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Extinction: How Life on Earth Nearly Ended 250 Million Years Ago
Douglas H. Erwin
Princeton University Press, 2006
320 pp., $24.95
Stephen O. Moshier
That Was a Close One
Rocks and fossils contain many lessons of life and death on ancient earth. A particularly impressive example is found in the Permian limestone of the Guadalupe Mountains in west Texas. Fossil sponges and algae embedded in the rocks formed perhaps the most spectacular barrier reef ever to exist in an ancient sea. Hikers on the trail up McKittrick Canyon follow the traverse of an imaginary paleo-scuba diver swimming up from deep water to the reef crest and then landward across a wide shallow lagoon.
All of the fossil species in the Permian reef are long extinct. This is true of most fossils in the geologic record of life. Paleontologists reckon that most fossil species survived no more a than a few hundred thousand to a few million years before extinction (often to be replaced in overlying strata by similar species with modified features). Overlapping ranges of species generally appear to have kept the ecosystems of ancient earth filled and functioning like an exceedingly long baseball game in which generations of players gradually replace their predecessors. But the Permian reef teaches us another lesson about earth history, for the lords and tenants of this reef were doomed to a mass extinction that would leave few descendants in the overlying strata. Game called on account of global holocaust.
The mother of all extinctions happened at the end of the Permian Period, some 251,400,000 years ago. Ninety-five percent of all marine species were vanquished, with profound effects on emergent terrestrial life. An asteroid that hit the earth more recently, a mere 65 million years ago, managed to take out only 50 percent of marine species while wiping out that hearty group of terrestrial beasts we call dinosaurs. There is substantial evidence for that end-Cretaceous scenario, including a crater buried beneath the Yucatan coast and a thin layer of clay around the world containing abnormal concentrations of iridium, a rare element that is found in meteorites and "cosmic dust." A ...