Plant evolution viewed through a functional and paleoclimatic prism

The fossil record of plants offers a spectacular window on the trajectories of biotic replacement, failed and triumphant evolutionary experiments, and the responses of life to environmental catastrophes. Consequently, plant fossils have long inspired hypotheses about the inner workings of evolution and the possible genetic and environmental catalysts provoking bursts of evolutionary change. Although necessarily a biased lens (Behrensmeyer, Kidwell, and Gastaldo, 2000), fossils do offer a unique perspective on the temporal and spatial scales of evolution, while framing these in the context of a dynamic Earth. However, study of the "dead" is only one line of inquiry, because no matter how well-preserved a particular fossil is, the past plant it marks cannot be greened and coaxed to grow from the rocks. To fully understand the mechanisms driving evolution, life's genealogy, development, as well as ecology must be infused into the geological pattern. The field of paleobotanical research has recently accelerated on several fronts, fueled by advances in rock dating methods (Knoll and Carroll, 1999), mega-analyses to study patterns of biotic replacement (Lupia, Lidgard, and Crane, 1999), microscale and macro-scale biochemical analyses of fossils (Arens, Jahren, and Amundson, 2000; Briggs, Evershed, and Lockheart, 2000; Boyce, Hazen, and Knoll, 2001), plus new approaches to taphonomy (Behrensmeyer, Kidwell, and Gastaldo, 2000) and clarification of phylogenetic patterns as well as developmental rules and functional biology of extant groups (Mathews and Donoghue, 1999; Chaw et al., 2000; Cronk, 2001; Pryer et al., 2001; Boyce and Knoll, 2002). And all the while, the fossil record of plants grows richer and richer by the year, exposing new interfaces for the application of techniques and opportunities for addressing questions. Taken by this spirit, and a belief in the importance of combining studies of the dead plants with living ones, K. J. Willis and J. C. McElwain, two players in the paleobotanical renaissance, have fashioned a new, undergraduate-targeted text on the evolution of plants. The content of the text, The Evolution of Plants, is colored by both Willis' and McElwain's interests in paleoclimatology and the functional biology of extinct plants. Willis' research, based out of the School of Geography and Environment at Oxford University, has largely focused on the long-term relationships between vegetation dynamics (largely Holocene processes) and environmental changes, as well as possible coevolution between early angiosperms and dinosaurs. At the Field Museum, McElwain's work centers on using structurefunction relations of fossil plants to reconstruct geologic-time scale changes in global carbon cycle dynamics. In particular, her pioneering work exploring the links between stomata structure and the interpretation of past climates has had significant impacts on our understanding of plant responses at times of animal mass extinction. Summarizing the evolutionary chronicle of plants is a difficult task by any measure. Crafting a plant evolution course into a format enticing enough to woo undergraduates away from other courses is even more daunting. Paleobotany can seem particularly far-removed from day-to-day affairs because of the strong attachment to terminology and study of bits of seemingly insignificant plant detritus. But in everyday life, plants loom large, providing the raw materials for most of the clothes we wear and filling our stomachs and lungs with lifegiving energy and oxygen. But it's more than that, most of us will see more plants in our lives than animals, in forests to weeds growing at the bus stop--you can't easily get away from plants. To enrich our understanding about the natural world, just about everyone needs to know a thing or two about plants. So how did the particular cohort of plants that pervade our world today come to be? Was the world always like this? It turns out that things get even more interesting, when the layers, or rather the rocks, are peeled back, and we begin to examine the 90% of all the plant species that ever lived. The text by Willis and McElwain effectively serves as a good introductory analysis of plant evolution, to get neophytes generally set off on the right foot. S veral recent texts have touched on the broad topic of plant evolutionary biology, in particular covering aspects of the fossil record. Most notable is K. J. Niklas' book Evolutionary Biology of Plants (1997), which is well written and broad in scope. This text does an excellent job at marrying insights from the living and the dead to examine plant evolution. Also, th re is L. E. Graham's Origin of Land Plants (1993), which is particularly fun to read and focuses on the transition of plants onto land and into air from green algal ancestors. Finally, W. N. Stewart and G. W. Rothwell's Paleobotany and the Evolution of Plants (1993) as well as T N. Taylor and E. L. Taylor's The Biology and Evolution of Fossil Plants (1993) both showcase how deep the rabbit hole of plant evolution goes, providing all the details one could want about what groups lived when and where and who may have been related to whom. Besides extensive and clear coverage of the rogues gallery of plant evolution and review of the major mechanisms underlying evolutionary change in plants, what are the real advances made in The Evolution of Plants by Willis and McElwain? There are several. Overall, the book is lucidly written and produced fairly well, with copious use of well-chosen and clearly labeled figures. There are all of the usual reconstructions (although rendered ' The evolution of plants. K. J. Willis and J. C. McElwain. Oxford University Press. 2002. ISBN 0-19-850065-3. 2 Tel.: 510-528-4070; e-mail: feild@botany.utoronto.ca