Plants and the K–T Boundary

The ‘K–T Boundary’ or ‘end Cretaceous event’, which marks the extinction of the dinosaurs on land and the ammonites and other invertebrate fauna in the marine realm, is one of the major punctuation marks in the history of life. Its impact on plant life appears to have been of a much lesser magnitude. This book sets out to review the evidence from a wide geographic range of sites for the nature and timing of the effect of this ‘event’ on plant life. The authors, both on the staff of the Denver Museum of Nature and Science, have published extensively on fossil plants of Tertiary and Cretaceous age. Nichols has been mainly concerned with the palynology (the microfossil record), while Johnson has concentrated on leaf assemblages (megafossils) of this age span. The actual cause of whatever produced that far-reaching mass extinction in the animal Kingdom has long been debated, and this took on new significance for palaeobiologists with the discovery of the ‘iridium anomaly’, peculiar to sedimentary sequences across the K–T transition (Alvarez et al., 1980). The Alvarez father-and-son team argued that the cause of the peak occurrence of that element was the result of the impact of an extra-terrestrial body. The diverse and disastrous consequences of such an impact were, they claimed, the most likely cause for the extinctions occurring at the close of the Cretaceous Period (the K–T boundary). This thesis has subsequently become widely, but not universally, accepted. The most likely site for the impact is generally agreed to be close to Chicxulub, on the Yuccatan Peninsula of Mexico. While the effect of the end-Cretaceous event is very evident in the fossil record of animal life, both marine and terrestrial, the extent to which plant life was significantly affected has been less generally agreed. The information from these two types of plant fossils (microfossils and fossil leaves) seemed at first to be in conflict. As Nichols and Johnson record, ‘early responses to the Alvarez hypothesis from the fields of palaeobotany and palynology were ambiguous if not negative’. In this book the authors set out to review the entire field of fossil plant evidence, both palynological and macrofossil, for the nature and causality of events at the K–T boundary. The authors record that some 500 papers making reference to changes in the flora through the K–T sequence have been published over the last 50 years. They attempt to examine and summarize the range of evidence, but most particularly that from the fossil spore/pollen record, for extinction of plants across that boundary. They briefly consider some of the main books discussing the K–T boundary, most of which deal largely or exclusively with the animal record, especially the dinosaurs, and remark: ‘ … some of them overlook plants entirely. This short shrift or omission from prominent books on the subject served as our inspiration for writing this book’. A large part of the book is a detailed survey of the record of these two principal types of plant fossils across the K–T boundary, mainly from western North America, but including a review of published material from Europe and Asia, and also including a number of Gondwanan sites. Nichols and Johnson emphasize that to study events close to the K–T boundary, very precise stratigraphic control (dating of the rock sequence) is needed: ‘Essentially, all non-American terrestrial K–T boundary sections are wanting when it comes to this resolution … For this reason our summary will focus primarily on the North American record’. Hypotheses on K–T extinction focus on four main causes: climate change, marine regression, volcanism and an impact event. Does the fossil plant evidence support an impact as the cause? The authors conclude that ‘in western North America it does’. But more importantly globally, ‘nearly all of the K–T boundary sections that contain evidence of the impact event are in North America … without the North American record we would be hard pressed to argue for major floral change at the K–T boundary’. According to the Alvarez impact scenario, ‘darkening of the skies was caused by dust and sulphate aerosols injected into the stratosphere by the impact … The cold of an impact winter would ensue. Our review of the global reaction of vegetation at the K–T boundary shows that plant communities in North America were affected in a way consistent with this mechanism, but outside North America the evidence is less clear or lacking’. Rather surprisingly the authors seem more ready to attribute this to imperfections in the record elsewhere, rather than accepting that the occurrence of the impact in North America evidently had a greater terrestrial ecological effect there than elsewhere. The occurrence of the ‘fern spike’ (an abrupt rise in abundance of fern spores, evident above the K–T boundary at many North American sites) has been used by some authors to suggest a widespread or even global forest fire, resulting from the bolide impact. However, Belcher et al. (2003) point out that the lack of a charcoal accumulation at or above the K–T boundary is strong evidence against the occurrence of extensive forest fire following the impact event, and the evidence from Nichols and Johnson's numerous K–T boundary sections supports Belcher's conclusion. They explain the ‘fern spike’ as due to impact (blast) ‘knock down’ of Late Cretaceous forests, rather than destruction by forest fire. Perhaps the single most important conclusion of these authors is that ‘no major plant groups disappeared at the boundary, and the damage primarily occurred at the species level’. This of course contrasts strikingly with the much more extensive extinctions occurring at that horizon in the animal kingdom. Some 20 years ago, Traverse (1988) argued that the factors influencing terrestrial plant evolution and extinction have always been different from those affecting animals. The resilience of the (autotrophic) terrestrial plants contrasts with the more vulnerable heterotrophy of the terrestrial vertebrates that depend upon them. My only criticism of this book is that no space is given to this much wider debate, which confronts issues concerning the entire geological record of plant life on land. Nonetheless, this book is an important contribution to the literature of both palaeobotany and palynology, and is of interest to any plant scientist concerned with the capacity of the terrestrial flora to respond to major environmental change.