The Extinction of the Dinosaurs

The Extinction of the DinosaursGeologists define the boundary between sediment layers of the Cretaceousperiod (144..C65 million years ago) and the Paleocene period (65..C55 millionyears ago) in part by the types and amounts of rocks and fossils they containor lack. Before the limit of 65 million years ago, marine strata are rich incalcium carbonate due to accumulations of fossils of microscopic algaedeposited on the sea floor. Above the 65-million-year limit, sea-floor sedimentscontain much less calcium carbonate, and fossils of several families ofmollusks are no longer found. In continental sediments, dinosaur fossils,though frequent before 65 million years ago, are totally absent. By contrast,new families of mammals appear, including large mammals for the first time.Scientists wondered for many years about what could have caused thedinosaurs’ rapid disappearance at the end of the Cretaceous period, comingup with a great variety of theories and scenarios. For some, it could have beendue to unfavorable genetic changes triggered by a dramatic increaseby afactor of 10, 100, 1,000 in cosmic-ray particles reaching the Earth after asupernova explosion somewhere in the neighborhood of the solar system. Forthese high-energy particles to affect life, they would have to get through theprotective barrier of the Earth’s magnetosphere, the region of the upperatmosphere controlled by Earth’s magnetic field. That could have happened ifthe cloud of particles from the supernova explosion reached the Earth during aperiod when the magnetosphere was weakened, something that may happenwhen the Earth’s magnetic field changes direction. And we know that themagnetic north and south poles of the Earth switch on the average twice everymillion years. However, this is not the only possible explanation for dinosaurdestruction.According to paragraph 1, which of the following is true of PaleocenesedimentsA.TheylackfossilsfromsomefamiliesofmammalsfoundinCretaceous sediments.B.They contain fossils of dinosaurs.C.They contain fossils of some animals that did not exist during theCretaceous.D.They contain fossils of more kinds of mollusks than are found inCretaceous sediments.The word “unfavorable” in the passage is closest in meaning toA.UnusualB.DisadvantageousC.SuddenD.majorOther theories have raised the possibility of strong climate changes in thetropics (but they then must be explained). Certainly, if climate changes, thechanged distributions of temperature and rainfall modify the conditions thatfavor one ecosystem over another. The extinction of a particular family, genus,or species may result from a complicated chain of indirect causes and effects.Over thirty years ago, scientist Carl Sagan quoted one suggestion that thedemise of the dinosaurs resulted from the disappearance of a species of fernplant that was important for dinosaur digestion. Other theories involved aworldwide cold...