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Life and Death in the Mesozoic
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19341 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
6 / 1991 |
2,480 Words |
| Author
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Peter Douglas Ward
Peter Douglas Ward is professor of geological sciences at the
University of Washington. He has been investigating the late
Mesozoic extinction for over a decade, both documenting the
ranges of species that became extinct and studying the
survivors of this crisis. His most recent book, The
Methuselahs, dealing with the survivors of mass extinction,
will be published by the W.H. Freeman Company in September. |
There is no doubt that a mass extinction of almost unprecedented magnitude took place some 65 million years ago, at the end of a period known as the Mesozoic era. The flora and fauna were very different from those of the present day: The land was covered with ancient plants, such as ginkgos, redwoods, and giant tree ferns, while immense herbs of dinosaurs roamed the plains. Archaic species of fish and shellfish flourished in the oceans, both in the depths and in the shallow seas that covered many of the continents. This assemblage of animals and plants, which had existed for many tens of millions of years, underwent an extinction so massive that it divides the last 200 million years of earth history into two parts.
Exactly what caused this extinction, and how long a period was required for it to take place, is one of the great scientific debates of our time. In the last few years, research has confirmed suspicions that extinction was abrupt, not gradual, for a broad range of plants and animals.
Many hypotheses have been offered to explain the extinction that wiped out the dinosaurs (and much else) about 65 million years ago, with causes such as disease, predation, competition, and even increased radiation being offered as possible culprits. Today, however, these competing theories have been narrowed to two:
1.The extinctions took place over several millions of years and were caused by gradual climate change and cooling; the advent of this cooling may have been brought about by a combination of a global lowering of sea level and increased volcanic activity, which disrupted global circulation patterns of the atmosphere and sea. According to this theory the extinctions were gradual, a fact that should be demonstrated in the fossil record by slowly dwindling stocks of animals and plants being replaced through time by hardier forms.
2.The extinctions were rapid and were caused by the effects of a large asteroid colliding with the earth. In this case (from a geologist's point of view), the extinctions were virtually instantaneous, taking place over a period of years or even months.
The scientific debate over these alternatives has raged for more than a decade now, at times bitterly dividing a previously undemonstrative lot of scientists [see "The Mass Extinction Controversy," THE WORLD & I, January 1988]. The ultimate resolution of this debate must come from detailed examination of the fossil record of 65 million years ago. Several such
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