Steven M. StanleyExtinctionScientific American Books, New York, 1987Monika says:
The history of life on Earth is a constant coming and going of species. Ever since life began (or arrived) on our planet ca. 3.6 billion years ago there were smaller and greater waves of extinctions. The fossil record shows that new species suddenly appeared after each of those waves. The most distinct among the extinction waves are the so-called Big Five, each of which made more than 50 % of the existing species disappear. In the book Steven Stanley published in 1987 he takes the reader on a trip through these crises of evolution. Apart from a chronological list of the extinction events the book also offers a look at what caused them. Keep in mind, though, that the results presented here are more than ten years old and therefore sometimes outdated even if some debates are not settled to this day. The so-called cosmic hypothesis, claiming that some of those crises were caused by comet or asteroid impacts, now is widely recognized which was not true when this book was published. Which doesn’t mean that nobody opposes the theory anymore. Once more the crisis that ended the Mesozoic 65 million years ago serves as an example. The Chicxulub crater in the Gulf of Mexico was discovered in the early 90ies. In the late 80ies it was still controversial whether the iridium anomaly in the K/T boundary layer really was of extraterrestrial origin. Opponents argued that iridium may well have been produced on Earth and reached the surface by volcanic activity. Giant basalt deposits on the Indian subcontinent, the so-called Deccan Trapps, seemed to support this hypothesis. The age of the deposits corresponded with the time of the K/T crisis. Advances in rock dating technology refuted this theory. The Deccan Trapps are a couple of million years older and the K/T extinction took place in a break between two phases of heavy volcanic activities. During those phases no mass extinction took place. Considering our recent knowledge about the Chicxulub crater Stanley’s remarks about the extinction patterns on the North American continent are most interesting: he states that the loss of species was highest in the area of the crater – unknown at the time the book was published – and gradually declined to the North. His explanation that tropical species register higher losses than species better adapted to a rough climate sounds plausible. But if you analyze this data knowing of a crater about 200 km in diameter there is a much simpler explanation that jumps to the eye. Of course at the site of the impact the consequences for the environment must have been the most devastating and that they gradually diminish with growing distance to Point Zero. But not every extinction event can be linked to a cosmic impact, nor can a change of climate be held (solely) responsible. This kind of explanation seems oversimplified and leaves the reader with a feeling that the author wasn’t certain himself but felt compelled to make a definite statement. But there still is a lot of room for future research. EXTINCTION is a classic, a must-read for everyone interested in the subject of extinction. |
|
|||
|
Last changes: 02-09-03 Copyright 2001 Christina Gross & Monika Hübner |
||||
