Kt boundary4/10/2023 ![]() ![]() Does the fossil plant evidence support an impact as the cause? The authors conclude that ‘in western North America it does’. Hypotheses on K–T extinction focus on four main causes: climate change, marine regression, volcanism and an impact event. 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’. This short shrift or omission from prominent books on the subject served as our inspiration for writing this book’.Ī 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. 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. ![]() 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. 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. 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. As Nichols and Johnson record, ‘early responses to the Alvarez hypothesis from the fields of palaeobotany and palynology were ambiguous if not negative’. The information from these two types of plant fossils (microfossils and fossil leaves) seemed at first to be in conflict. 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 most likely site for the impact is generally agreed to be close to Chicxulub, on the Yuccatan Peninsula of Mexico. This thesis has subsequently become widely, but not universally, accepted. 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). 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 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). Nichols has been mainly concerned with the palynology (the microfossil record), while Johnson has concentrated on leaf assemblages (megafossils) of this age span. 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. 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. Its impact on plant life appears to have been of a much lesser magnitude. 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. ![]()
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