Extinction of the Dinosaurs
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Paleozoic Era 334 to 248 million years ago
Mesozoic Era 245 to 65 million years ago
—Triassic Period
—Jurassic Period
—Cretaceous Period
Cenozoic Era 65 million years ago to thepresent
Paleontologists have argued for a long timethat the demise of the dinosaurs was caused by climatic alterations associatedwith slow changes in the positions of continents and seas resulting from plate tectonics.Off and on throughout the Cretaceous (the last period of the Mesozoic era,during which dinosaurs flourished), large shallow seas covered extensive areasof the continents. Data from diverse sources, including geochemical evidencepreserved in seafloor sediments, indicate that the Late Cretaceous climate wasmilder than today’s. The days were not too hot, nor the nights too cold. Thesummers were not too warm, nor the winters too frigid. The shallow seas on thecontinents probably buffered the temperature of the nearby air, keeping itrelatively constant.
At the end of the Cretaceous, thegeological record shows that these seaways retreated from the continents backinto the major ocean basins. No one knows why. Over a period of about 100,000 years,while the seas pulled back, climates around the world became dramatically moreextreme: warmer days, cooler nights; hotter summers, colder winters. Perhapsdinosaurs could not tolerate these extreme temperature changes and becameextinct.
If true, though, why did cold-bloodedanimals such as snakes, lizards, turtles, and crocodiles survive the freezing winter’sand torrid summers? These animals are at the mercy of the climate to maintain alivable body temperature. It’s hard to understand why they would not beaffected, whereas dinosaurs were left too crippled to cope, especially if, as somescientists believe, dinosaurs were warm-blooded. Critics also point out thatthe shallow seaways had retreated from and advanced on the continents numeroustimes during the Mesozoic, so why did the dinosaurs survive the climaticchanges associated with the earlier fluctuations but not with this one?Although initially appealing, the hypothesis of a simple climatic changerelated to sea levels is insufficient to explain all the data.
Dissatisfaction with conventionalexplanations for dinosaur extinctions led to a surprising observation that, inturn, has suggested a new hypothesis. Many plants and animals disappearabruptly from the fossil record as one moves from layers of rock documentingthe end of the Cretaceous up into rocks representing the beginning of theCenozoic (the era after the Mesozoic). Between the last layer of Cretaceousrock and the first layer of Cenozoic rock, there is often a thin layer of clay.Scientists felt that they could get an idea of how long the extinctions took bydetermining how long it took to deposit this one centimeter of clay and theythought they could determine the time it took to deposit the clay bydetermining the amount of the element iridium (Ir) it contained.
Ir has not been common at Earth’s since thevery beginning of the planet’s history. Because it usually exists in a metallicstate, it was preferentially incorporated in Earth’s core as the planet cooledand consolidated. Ir is found in high concentrations in some meteorites, inwhich the solar system’s original chemical composition is preserved. Even today,microscopic meteorites continually bombard Earth, falling on both land and sea.By measuring how many of these meteorites fall to Earth over a given period oftime, scientists can estimate how long it might have taken to deposit theobserved amount of Ir in the boundary clay. These calculations suggest that aperiod of about one million years would have been required. However, otherreliable evidence suggests that the deposition of the boundary clay could nothave taken one million years. So the unusually high concentration of Ir seems torequire a special explanation.
Inview of these facts, scientists hypothesized that a single large asteroid, about10 to 15 kilometers across, collided with Earth, and the resulting falloutcreated the boundary clay. Their calculations show that the impact kicked up adust cloud that cut off sunlight for several months, inhibiting photosynthesisin plants; decreased surface temperatures on continents to below freezing;caused extreme episodes of acid rain; and significantly raised long-term globaltemperatures through the greenhouse effect. This disruption of food chain andclimate would have eradicated the dinosaurs and other organisms in less thanfifty years.
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