The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis. George R. McGhee, Jr. xiii + 303 pp. Columbia University Press, 1996. $29 paper.
Five major mass extinctions stand out in plots of the number of families (or genera) through Phanerozoic time (the past 550 million years). These crisis intervals occur rapidly and are marked by substantial drops in global biodiversity and broad taxonomic impact. The temporal and geographical patterns of extinction, potential causes, and evolutionary consequences are topics of considerable interest. Several recent books on the end-Cretaceous episode (the smallest of the five) continue the debates about the disappearance of the dinosaurs and the effects of an asteroid impact 65 million years ago. The end-Permian mass extinction (the most severe in the history of life) and its dramatic impact are considered in an earlier volume of Columbia University Press's Critical Moments in Paleobiology series. Now George McGhee has provided an excellent synthesis of one of the less widely known of the "big five" crises: that of the Late Devonian.
About 365 million years ago, 22 percent of all families of marine animals (57 percent of the genera, and at least 75 percent of the species) disappeared between the two stages of the Late Devonian, the Frasnian (pronounced Franian) and Famennian. The recently evolved terrestrial plants and vertebrates also suffered significant extinctions. In 1969, Canadian paleontologist Digby McLaren suggested that an asteroid impact was the prime cause of this faunal turnover and destruction of ecosystems. McGhee supports the argument that extraterrestrial objects played a critical role in these extinctions. But rather than simply construct a case for a particular explanation, he uses careful field observations and hypothesis testing to present a thoughtful, thorough and engaging review of possible causes of the mass extinction.
After identifying the major Phanerozoic extinction events, McGhee focuses on the Devonian time scale and the biostratigraphic framework for determining when the extinctions took place. He next calculates the extinction intensities and magnitudes of the Late Devonian losses at different taxonomic levels. Addressing the duration of the mass extinction, he concludes that it consists of several distinct events through an interval of, at most, three million years.
Next, McGhee documents the breadth of the Late Devonian crisis by discussing the victims and survivors in each of the marine and terrestrial groups. He concludes that survivors generally represent more primitive or ancestral morphologies. The data are presented with the highest level of taxonomic and stratigraphic resolution currently available. Unfortunately, the degree of resolution varies among groups, which complicates comparisons of the magnitudes and timings of the losses of different taxa. To provide the environmental context of the extinctions, McGhee offers a concise historical summary of paleomagnetic and paleoecologic data and their application to reconstructions of Devonian paleogeography. Employing our current map of the Late Devonian world, McGhee identifies five ecological signals in the extinction data: global scope, greater extinction in equatorial regions, latitudinal compression of the ranges of low-latitude survivors, higher extinction in shallow-water marine environments, and more severe losses in marine than freshwater environments. He finds that factors such as the size of geographic ranges, trophic level and reproductive mode have no effects on survival. Because McGhee relies on apparent diversity patterns, his conclusions may be weakened by the little attention he gives to possible effects of preservational and sampling biases. These are of particular importance in evaluating extinction rates and claims of separate extinction events.
In the second half of the book, McGhee considers the causes of the mass extinction. Applying his extensive knowledge of the Devonian, which is based on two decades of research and careful field observations, he reviews an extensive list of possible killing mechanisms. McGhee concludes that a sharp, global temperature decline is the most likely explanation for the mass extinction. He rejects oceanic overturn and resulting anoxia because they do not fit the first two ecologic signals. But his argument requires a universal cause which may not be appropriate, and other workers have seen anoxia as a more significant factor. Although explanations invoking multiple causes may be much harder to test, I would have appreciated an explicit consideration of the possibility that the severity of the Late Devonian extinction stems from the simultaneous or sequential effects of independent factors.
Because glaciation appears only toward the very end of the Devonian, McGhee rejects it as the cause of the temperature drop. He favors impact-induced global cooling, and notes that the temporal pattern of extinctions requires either multiple impacts or that the global effects of a single impact persist for several million years. Two chapters are devoted to a thorough discussion of the evidence of craters, microtektites, geochemical signals, and shocked quartz, which suggest that several impacts occurred during the Late Devonian. Because none of these events appear at the major extinction horizon, he recognizes that the data allow only a circumstantial case for the multiple impact scenario he prefers. Thus, as McGhee continues to search for evidence, he leaves the reader with the uncertainty over the causes of the mass extinction which accurately reflects recent research.
These chapters provide an extensive background for the various aspects of the Late Devonian extinction. The many details are clearly explained, well illustrated, and concisely referenced and supplemented with a short glossary and an appendix of Devonian biota. The author's engaging synthesis of our current understanding is easy to follow. The only significant problem with the presentation is that reduction of some figures taken from other sources has left some patterns indiscernible and accompanying text unreadable.
The book is an excellent choice for a seminar for upper-level undergraduates or graduate students, and it will reward anyone with interests in mass extinctions or the Paleozoic. It may even prompt some of those unhappy with the uncertainties (or McGhee's interpretations) to resolve remaining questions about one of the most important and intriguing crises in the history of life.—Sherman J. Suter, National Museum of Natural History, Smithsonian Institution