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| Mass Extinctions |
Imagine a meteor ten kilometers wide hitting Earth. The resulting impact would cause ferocious tidal waves and massive earthquakes. Sulfuric acid would be released into the air, leading to intensely acidic rain. Later the atmosphere would dramatically cool because of the dust. The impact would affect nearly all life to some extent, and almost certainly there would be a significant decline in biodiversity.
Such a scenario is not just the plot of a Hollywood movie like Deep Impact. A meteor that size actually did hit Mexico's Yucatan peninsula sixty-five million years ago (Fig. 3). The consequences of the impact led to the extinction of many major groups of animals, most notably the dinosaurs. This mass extinction marked the end of the Cretaceous (K) period and the beginning of the Tertiary (T), and is known as the K/T extinction.
Although the K/T mass extinction is the best known, it was not the largest. That honor belongs to the mass extinction at the end of the Permian period, 250 million years ago. It is often exceedingly difficult to distinguish species in the fossil record, so paleontologists studying extinction usually examine the disappearance of larger taxa (like genera or families). At the end-Permian extinction, sixty percent of families went extinct. Based on the family extinction data, David Raup extrapolated that up to ninety-six percent of species went extinct at this time. Most paleontologists recognize three other mass extinctions, for a total of five (Fig. 4).
Although these mass extinctions happened during a short period by geological scales, they were not instantaneous. In fact, the extinctions probably actually occurred over a period of a few million years.
What were the causes of the mass extinctions? We know the most about the asteroid-caused K/T extinction. Based on changes in the floral composition around the K/T boundary, some paleobotanists have speculated that there was global cooling after the extraterrestrial impact. Oceanic cooling may have led to the disappearance of reef-building organisms. We know less about the other extinctions, but it likely that they were marked by periods of global climate change as well.
Species extinctions during mass extinction events account for only a few percent of total extinctions. Indeed, some paleontologists have wondered whether there is anything special about mass extinction events; species extinctions occur often but at different rates across time. Perhaps mass extinctions are merely the tail-end of the distribution of extinction rates.