Mass Extinction a general view

Ashraf M. T. Elewa

Geology Department, Faculty of Science, Minia University, Minia 61519, Egypt, [email protected]

Mass extinction is considered as the most subject matter in paleontology that received several debated input. When looking to the past, we find five major mass extinctions, I believe, in the fossil record (e.g. Late Ordovician, Late Devonian, Late Permian, Late Triassic and Late Cretaceous). Some authors believe in six mass extinctions by adding Cambrian to the previous five events, some others speak on cycles of mass extinctions up to 23 events since the Cambrian. Nevertheless, patterns and causes of these mass extinctions still disputable. It is notable, however, that there is a variety in degree of diversity loss between the minor and major biotic disaster. Yet, the scientists stress on the five major mass extinctions with more focus on the Cretaceous/Paleogene event. This focus is normally due to the extinction of dinosaurs during this interval, which can make the cover of Time magazine, but not productids or fusulinids as stated by Prothero (1998).

The ordinary questions of the scientists interested in this field are lying in the search for common causes and patterns that may lead to a general theory of extinction. The most frequent arguments are related either to bad genes or to bad luck. Whatever were the causes, it is logic to give an idea on each of the five major mass extinctions in the fossil record.

The first major mass extinction event started in the Late Ordovician time. Despite the rigorous crisis affected this extinction, for no reason it has so far received little attention from scientists. Sepkoski (1989) affirmed that 57% of the marine genera disappeared in this crisis (some authors stated that it is the second biggest extinction of marine life, ranking only below the Late Permian extinction). Scientists assigned this extinction to global cooling that triggered glaciation and significant lowering of the sea level. As a result, one hundred families of marine invertebrates died, including two-thirds of all brachiopod and bryozoan families, and once-flourishing trilobites as well as archaic groups of echinoderms also died out (for more information see Sepkoski 1984, 1989; Hallam and Wignall

1997; Prothero 1998). It is notable that scientists excluded the single extraterrestrial iridium anomaly from the causes of this big mass extinction.

The Late Devonian extinction event was almost as remarkable, obliterating 75% of the species and 50% of the genera in the marine realm (McGhee 1995). Some specialists clued that 19% of all families have been died and went extinct during this event. Again global cooling was the most important factor affected this mass extinction according to different authors. On the other hand, the effect of extraterrestrial impact on this event is strongly debated.

The third major mass extinction occurred in the Late Permian-Early Triassic and is considered as the biggest within the five major extinction events. In this event, about 57% of all families, 83% to 90% of all genera, and 96% of the species in the marine realm have wiped out. Even though, the causes of this biggest mass extinction still controversial. Benton (2003) assigned this event to the rise of carbon in the atmosphere. Some others referred to global warming and a depletion of oxygen in the atmosphere due to massive volcanic eruptions in Siberia. A third group blames an asteroid impact, and the fourth group suggested global cooling. Erwin (1993) and Prothero (1998) believe that a combination of different disasters caused this extinction.

The Late Triassic extinction event is considered as the smallest of the big five extinction events. In this event about 23% of all families and 48% of all genera went extinct (Sepkoski 1989). Most authors assigned this event either to global cooling or to volcanic activity.

The fifth major mass extinction started in the Late Cretaceous. It eliminated about 17% of all families and more than 50% of all genera. No doubt, it is the most interesting and attractive event in the fossil record, as it wiped out the dinosaurs. As Prothero (1998) said, prior to 1980 there were a number of ideas for the extinction of the dinosaurs (cooling, warming, disease, inability to digest angiosperms, mammals ate their eggs). I add to this the possibility of dinosaurs to mistakenly kill themselves by their rear spines. However, all these causes, of course, did not kill other groups of organisms!! Therefore, scientists searched for other reasons to this mass extinction, which is apparently started in 1980 when Alvarez et al. suggested that this extinction is due to an extrateresstrial bolide impact with about 10 km in diameter. Since then, controversial evidences have been proposed for this extinction with two significant schools. The first one presumes the effect of the bolide impact (catastrophe), whereas the second believes in gradual extinction (volcanic activity, environmental and climatic changes). This situation changed in 1996, when Molina et al. supposed that both catastrophic and gradual extinctions might have occurred. Elewa and Dakrory (submitted) ensured the results of Molina et al. (1996).

After all, it seems that the sixth major mass extinction is underway. If this is true what are the causes and evidences? The book in hand may give important information on this area under discussion.

In summary, usually, most of the published books on the subject just focus on restricted groups of organisms and could not answer several questions relating to mass extinction. However, the present book is different in combining two main aspects:

1. Five major mass extinctions of the fossil record; and more importantly

2. Contributions on minor extinctions and current mass extinction

These two aspects are introduced through interesting studies of mass extinctions in diverse organisms ranging from small invertebrates to big vertebrates, and take account of the most admired subjects discussing mass extinctions in attractive groups of organisms like insects and dinosaurs.

Moreover, as in my previously edited books with Springer, I selected an exceptional group of specialists working on this phenomenon to explain and write about the subject.

Regarding addressees, mass extinction is one of the most popular topics for students, at all levels, researchers, and professionals. Besides, this book project represents advanced ideas and useful synopsis on this subject stuff.

Finally, I would like to thank Dr. P. David Polly (USA) for writing the foreword. The contributors and the publishers of Springer-Verlag are deeply appreciated. As usual, I am much indebted to the staff members of Minia University of Egypt.

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