Few palaeontologists appear to be greatly concerned as to the causes of the extinction of dinosaurs and other Mesozoic taxa. Extinction is inevitable and it is more interesting to study how the archosaurs lived than why they died. 'The Age of Reptiles' began about 250 mya and ended some 185 my later. It seems probable from a biological viewpoint that the forms which became extinct then went, 'not with a bang but a whimper', to quote T.S. Eliot's The Hollow Men (published in 1925).

Many people now accept that there may have been a bolide impact about the time of the K-T boundary. There was also a considerable amount of volcanic activity, mountain building, marine transgression, recession and so on, associated with continental drift. The evidence supporting both catastrophic and gradualist explanations of the end-Cretaceous reptilian extinctions has been enumerated by Russell (1971),McGowan (1991),Milne (1991),Benton (1996),Fastovsky and Weishampel (1996), Russell and Dodson (1997) and many others.

Among these, Archibald (1996,1997) believed marine regression, extraterrestrial impact and, to a lesser extent, massive volcanism to have been responsible. To biologists and ecologists, who realise that natural selection operates at several different levels and on many morphological and physiological functions simultaneously, it seems impossible that the numerous selective proximal extinctions that took place could have been the result of a single cause, however dramatic, or even of a limited number of causes. Benton (2004) summed up the matter as follows: "available killing models are either biologically unlikely, or are too catastrophic ... 70-75% of families survived the K-T event, many of them seemingly entirely unaffected! Some palaeontological evidence supports the view of instantaneous extinction, but the majority indicates longer-term extinction over 1-2 my." The event can be understood better when the time scale is broadened (Fig. 127).

Multiple proximal factors ultimately associated with tectonic activity, and operating differently on land and in the oceans, must surely have been mainly responsible for the K-T extinction. Impact with a bolide might also have contributed to the extinctions. For a wide variety of reasons, changing environmental

Fig. 127. Phylogeny of the major groups of vertebrates. (Benton and Harper 1997)

and climatic conditions tend to make certain groups somewhat less competitive than others and, consequently, they are replaced. It will probably be more profitable in the future to study the recovery of the taxa that outlived the Mesozoic Era than to investigate the disappearance of those that did not!

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