Death on a lost horizon

Extinction. The very word conjures up visions of destruction and an abiding sense of unease in many thoughtful minds. Virtually everyone within range of western media sources understands that modern climate changes are predicted to result in extinction levels that rival episodes of so-called "mass extinction" in the geological past. Moreover, thanks to the influence of popular media everyone seems to know that asteroid/comet impacts have something to do with these ancient mass extinctions, although the whole affair is typically reported as being the subject of continuing debate among "scientists".

As is so often the case with contemporary scientific controversies, a single, authoritative account of the topic and the controversy has been sorely lacking. Andrew Hallam and Paul Wignall's Mass Extinctions and their Aftermath seeks to provide this much-needed introduction to the study of extinctions from a paleontological point of view. Their book is published (appropriately) in the 201st year of the scientific community's acceptance of biotic extinction as a fact.

In January 1796 Baron Georges Cuvier delivered a lecture at the National Institute of Sciences and Arts in Paris where he demonstrated that a fossil mastodon collected from the bank of the Ohio River in North America was morphologically unlike any living probiscidian. Arguing that a population of such large animals could hardly have escaped the notice of explorers and naturalists, Cuvier concluded that the fossil must represent an extinct species that had succumbed to one of a number of global environmental catastrophes. Soon after, geologists in Europe and Britain identified several stratigraphical horizons at which large or "mass" extinctions seemed to have occurred.

The most prominent of these came to mark the boundaries between the major time intervals in the earth's past. However, until the past 20 years the environmental and evolutionary study of mass extinctions languished as a backwater of paleontology, largely because most of the scenarios advanced to account for such events were either untested (sea-level change), untestable (radiation from a nearby supernova), or far-fetched (culling by aliens).

This situation changed in 1980 with the discovery of an anomalously large concentration of the rare-earth element iridium at one of these mass extinction horizons: the Cretaceous-Tertiary (K-T) boundary. The University of California (Berkeley) research group that made this discovery, led by Luis and Walter Alvarez, interpreted the K-T iridium anomaly as evidence that an asteroid 15 km in diameter had struck the earth, generating environmental perturbations that wiped out up to 75 per cent of all fossilisable species, including the dinosaurs, ammonites and several groups of marine plankton. Regardless of its cause, the earth never really recovered from the effects of the K-T mass extinction because it eliminated many of the previously diverse and widespread biotic groups whose demise permitted the rise of the modern biota's ancestors.

These propositions serve as the starting point for the modern extinction debates and, in turn, the starting point for Hallam and Wignall's book. The authors attempt to integrate available paleontological, stratigraphical, geochemical, oceanographical, geological, climatological and physical data from both mass and background extinction intervals and compare these against predictions derived from the leading causal models: asteroid/comet impact, massive volcanism, sea-level change and marine anoxia (decreases in the oxygen content of marine waters).

Although descriptions of the five largest mass extinctions comprise the bulk of Mass Extinctions and their Aftermath, the book is unique in that all major extinction intervals are considered. Organisation is chronological with material on smaller extinction events lumped together into single chapters and the "big five" accorded chapters of their own. This introduces an understandable asymmetry in the depth of the coverage between large and small events. However, a pronounced asymmetry also exists among the main chapters. For example, highly useful cause-and-effect "box models" are presented and discussed for the Late Devonian and Late Permian mass extinctions, but not for the similarly sized Late Ordovician, Late Triassic, and Late Cretaceous events, despite the fact that many important feedback relations between aspects of the global environment are described in each chapter.

Similarly, descriptions for various well-studied geological successions that include the mass extinction horizon are provided for the Late Ordovician, Late Devonian and Late Permian events, but not for those of the Late Triassic or Late Cretaceous. These are not major oversights, but do needlessly disrupt the consistency of the narrative and reduce the book's value as a quick and easy-to-use reference.

A more serious problem is the writing. The authors have covered a broad range of material in considerable depth. The price for this commendable brevity has been paid in terminological complexity. The book is so laden with technical jargon that it is likely to be immediately accessible only to specialists. Given the large potential audience, it was disappointing to see that even the most basic nods to the non-specialist (a glossary) have been omitted.

Mass Extinctions and their Aftermath will richly reward the efforts made by students and laymen to come to grips with the material presented, but I worry about how many will stay the course. Irrespective of these caveats, the book is well-written, comprehensive, copiously referenced and closely reasoned. It succeeds in its aim to be the foremost scientific introduction to the phenomenon of mass extinction on virtually every level.

After looking closely and more-or-less consistently at more than 20 different extinction episodes and half-a-dozen explanatory models, does any overarching pattern emerge? Hallam and Wignall predictably come down in favour of death from the sea as opposed to death from the sky. Both authors have long been advocates of the association between episodes of mass extinction and fluctuations in global sea levels; especially insofar as these fluctuations affect the marine anoxia.

Hallam and Wignall review the evidence for and against extraterrestrial causality in detail and conclude that, irrespective of their media profile, asteroid/comet impacts have played a very minor role in either precipitating or contributing to an overwhelming majority of geological extinction events. For them, "only for the Cretaceous-Tertiary boundary can a really convincing case be made out for (the association) with extraterrestrial impact (and) even here it remains by no means clear what role impact had in causing environmental deterioration, as opposed to other earth-bound factors".

Since many climatologists predict sea-level fluctuations as an inevitable consequence of recent global warming trends, Hallam and Wignall's conclusion is somewhat disconcerting, though no less important for being so. In sum, this volume fulfils its stated aim as a guide to the world of the geological past and the mass extinctions that have occasionally disrupted that world. Many books are now available that focus on particular extinction events and the fates of organismal groups within particular extinction events. In addition, many collections of papers have been published that discuss aspects of different extinction events. Regardless, Hallam and Wignall's volume is the only comprehensive, "single author" treatment of the major features of the entire paleontological extinction record. It deserves a wide audience within and outside the earth science community.

Norman MacLeod is senior staff scientist, department of paleontology, Natural History Museum, London.