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Uprooted Findings

Philip Gingerich

The Terrestrial Eocene-Oligocene Transition in North America. Donald R. Prothero and Robert J. Emry, eds. 688 pp. Cambridge University Press, 1996. $95.

The Eocene-Oligocene transition was a turning point in the history of life in many environments. In the Cenozoic age of mammals, the Eocene is best known for the appearance of modern groups like our own order Primates, dawn horses of the order Perissodactyla, and archaic whales of the order Cetacea. During the Oligocene period, more advanced fossil apes and diverse gigantic forms of Perissodactyla appeared. In addition, the first differentiation of baleen and simple-toothed Cetacea occurred. But a sense of progression from the Eocene to the Oligocene does not fairly represent the transition. Mammalian faunas of the Cenozoic have long been thought to be divided by a "Grande Coupure" or "Terminal Eocene Event" of extinction and origination at or near the Eocene-Oligocene boundary, and more recent studies of continental climate have shown this to be a time of rapid transition from an equable "greenhouse" environment to our present highly seasonal "icehouse" conditions.

The Terrestrial Eocene-Oligocene Transition depicts North American Eocene-Oligocene mammalian biostratigraphy and chronostratigraphy as complicated and unstable. With many long stratigraphic sections producing mammals by the thousands, many distinctive taxa, and many smart people studying them, why doesn't it make more sense? In a book like this, one expects (but does not find) an introductory chapter outlining the kinds of evidence available for understanding a terrestrial Eocene-Oligocene transition. Moreover, such an introduction should describe how evidence from local sections can be combined to interpret local faunal history, correlate such histories from place to place to gain a regional perspective and, finally, calibrate events radiometrically.

The first 15 chapters of this book cover Eocene-Oligocene vertebrate biostratigraphy and chronostratigraphy in sedimentary basins spanning the western interior of North America from California to Colorado and Saskatchewan to Texas. Editor Prothero is an author or coauthor of 10 of these chapters. The next 14 chapters cover the White River Chronofauna. Two of these chapters are by Prothero; coeditor Emry is a coauthor of two others. All are valuable, if somewhat specialized, technical reviews. Still searching for an overview, I turned to the final summary chapter by Prothero and Emry. This includes new correlation charts with some simplified taxonomic ranges for the Uintan, Duchesnian, Chadronian, Orellan and Whitneyan land-mammal ages. The charts are revised from recent publications by the authors and others, with changes said to be based largely on new argon 40/ argon 39 radiometric ages. None of the charts includes an Eocene-Oligocene boundary. I read this summary as saying the Eocene-Oligocene transition was a series of nonevents. This would be more credible if supported by quantification of faunal change through time.—Philip D. Gingerich, Geological Sciences and Museum of Paleontology, University of Michigan

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