1 of 3
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 satisfactory explanation for the end-Permian extinction remains up for grabs. Solving this ultimate murder mystery is the preoccupation of an international cast of earth scientists using all available technology for wringing history out of rocks.
Paleontologist Douglas Erwin has hiked up McKittrick Canyon and other trails around the world searching for clues. In Extinction: How Life on Earth Nearly Ended 250 Million Years Ago, Erwin offers a thorough overview of one of the most interesting problems in earth history. A curator of fossils at the Museum of Natural History in New York City and author of numerous professional papers related to the Permian extinction, Erwin takes the reader on an insider's journey that includes adventures in the field, tedious hours in the laboratory, and stimulating but sometimes contentious exchanges among colleagues at scientific meetings. He gives rigorous consideration to every reasonable hypothesis, which means the reader is presented with data and interpretations from paleontology, paleoecology, sedimentology (the deposition of strata), geochronology (radiometric dating), and geochemistry (chemical tracers in rocks that relate to environmental conditions).
Erwin interweaves the history of paleontological work on the Permian mass extinction with an account of contemporary work, including his own multidisciplinary and international investigations. He describes travels to China with MIT geochronologist Sam Bowring and Nanjing Institute paleontologist Jin Yagan to look at marine deposits spanning the Permian-Triassic boundary. Their detailed sampling and dating of volcanic ash deposits in the strata narrowed the duration of the end-Permian extinction event to less than a few hundred thousand years, a mere instant in geologic time. Erwin was guided by South African paleontologists to study the impact of the extinction on terrestrial animals, plants, and fungi (yes, fungi) preserved in the strata of the Great Karoo Desert. But it is not enough to collect ash and bones; even the isotopes of carbon in sediments contain information related to the health of the Permian ecosystem. Erwin not only reports the results of scores of scientific studies but also describes the investigators with collegial grace. More than eighty contemporary scientists are mentioned by name in the text, and even more in footnotes and references. This is a mystery with more detectives than suspects.
share this page
RSS Feed
