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.
If asked what they understood by the word ‘catastrophe’, most people would probably agree that it was something big, bad, and sudden, and involved damage to organisms. In the natural world today, perhaps the most striking catastrophes result from major earthquakes, in which thousands of people can be killed within minutes. Going back through human history, we allow for greater stretches of time. Thus, in the middle of the fourteenth century, over a period of five years, an estimated one-third of the European population died directly as a result of catching the plague: the ‘Black Death’. By any reckoning this ranks as a catastrophe. It had a dramatic effect on European society for many years. When we extend our consideration to geological time, in which it is routine to deal with changes taking place over millions of years, events lasting only a few thousand years may be regarded as catastrophic if the contrast with the ‘background’ is sharp enough. Various definitions have been proposed for a mass extinction. A conveniently concise if imprecise one that I favour is that it is the extinction of a significant proportion of the world’s living animal and plant life (the biota) in a geologically insignificant period of time. The imprecision about the extent of an extinction can be dealt with fairly satisfactorily in particular instances by giving percentages of fossil families, genera, or species, but the imprecision about time is more difficult to deal with. An important question about mass extinctions is to assess how catastrophic they were, so we also require a definition of ‘catastrophe’ in this context. One thought-provoking attempt at such a definition is that a catastrophe is a perturbation of the biosphere that appears to be instantaneous when viewed at the level of detail that can be resolved in the geological record. At this point more needs to be said about the nature of the geological record. The material that geologists and palaeontologists deal with occurs in the layered successions of sedimentary rocks, mainly sandstones, shales, and limestones, that can clearly be observed in good rock exposures, either natural ones, as in coastal cliffs or mountains, or artificial ones, as in quarries or borehole cores.
In Catastrophes and Lesser Calamities, renowned geologist Tony Hallam takes us on a tour of the Earth's history, and of the cataclysmic events, as well as the more gradual extinctions, that have punctuated life on Earth throughout the past 500 million years. While comparable books in this field of study tend to promote only one likely cause of mass extinctions, such as extraterrestrial impact, volcanism, and or climatic cooling, Catastrophes and Lesser Calamities breaks new ground, as the first book to attempt an objective coverage of all likely causes, including sea-level and climatic changes, oxygen deficiency in the oceans, volcanic activity, and extraterrestrial impact. Hallam focuses on the so-called 'big five' mass extinctions, at the end of the Ordovician, Permian, Triassic, and Cretaceous periods, and the later Devonian, and he also includes less well-known examples where relevant. He devotes attention especially to the attempts by geologists to distinguish true catastrophes from more gradual extinction events, and he concludes with a discussion of the evolutionary significance of mass extinctions, and on the influence of Homo sapiens in causing extinctions within the last few thousand years, both on land and in the seas.
In earlier times many geologists clearly became cynical about what they had learned as students about Earth history from their stratigraphy courses. ‘The sea comes in, the sea goes out.’ This is indeed one of the most striking signals that emerges from study of the stratigraphic record in a given region: a succession of marine transgressions and regressions on the continents. Little scientific rigour was, however, applied to the subject, and students were left with no overarching explanation to provide any intellectual stimulation. Since the 1970s things have begun to change for the better, as less emphasis has been placed on learning the names of rock formations and fossil zones and more on the dynamic aspects of what to many ranks as a fascinating subject. This entails studying changing geographies and climates within a framework supplied by plate tectonics, the successions of strata being subjected to ever-more-rigorous sedimentological and geochemical analysis, and global correlation continually improved by further study of stratigraphically useful fossils. How do we infer sea-level changes from a given succession of sedimentary rocks? In essence we use facies analysis, which is based upon a careful study of the sediment types and structures, together with a study of the ecological aspects of the contained fossils, or palaeoecology. These features can be compared with those of similar sediments that are being deposited today, or similar organisms living today. Comparisons of this kind were practised by the likes of Cuvier as well as Lyell. Consider, for example, the Cretaceous succession in southern England. The oldest strata, well exposed on the coast from Sussex to Dorset, are known as the Wealden, and consist mainly of sandstones and siltstones that were deposited in a coastal plain (non-marine) setting. They are overlain by the Lower Greensand, a sandy unit of Aptian–Lower Albian age laid down in a very shallow marine environment. These conditions are revealed, not just by the types of fossils, which include the exclusively marine ammonites, but also by the distinctive green clay mineral glauconite, which gives its name to the rock formation and occurs today only in marine settings.
Unlike the other factors that have been invoked to account for mass extinctions, climate change is manifest to us all, whether we travel from the tropics to the poles or experience the seasonal cycle. Over a longer timescale, the issue of global warming in the recent past and likely future, and its probable consequences for other aspects of the environment, has occupied a considerable amount of media attention. Those people who are unaware of the likely consequences of the burning of fossil fuels cannot count themselves as well educated. Over a longer timescale, geologists have been aware for many decades of significant climatic changes on a global scale leading to the appearance and disappearance of polar ice caps on a number of occasions. Steve Stanley, the distinguished palaeobiologist at Johns Hopkins University in Baltimore, has actively promoted the view that episodes of climatic cooling are the most likely cause of mass extinctions. However, we must consider also the significance of global warming, and for the continents, at any rate, the possible effects of changes in the humidity–aridity spectrum. Before examining the relationships between climatic change and mass extinctions we need to examine the criteria from the stratigraphic record that geologists use to determine ancient climates, or palaeo-climates. The most obvious way of detecting cold conditions in the past is to find evidence of the presence of ice. At the present day the sedimentary deposits associated with glaciers and ice sheets, which occur where melting ice dumps its rock load, range in grain size from boulders and pebbles to finely ground rock flour. Such deposits are known as boulder clay or till, and ancient examples consolidated into resistant rock as tillites. The surfaces of hard rock that have underlain substantial ice sheets bear characteristic linear striations indicating the former direction of ice movement, such as glaciers moving up or down a U-shaped valley. The striations are produced by pebbles embedded in the ice, and are a unique marker for glacial action. In the 1830s Louis Agassiz, the great Swiss naturalist, extrapolated from his knowledge of the margins of Alpine glaciers to propose that the whole of northern Europe had been covered by one or more ice sheets in the recent geological past.
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