be clusters of episodes of extinction only roughly associated in geological time. As pointed out by Raup (1992), an analysis of the extinction-intensity curve shows extinction intensities to be distributed continuously. At the same time, most authorities agree that no fewer than five genuine major extinctions are apparent in the fossil record. These occurred at the end of each of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods (Table 3). The Cretaceous-Tertiary extinction during the Maastrichian stage at the close of the Mesozoic Era is known to geologists as K-T (Kreta is the Greek for chalk) to distinguish the word Cretaceous from Cambrian or Carboniferous. It was not the largest of the major extinctions although it has attracted by far the greatest popular interest. The reptilian groups or taxa that died out then included the mosasaurs, plesiosaurs, dinosaurs (except birds) - including the ankylosaurs, ceratopsians, coelurosaurs, hadrosaurs, hypselophodonts, tyrannosaurs and others (Archibald 1996) - but the ichthyosaurs, most pterosaurs, the pliosaurs and many other taxa had disappeared long before the K-T boundary.
The dinosaurs did not all become extinct at the same time but, from a peak of about 120 genera, numbers dwindled steadily during the final 300,000 years of the Mesozoic Era when the remainder finally vanished. Giant sauropods, including Diplodocus (Fig. 66), Brachiosaurus (Fig. 109) and Apatosaurus (Fig. 110), had become extinct long before the conclusion of the Cretaceous, while ceratop-sians, such as Triceratops (Fig. 72) and the carnosaur Tyrannosaurus (Fig. 123) were among the last dinosaurs on earth (Weishampel et al. 1990; Currie and Padian 1997; Farlow and Brett-Surman 1997).
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